Article on Scheimpflug in latest View Camera [Archive]

Leonard Evens

2-Feb-2008, 14:58

The latest View Camera, which I just received has an article entitled "What Scheimpflug Didn't Tell You". I wonder if anyone has figured out what the author is trying to say.

It looks as if it might contain some kernels of useful truth, but I've been having an awful time trying to decipher it. Part of my problem may be that I am a mathematician and have studied and taught geometry for over 50 years, so for me certain terms have definite, well established meanings. What he says is often either false or meaningless with the conventional meaning of the words. . For example, he says that the plane of sharp focus in subject space "contains" two other planes, but when one plane contains another, in the usual meaning of the word "contains", the two places have to be identical. It is possible he means "intersects" rather than "contains". But perhaps a non-mathematician might read these words differently.

I don't mean to dump on the article because writing about mathematics, including geometry, can be difficult, and sometimes even skilled mathematics teachers have difficulty doing it clearly. I used to spend hours using preparing colored 3D diagrams for my classes. A non-mathematician may find it even more difficult to say what he means. I would just like to know if there is anything in this article which I don't already know. I am about halfway through, and I still don't see what the point is. I plan to dig my way through it, but if anyone has any clues as to what it is about, I would be glad to hear about it.

In trying to figure out what he is saying, i did come across one useful bit. Namely, if you use the distance from the lens to the image plane along the (tilted/swung) lens axis, some formulas take on a simpler form. Also, that image point always corresponds to a point in subject space (possibly at infinity.) T.here is another useful image point often considered, that along the original (un-tilted/swung) lens axis, perpendicular to the image plane. Its distance to the lens is useful because it is what you control directly as you focus, but there may or may not be a corresponding subject point, depending on how far you tilt/swing.

Louie Powell

2-Feb-2008, 15:14

I'm glad I'm not the only one who found that article incomprehensible.

Doug Howk

2-Feb-2008, 15:23

Harold Merklinger's explanations are more comprehensible than this article. The article's accompanying photos & sketches may be part of the problem.

phil sweeney

2-Feb-2008, 15:24

Hi Leonard,

I've read some of your posts and would consider you an expert. For me however, the first couple of paragraphs were input overload - I knew I would not be able to digest all to follow.

I did enjoy all the photographs this issue.

Matt Blaze

2-Feb-2008, 16:11

I, too, found that article incomprehensible, but I'm also in a mathematically pedantic line of work so perhaps that puts me at the same disadvantage. Aside from the non-standard (or incorrect) geometric terminology, I was confused by whether the piece was intended to be about theory (in which case I'd be reading to learn principles) or practice (in which case I'd be reading to learn techniques I could put to direct use).

Not trying to be snide or picky here, but that article definitely did give me a headache.

Eric Biggerstaff

2-Feb-2008, 16:16

I had a tough time with this one, but these types or articles always leave me a bit lost! However, if I get a few hints and a little infor from them, all the better.

Steve mentioned in another thread that the article did have some printing issues, for example, the images were not cropped correctly. He tried to correct it before the printing ran the article (if I remember correctly) but it was too late.

He did print the corrected article on the free articles section of the VC website. I have not gone there to read it (heck, it wouldn't matter for me!) but a few of you might check it out, perhaps it would help with some of the confusion.

Bruce Watson

2-Feb-2008, 16:26

The latest View Camera, which I just received has an article entitled "What Scheimpflug Didn't Tell You". I wonder if anyone has figured out what the author is trying to say.

I'm a mechanical engineer. One of the things engineers learn is how to quickly find useful information. After the first paragraph I could tell that the author wasn't going to be sufficiently clear and concise to make the article useful for me. Sorry, I just gave it a pass, precisely because I couldn't figure out what the author was trying to say.

steve simmons

2-Feb-2008, 17:22

Yes, the photos were incorrectly cropped. As the publisher the buck stops at my desk and I apologize. We have placed a corrected version in the Free Articles section of the view camera web site.

I am curious about one thing. I have asked Leonard Evens on more than one occasion to write this type of article for View Camera. He either does not respond to my invitations or declines. Yet everytime we do something like this, we did an article two years ago on movements and landscape photography, he comes on and writes a long critique telling us about the mistakes in the other person' article. In the case of the movements and landscape he told us there was an error, never told us what it was, and then said it didn't matter anyway, but it took him many paragraphs of writing to do all of this. ?????????????????????????

Anyway, I do apologize for the errors in this issue and we are working with our printer to try and decide what to do. In the meantime this article, with the correctly cropped photos, is in the Free Articles section of the view camera web site.

www.viewcamera.com

steve simmons

Peter Lewin

2-Feb-2008, 17:29

Steve: When I go to the Subscribers Only section of the site, the page that comes up says "Last updated Nov 1, 2007." Any ideas what could be happening? By the way (and judging from prior posts, maybe this is a failing for all of us with degrees in mathematics :) ) I also found the article incomprehensible. Was it meant to illustrate theory, application of Scheimpflug, or both?

steve simmons

2-Feb-2008, 17:34

My web person is out at PMA but will be back Monday. At that time we will add

a slightly expanded version of the 7x17 article part 2 and a very thorough comparison between the old and new versions of T-Max 400 done by Sandy King.

The math in the article is not my thing - my mantra is "The ground glass is truth", but I can't just do a magazine for me and many people enjoy this kind of depth.

steve

Nathan Potter

2-Feb-2008, 18:04

Yeah Steve, this article is rather complex and I've been trying to understand what Schiempflug didn't say as explained by the author. The problem is that the terms are not clearly defined as pointed out above. On the other hand these kinds of technical articles require a lot of work on the part of the reader even when they are easily intelligible. I for one like the challenge though so keep a few coming - much appreciated. Great issue - thanks.

Nate Potter

Jeff Conrad

2-Feb-2008, 18:28

I've analyzed the Scheimpflug principle extensively (as Leonard can attest), and think I understand it quite well, yet I can make no sense whatsoever of this article. Some of the photos and later diagrams are interesting, but like Bruce, I didn't want to invest the time wading through the material at the beginning.

Leonard Evens

2-Feb-2008, 19:27

I've now finished the article, but I still don't see its point.

Be that as it may, I did find that while struggling through it I started to think about some geometric issues that I had previously ignored, and I did find that useful. I mentioned one in my previous remarks. Another concerns what happens to the relation between the frame and the region of lens coverage when you tilt. The article makes an oversimplified calculation, which ignores the actual geometry . In reality, when you tilt, the cone of lens coverage intersects the image plane in an ellipse, the size and shape of which depends on the tilt angle and bellows extension. Where you can fit the frame within that ellipse through rise/fall depends on the dimensions of the frame and whether it is in portrait or landscape orientation. There is also a lower level in the image plane below which you can't image any points in the exact subject plane, which might limit further where you can place the frame. It becomes even more interesting if you do some additional ray tracing for points in the DOF wedge about the plane of exact focus. It should be an interesting problem to do the calculations. So reading the article wasn't a waste of time.

There is supposed to be a follow-up article. Perhaps that will say something interesting. Maybe he will even get to the hinge line which as far as I can tell didn't appear is the first article.

I've done some google searching for the "Rules of Optical Rectification", but I couldn't find very much on the subject. The author describes the Scheimpflug principle as Rule II in that subject. I'm curious what the other rules might be. I think the subject area in which this might all fall is photogrammetry, but I wasn't able to find out much about that on the web. If I have time I might go to my University's Library and try to see if I can find anything on the subject in a book.

I'm considering writing an article myself which summarizes various things which have cropped up in our discussions here, but I am deterred by the difficulty of producing adequate diagrams. Also, I would need a crew of volunteers to assess how understandable it is. It is alwasy difficult to get the right balance between comprehensibility and precision.

Helen Bach

2-Feb-2008, 20:20

Aside from the general geometric confusion, there seem to be quite a few optical terms used in the article that do not have the meanings I understand them to have. The principal planes, for example. How much peer review is there for these technical, or pseudo-technical articles?

Best,
Helen

cotdt

2-Feb-2008, 20:23

I didn't understand this article either. But I'm new to large format so that's understandable.

Capocheny

3-Feb-2008, 00:02

Hi Leonard,

Of all the people on this forum (and, probably, ANYWHERE,) who could pen an understandable article on Scheimpflug, IMHO, it would be you.

I'm sure it would be terrific (and comprehensible!) :)

Cheers

Michael Kadillak

3-Feb-2008, 08:18

phil sweeney

3-Feb-2008, 09:06

Hi Michael,

I agree and hope my post is only seen as idle chatter.

Helen Bach

3-Feb-2008, 09:21

The act of assistance that I would consider as being the most valuable to an author of a technical article is a thorough peer review. This is something that can distinguish printed material from personal pages on the internet.

In a way, participation in forums like this one is a process of open peer review - we are sharing our photographic knowledge and experience, and allowing it to be contradicted, discussed, criticized and even ridiculed. Taking refuge in a monologue with no opportunity for counter argument is not what some of us wish to do. I have great respect for those authors who do write articles for their websites and then respond to technical comment. You can do that with a live thing like a webpage, but once something is in print it's pretty much done. The review needs to be done before publication.

Finding someone willing to review work properly and intelligently is not easy - it may be more difficult than finding someone to write the article in the first place. People who have a desire to craft an article that is accurate and clear will have to put a lot more effort into it than someone who is content with muddy, inaccurate confusion.

I hope that Michael is not accusing Leonard of Monday-morning quarterbacking. Leonard has made significant contributions to the community, and put considerable effort into those contributions.

Best,
Helen

Scott Knowles

3-Feb-2008, 10:20

I guess after reading the article and the responses, I'm curious what happened to the pre-publication peer review? The problems I saw in the article was its complexity not easily explained in the text and the presentation which added confusion to the text. What it may have had in a qualified peer review for technical merit, it thoroughly lacked in a lay review for presentation and understanding. It may be that there is simply too much in it to describe and explain in the alloted pages, and it should have been reduced to some initial or general ideas with suggestions for later Website or publication articles.

In short, to me, it lost the perspective for its audience, and anyone not knowlegeable about Scheimpflug's rule will simply find it a place to park the coffee cup so you don't ruin the other pages of the magazine. Just my thoughts.

Leonard Evens

3-Feb-2008, 10:23

I am with 100% with Steve on this one. If someone takes the bull by the horns and writes an article, at least he should be given credit for accomplishing what others only contemplated. Plus, he should be at least given an opportunity to be involved in the discussion here to defend his position to the degree necessary. It is wonderful that this evolved into a discussion on the subject, but there is a lot of Monday morning quarterbacking going on here.

The lesson here is that if you have considered sharing some of your photographic knowledge or experience on a particular subject then reach our for assistance along these lines and/or get on the keyboard and get it done. I procrastinated for a log time myself and am working on several articles as we speak. You would be surprised at the assistance that this gracious audience would provide in all aspects of the process as necessary if you would only take the first step.

Cheers!

Look, my motivation was not to tear the author of the article apart nor to criticize View Camera for publishing it. Had I wanted to do that I would have gone about it very differently. I really did have great difficulty understanding what the author was getting at. I thought that might have been because my background led me to interpret certain terms differently. I hoped that someone could give me some further insight about something I missed. From the responses, it seems that no one had any greater luck than I did.

Getting back to the point, it is not as though I haven't regularly posted explanations in this forum and elsewhere, and I even put an article on my website, which some people have found useful. I also am far from perfect and I make mistakes. When they are pointed out to me, either directly or indirectly, I try to correct myself. On occasion people ridicule my explanations, but I hope I don't respond by attacking them for not putting out their own explanations. When you publish something in any form, you should expect people to criticize it. If you feel the criticisms are mistaken or unfair, you should explain why, not attack the critic.

As to the author of the article (or anyone else) being able to respond to my "criticisms", he is of course free to do so. If he doesn't know about this forum, perhaps some kind soul could tell him. If he provided further explanations of the things that confused me, that would be great. I would start by asking what he means by saying that one plane can contain another when they are not the same plane. After that I would ask what he means by saying a plane contains various points that appear not to be coplanar. That would be a good start.

As to publishing an article of my own in View Camera Magazine, I am considering it, but there are some obstacles. Anything about camera movements requires preparing elaborate three dimensional diagrams. A while back I posted in this forum a few such diagrams ( 3D visualization of movements ) having to do with combining tilt and swing. It took me many, many hours to do the calculations. Also, perhaps I am old fashioned, but I feel anything that I publish in print should not only be correct but should be appropriate for the intended audience. Much of the fiddling I do with camera movements involves elaborate algebraic calculations. I doubt if any of that would be relevant for View Camera, and I' not sure I know how to present the gist in words. For me to do that would be a considerable undertaking. I am 74 years old, have many unfinished projects which I will never finish, and I don't know if I want to devote the necessary time to this one. Finally, a while back when I knew far less, I did respond to a request from Steve Simmons and submitted something to View Camera. I won't go into details, but we couldn't agree about the meaning of certain crucial terms. He wanted me to make certain changes which didn't make any sense to me. I found the whole experience rather off putting. Instead I produced the article I put on my website, (which is somewhat out of date now and may have some inaccuracies that I should correct).

I appreciate the kind words about me, but I think there are several others who contribute regularly to these forums who know as least as much as I do and might do a better job explaining it, e.g., Jeff, Helen, Emmanuel, and others. I don't mean that as a suggestion that any of them should undertake such a task beyond what they've already done.

Let me end by saying that despite the fact that View Camera magazine is not perfect, I continue to subscribe to it and read each new issue with interest. I believe Steve is to be congratulated for doing such a good job with it. I just wish he wouldn't take every comment about every article so personally.

Michael Kadillak

3-Feb-2008, 11:32

I hope that Michael is not accusing Leonard of Monday-morning quarterbacking. Leonard has made significant contributions to the community, and put considerable effort into those contributions.

Best,
Helen

Goodness no - Leonard is a highly skilled and valued contributor to this forum and we all value his presence and participation. The subject was mentioned to discourage others from continuing the criticism and keep it from spinning down to the Monday morning quarterbacking type of analysis that so regularly happens.

I would like to encourage others that may have been thinking about authoring an article to do so. I am in the process of working on several more myself as it is a great feeling to contribute to such a marvelous art form we are all so passionate about. And if writing is not your thing maybe there are other ways to assist. I am really looking forward to Foto3 as a very positive product of this conference is to make as many personal connections as possible as we all continue to advance our craft.

Cheers!

Leonard Evens

3-Feb-2008, 11:51

I've been thinking some more about the article, and I think I may see what the main thrust is. Obsessives like me just can't let go of something. Let me try to put it in my own words.

If you assume the camera is leveled with the standards parallel, the Scheimpflug line (called by the author the vanishing line) is at infinity and parallel to any horizontal line in the image plane, all of which are parallel. Choose one such, such as the line in the film plane imaging the optical horizon., and call it the image horizon. If you now tilt the front standard forward, you move the Scheimpflug line up from infinity so it is now at a finite distance. (If you also change the position of the rear standard with respect to the lens, you further change the position of the Scheimpflug line as the plane of exact focus rotates on the hinge line.) The Scheimpflug line is still parallel to the image horizon, so it meets it at infinity. If you now swing, you bring that intersection at infinity in to a finite distance. If you swing to the right---meaning swinging clockwise when looking from above--- you bring that point at infinity in on the right, and if you swing to the left, you bring it in from the left. When you are done, the Scheimpflug line, through which the plane of exact focus passes is at a skew angle, neither horizontal as it would be for a pure tilt nor vertical as it would be for a pure swing. If you keep track of that scheme, it will help you visualize some of the effects of different movements. I think most experienced large format photographers do that instinctively, but it doesn't hurt to spell it out.

One point that is confusing in what I just said is that all parallel horizontal lines intersect in a single point at infinity, at least in the context of projective geometry, which I believe is the appropriate framework. If you consider just a one dimension, a single line, you can view it as being completed at infinity with a single point, so topologically, it is a circle, or as having two endpoints "infinitely" far away. But there is no good way to do this for two, three or higher dimensions without running into difficulties with the concept of parallel lines. The point is that any two different non-parallel coplanar lines intersect in a single point. It is that property which you want to preserve by adding points at infinity. In the projective plane, any two different lines intersect in a single point, and in projective space any two different planes intersect in a single line, and different coplanar lines intersect in a single point. Trying to attach two different intersection points for a pair of parallel lines at infinity leads to difficulties with the parallel postulate for the finite part of the plane or space, so you would end up trying to do optics in some sort of bizarre non-Euclidean space.

3-Feb-2008, 14:02

I assumed the author knew what he was writing about, even if I couldn't follow the author's line of thought. After a few brief tries at it I found the writing so convoluted I went on to the plentiful green pastures elsewhere in the issue.

I think it speaks to the magnitude of the work Steve Simmons has undertaken in keeping View Camera in print, that one time, an article wasn't up to snuff. I can't imagine publishing a magazine myself and am grateful someone has the fortitude to persevere.

So, I prefer to think of all the almost unbroken string of successes of View Camera. Steve Simmons' efforts almost always bear fruit. That's what counts for me.

So I'll just thank Steve for so many works that benefit the world of LF.

Another thought. And here I'll start by offering apologies to the mathematicians on this forum who might feel slighted, but if I have the choice between using a complex theoretic approach yielding perfection to a photographic problem, or using an real world approximation or practical guideline, I'll almost always go for fast and good V.S. complex and perfect.

How do I navigate the Scheimpflug rule? I have a mental model that makes it pretty simple. It takes longer here to write it out than to conjure up the image as I work. Think of a book with just one rigid cardboard page, and rigid front cover and back covers. If you open and fan out the covers and the one page, you will have a model of three geometric planes that all intersect in a single line. A carpenter might think of it a hinge with three "leaves" and a single hinge pin. The hinge pin would define the line where the three planes or leaves of the hinge intersect.

The front cover is the plane of the subject I would like to have in focus. The inside page is the plane of the lens standard. The back cover is the plane of the film. Scheimpflug's rule says the three planes of subject plane, lens plane and image plane always converge on a line.

The orientation of the film back determines the perspective rendering of the image, so for me, that is a decision I make first. Then I sort out the plane I wish to be in focus in the subject. Those two planes intersect in a line, Scheimpflug tells me the lens plane must also enclose that line, so I tilt or swing the lens to bring the three planes into a "three plane hinge".

After that, it's just practice.

Best,

C

Charles Hohenstein

3-Feb-2008, 14:18

I'm glad that I'm not the only person who couldn't make any sense out of this article.

Capocheny

3-Feb-2008, 14:51

I'll second and third Darr and Michael's comments about Leonard... he has contributed a LOT of great information here on the forum and continues to do so.

I can still recall a number of conversations when I first started in LF... and his explanations always managed to "turn the light on" for me. He took the time and effort to ensure I understood the gist of the topic and, for that, I am eternally in debt to him for his kind assistance. Thanks again, Leonard. :)

So, to that end, I think it would be great to see an article on the subject from Leonard and I'm sure many others would find clarification and enlightenment from reading it.

As for the article in View Camera, people have different writing styles (and thought processes) and if he were offered the chance to respond here, I'm sure he would be able to explain his thoughts.

Like Leonard, I also hope someone will let him know about the ongoing discussion here on the forum and that he would join us to discuss his article.

Cheers

Nathan Potter

3-Feb-2008, 15:01

Leonard, thanks, I follow your comments but I'm not sure that is what the author was trying to say - could be. I liked your earlier comment about the circle of confusion shape that is imaged on a film plane that is in tilt or swing - it becomes an ovoid. Hadn't thought much about that in the past.

Nate Potter

Leonard Evens

3-Feb-2008, 16:03

Another thought. And here I'll start by offering apologies to the mathematicians on this forum who might feel slighted, but if I have the choice between using a complex theoretic approach yielding perfection to a photographic problem, or using an real world approximation or practical guideline, I'll almost always go for fast and good V.S. complex and perfect.

How do I navigate the Scheimpflug rule? I have a mental model that makes it pretty simple. It takes longer here to write it out than to conjure up the image as I work. Think of a book with just one rigid cardboard page, and rigid front cover and back covers. If you open and fan out the covers and the one page, you will have a model of three geometric planes that all intersect in a single line. A carpenter might think of it a hinge with three "leaves" and a single hinge pin. The hinge pin would define the line where the three planes or leaves of the hinge intersect.

The front cover is the plane of the subject I would like to have in focus. The inside page is the plane of the lens standard. The back cover is the plane of the film. Scheimpflug's rule says the three planes of subject plane, lens plane and image plane always converge on a line.

The orientation of the film back determines the perspective rendering of the image, so for me, that is a decision I make first. Then I sort out the plane I wish to be in focus in the subject. Those two planes intersect in a line, Scheimpflug tells me the lens plane must also enclose that line, so I tilt or swing the lens to bring the three planes into a "three plane hinge".

After that, it's just practice.

Best,

C

It may surprise you to learn that we mathematicians think about it in much the same way you do. But you didn't mention one additional element, the hinge line, which is directly below the lens and different from Scheimpflug's line. It actually took me a while to discover its importance. I found it first in Bob Wheeler's Notes and then later in Merklinger's writings. To understand the importance of the hinge line, you really need an animation, such as can be found at
www.trenholm.org/hmmerk/HMbook18.html
which shows how the bellows extension is related to the exact subject plane.

The Scheimpflug principle by itself doesn't determine the position of the subject plane since there are an infinity of "pages" starting at the Scheimpflug line, but only one focuses in the image plane. To select that "page" you need to know it also passes through the hinge line, which is fixed by the tilt.

The only advantage that I have over you as a mathematician is that I can translate the geometric picture we both rely on into formulas from which I can make calculations. For most purposes those calculations are not too relevant, but every now and then a question arises where they can be helpful. I also rely on 3 D visualization skills I've developed from years of mathematical work, but I think many large format photographers also develop such skills by visualizing how their images are formed.

steve simmons

4-Feb-2008, 00:12

Finally, a while back when I knew far less, I did respond to a request from Steve Simmons and submitted something to View Camera. I won't go into details, but we couldn't agree about the meaning of certain crucial terms. He wanted me to make certain changes which didn't make any sense to me. I found the whole experience rather off putting.

I don't remember this at all but if I made a few suggestions it was with the intent of trying to clarify a few things, I am sorry you found this 'off putting.'

The gentleman who wrote the article is a retired aerospace engineer. The fact that he may not have used terms used by others does not make the article innaccurate. I will try and commnicate people's concerns to him and see if we can clarify things in the near future.

With regards to my reactions to Evans' comments it is true that I am a little sensitive to his coming on this forum telling people that articles in View Camera are somehow flawed. He did so a couple of years ago about the camera movements and landscape photography article but he wouldn't tell us what the flaw was and then told us it didn't matter any way so what was his point.??? To be fair I will look at the article on Evans's site and read it carefully.

steve simmons

4-Feb-2008, 01:56

Emmanuel BIGLER

4-Feb-2008, 03:25

Hello from France
I have found the article difficult to read myself. But English is not my mother language.

However I think that I understand what the authors wants to tell us, at least in the title of the paper, i.e. questions of how the perspective in the object space is projected on the recording mediums, which is usually a plane (but could be a cylinder, remember some good ol' 620 film cameras of the past ;) )

Scheimpflug explains us how to get sharp the image of a slanted plane, but not much beyond that.
Even the most interesting question of DOF limits is not, strictly speaking, contained in Scheimpflug's rule and probably not described in the original patent. However in Leonard's paper on view cameras he proposes the 3-plane approch which solves the question (with some additional argumentations needed, I have tried to elaborate on that in my article in French,
http://www.galerie-photo.com/profondeur-de-champ-et-scheimpflug.html
and I absolutely need to incoporate this last version in the English version here as hosted generously by Tuan Luong)

My undestanding is that the vocabulary used in the recent View Camera article is the one in use in drawing perspective. According to the rules known since the Renaissance days by painters.
I would guess that "principal plane" is the terminogy used in perspective drawing. Sure I agree with Helen B., principal plane has another meaning in optical engineereing ! But scientists are specialists of using the same terms for different meanings in different specialities !

I missed by one year the last courses of what we call in French : Géométrie Descriptive (descriptive geometry ?) which was a part of the (mandatory, of course) course in mathematics that future engineers had to follow in order to enter one of our Grande Écoles. Including École Polytechnique.
So up to the sixties, all French engineers could make a correct perspective drawing or represent in perspective or by projections, say, not the View of Venice like Canaletto, but the representation of a slanted hole drilled inside a cylinder and the shape of the curve as visible along the cylinder surface (sure that Leoard loves this kind of exercise ;) ) The nice thing about this is that you could do it entirely by hand. No computer needed. Not even a traditional had computation with liogarithms.
That was magnificent. And magnificently difficult to understand.
-----
What is not explained in the article is the important fact that the projection we realize on our ground glass before recording it on the slanted, Scheimpflug-style, sensitive medium is a compound of a perfectly sharp image and many, many unsharp images.

Then enter our favourite heroes :

The Infamous Entrance & Exit Pupils ! (bells & whistles, please, while they enter the Stage,(plaude, cives)

The author says that the exit nodal point is the centre of perspective for the projected image, this is only true for quasi-symmetrical lens designs, hopefully most of our view camera lenses except telephotos. So forget about telephotes, the article is already complex ! (othewise I can see tomatoes aimed at the heroes on the stage, launched by the angry public !)

But I want to remind to our readers ready to purchase a ..mm wide-angle tilt & shift lens for their ***** [CENSORED BY THE MODERATORS] small format camera, that strong wide-angle retrofocus designs do not obey the classical rule : in order to get zero keystone distorsion, suffice to set the sensor plane parallel to the object plane whatever the tilt angle on the lens might be.
This is valid only for quasi-symmetrical lens designs as explained in all basic view camera primers.
With a strong retrofocus WA lens, even if the film plane is parallel to the objet plane, you get some keystone distorsion if you tilt the lens (then the image is blurred, sure, since Scheimpflug is no longer satisfied, but don't users of small format digital camera claim that they have a huge DOF ?)

And the reason for this ? Hmmm ? You bet ! Yes folks ! the Exit pupil is not where you think it should be !
(I stop here, tomatoes are starting to fly)

Helen Bach

4-Feb-2008, 04:58

Emmanuel,

Excellently written, thank you. If only the author of the original article had mentioned that this was based on drawing theory and terminology instead of optics. (Michael Nyman's strictly drawn An Eye for Optical Theory from Peter Greenaway's The Draughtsman's Contract has come to mind. Fortunately the drawing is done for us by the lens and we can learn its practical properties, including the complex behaviour of out-of-focus points*, by observation whether or not we choose to also attempt an understanding of the theory.)

Similarly, although we understand that the rear nodal point can be called the centre of perspective of the image because of its proximity to the entrance and exit pupils in the types of lenses usually used in large format, surely a comment on the applicability of the simplifications and assumptions made for the article would have been appropriate. The confusion over the properties of the entrance and exit pupils and of the front and rear nodal points appears to be widespread, and every author has the choice of maintaining the confusion, attempting to correct it, or at least acknowledging it.

Best,
Helen

*An aside, not in response to Emmanuel's post: The circle of confusion will probably not be an ellipse when the lens is tilted or swung - even disregarding other aberrations it will take on the projected shape of the exit pupil, which in turn may not be what simple theory would suggest.

Matt Blaze

4-Feb-2008, 09:42

Michael Nyman's strictly drawn An Eye for Optical Theory from Peter Greenaway's The Draughtsman's Contract has come to mind.

Ha; I love the connection!

Leonard Evens

4-Feb-2008, 10:51

Leonard,

Thanks for the comment. Maybe I'm going at it inefficiently, but I select the "page" by focussing, and use the Sinar iteratation of rotating the back to correct, then transferring the rotation to the front to get the camera set up. A couple of cycles of that get things in good shape. Most of my cameras, in one way or another, move asymmetrically, which facilitates the iteration.

The page you mention - www.trenholm.org/hmmerk/HMbook18.html - contains a reference to another page - www.trenholm.org/hmmerk/VuCamTxt.pdf On that second page is named the "front focal plane" of the lens, which seems part of visuallizing the "hinge line". Could you tell me what the "front focal plane" is?

Thanks,

Charlie

The front focal plane is the plane, in front of the lens, parallel to the lens plane, and separated from it by the focal length. The rear focal plane is the plane, in back of the lens, parallel to the lens plane, and separated from it by the focal length. All subject points at infinity produce images in the rear focal plane. The front focal plane can be thought of as the limiting plane for how close subject points can be and still produce an image at a finite distance from the the lens, and any points in it would, in principle, be imaged at infinity. (Of course, that assumes no limit on bellows extension. In a real camera, that is what establishes the practical limit for how close you can focus.) The front focal point is where the lens axis intersects the front focal plane, and if you put a point source there, it will produce a light beam of parallel light rays perpendicular to the lens plane emerging on the image side of the lens. That can be useful for certain kinds of lens tests.

The region between the two focal planes can be thought of as a forbidden region where on one side no subject point produces an image and on the film side no point ever shows up as an image point. As the article in your link indicates, the front focal plane intersects the exact subject plane in the hinge line, which plays such an important role in visualizing what happens when you tilt the lens.

The line where the rear focal plane intersects the image plane also plays a role, which is a bit subtler, and is not usually of any practical importance. Any point below it (when tilting forward) doesn't arise from any point in the exact subject plane, so in a certain sense, in delineates the lower limit of where you can place the frame by means of a rear drop. (Since usually you need to worry about points in the image plane not exactly in focus because of DOF considerations, it is more complicated than that.)

Of course, as Emmanuel and Helen have correctly pointed out, all this is for an abstract lens which is idealized as a single point. For a real lens, it gets considerably more complicated. Instead of a single lens plane, there are parallel rear and front principal planes, rear and front nodal points, and an entrance and exit pupil, which can be used collectively in ray tracing to determine where images are formed. Most of the time, in large format photography, particularly when discussing things like tilt and swing, we can use the simplified picture which treats the lens as a point and the lens plane as a single plane. But one can work it all out in gory detail if necessary. In certain cases, it is important to take the differences in the different points into account. For example, although one usually uses the principal planes to determine where image points lie, it is important to use the entrance and exit pupil when discussing parallax shifts as in panoramic photography. Usually the entrance and exit pupils are not that far from the other relevant points, but for some lenses, they can be some distance away.

Even this complex arrangement of planes, points, pupils assumes that certain approximations made in optical theory hold true, and it ignores a whole bunch of other issues such as lens aberrations. For example, the rear focal point is supposed to be where all light rays parallel to the lens axis come to focus. In reality, spherical aberration brings such light rays to focus at different points, so the concept of the focal point of a lens is only an approximation. Similarly, the concept of image and subject points as abstract points without extension doesn't reflect reality. For example, if you examined an "image point" with high enough magnification, you would find not a mathematical point but a complex diffractions pattern with most of the light concentrated in a small circular region, called an Airy disc. The diameter of the Airy disc depends on the aperture, and determines how far down we can stop down without degrading the image too much. In addition, as Helen points out, the aperture (also entrance and exit pupils) is not actually a disk but instead a polygon, with the number os sides depending on the diaphragm, and that can change the details of some analyses. Fortunately, most of the time, with exceptions such as diffraction, we can ignore deviations from the model described above.

Leonard Evens

4-Feb-2008, 11:41

Helen and Emmanuel,

I've always meant to learn more about perspective in drawing theory beyond what I learned in basic drawing classes in school. Can either of you recommend a good book to read on the subject.

My exposure to descriptive geometry is limited to what I learned in a mechanical drawing class in high school. I suppose it might be worth looking into that subject a bit more.

I've always thought that the best theoretical framework for understanding issues like tilt and swing---assuming we ignore the interesting lens theory and treat the lens as a point---is projective geometry. Indeed the author of the article makes the point that one is analyzing the projection through a point of one plane on another, called a perspective transformation by mathematicians. Projective geometry probably originated in trying to understand perspective in drawing and painting, but it goes considerably beyond that. In particular, I think you can't really understand the concept of "points at infinity" in photography without some ideas from projective geometry. Also, the Scheimpflug Principle is a consequence of Desargue's Theorem. (See Q.-Tuan Luong 's diagram at www.largeformatphotography.info/scheimpflug.jpeg.) I've been thinking of writing a short essay explaining the basics of the subject and how they relate to large format photography, but it may end up one more of those good ideas I've never brought to fruition.

I'm glad Helen brought up The Draughtsman's Contract I think I may have seen it sometime in the distant past, but while the plot sounds familiar, the details are hazy. I've just added it to my Netflix queue.

4-Feb-2008, 13:31

Leonard,

Thank you for a concise explanation!

In your quest for a source on drawing perspective, are you looking for a more technical treatment, or a down and dirty how-to-get-it-done, fast and well?

One direction to look for the how-to variety is at books and info for landscape designers, since they need results, now, yet must produce credible renderings.

Another possibility is the best writer on architectural design techniqe I have come across, Francis D. K. Ching. Over the years I have obtained a bunch of his books. I have in hand "Desing Drawing" by Ching and Steven P. Juroszek. It has a pretty broad section on perspective drawing construction, anatytic enough to delve well into measured spaces in perspective.

Ching is as good as I've found, but I've not yet got the whole package of perspective construction all in one source, so I've picked it up piecemeal from perhaps a dozen or more books.

Best,

Charlie

QT Luong

4-Feb-2008, 13:56

Leonard Evens

4-Feb-2008, 14:34

As elegant as descriptive geometry is, it was designed to deal with parallel projection, the kind of projection that projects 3D parallel lines into 2D parallel lines, used extensively in engineering diagrams. By contrast, projective geometry deals much better with central projection, which is the kind of projection a lens performs. That's why it is the best framework for investigating the geometry of imaging. However, this was not always obvious. Olivier Faugeras and I were the first to introduce this idea in the computational world in the 90s (http://mitpress.mit.edu/catalog/item/default.asp?ttype=2&tid=10195)

Thanks QT. It looks like must reading for someone like me. Of course, Northwestern doesn't have it, so I just ordered it from MIT Press. If your books sell like mine, I don't imagine you are going to get rich off it, but I hope my little contribution to your royalties helps.

BrianShaw

9-Feb-2008, 20:50

The gentleman who wrote the article is a retired aerospace engineer. ... I will try and commnicate people's concerns to him and see if we can clarify things in the near future.

(I've stayed out of this discussion because I didn't receive my copy of VC until yesterday. But after reading the article...)

Please remind the author that he isn't writing for an engineering journal. As my boss said the other day about something technical I authored: "Would you please 'dumb it down' so I can understand it and explain it to my boss." I hate that term - "dumb it down" - but that is a real necessity in some situations. VC might be one of them!

p.s. I'm totally impressed by the discussion in this thread amonst those who quite obviously understand the details of the mathematics and engineering!

steve simmons

12-Feb-2008, 11:20

Doug Dolde

12-Feb-2008, 12:28

Does anyone really read View Camera?

Eric Biggerstaff

12-Feb-2008, 12:50

Doug,

That is sort of a cheap shot isn't it?

Steve is trying to address issues many people had with the article and has proposed a couple of solutions while looking for feedback.

I would say there are MANY photographers around the country, and world, that read View Camera.

Bruce Watson

12-Feb-2008, 12:57

It might be better to just drop it and move on.

steve simmons

12-Feb-2008, 13:03

--------------------------------------------------------------------------------

Does anyone really read View Camera?

Yes, thousands read every issue and have done so for 20+ years.

thanks for asking.

steve

Alan Davenport

12-Feb-2008, 13:06

I guess my subscription must have run out.

steve simmons

12-Feb-2008, 13:09

As I have said before, if you have a problem with your subscription the most efficient thing to do is

e-mail

amiles@viewcamera.com

call 800-894-8439 from the US m-f 7-4 mountain time
or call 505-899-8054 from anywhere in the world 7-4 mountain time.

Now back to the topic......

steve

David A. Goldfarb

12-Feb-2008, 13:23

Vaughn

12-Feb-2008, 14:07

Does anyone really read View Camera?

I am going to give Doug the benefit of the doubt and assume he was attempting to be humorous (as in "I don't look at the photos, I only buy Playboy for the articles." Unfortunately, humor often falls flat when people are on the defensive. Personally, it gave me a bit of a chuckle.

Like David, I trust what is on the ground glass...simplified even more by me doing mostly landscape work. The article was one I tried to read, but since the time/effort to usability ratio was too high, I will perhaps try it again when I have more time on my hands than images to find, negatives to develop and prints to make. I certainly do not expect every article in the magazine to be personally interesting. As a subscriber since its inception, I have found VC to be a great value.

Vaughn

Hmmmm...I might have been mistaken. I do so like giving people a chance to be nice.

Eric Biggerstaff

12-Feb-2008, 14:28

I think Bruce is right.

As this has taken an ugly turn, lets move on.

Ole Tjugen

12-Feb-2008, 14:43

Thanks for all of your comments. I have copied this thread and am sending it to the author.

Here are the possibilities

1. reprint the article with the correct cropping on the photos
2. #1 with additional comments by the author
3.????????????

thanks

steve simmons

1. correct cropping on the photos won't help, as that is not where the (main) problem lies.

2. The "Additional comments" would then have to be a summary of the few valuable posts in this thread, debating the different terminologies and their derivation. I fear that this would take at least as much space as the original article.

3. Commision an explanatory article from one of the few people here who seem to really understand what's going on: Leonard Evens, Emmanuel Bigler, or Helen Bach (in utterly random order).

Even with their expert help, the original article is still an incomprehensible jumble of questionalby applied terminology to me. I fear it is unsalvageable as it is.

Ole Tjugen

12-Feb-2008, 14:47

Addendum:

I agree with David Goldfarb: An article of this sort should have been reviewed by a competent person or persons before publishing.

Christopher Nisperos

12-Feb-2008, 15:10

I have asked Leonard Evens on more than one occasion to write this type of article for View Camera. He either does not respond to my invitations or declines.www.viewcamera.com
steve simmons

Hi Steve, ... this is a terse message given in a friendly spirit.

It's me again. Christopher Nisperos. You know what? Hypocrisy gets on my nerves. I'm still waiting for your response to any of my many phone messages, emails, letters, faxes .. or whatever I've sent or left you over the years —I don't even remember anymore. In fact, naw ... I've stopped waiting. Don't bother.

Anyway, I find, as my dearly departed mom used to say, that "you have a nerve" ... and, I'd add, not much right to complain when others give you a taste of your own medicine. Odds would have it that you might respond to this open message ('cause it's not my first post to you on this subject). Please don't. I'll only give you the Steve Simmons treatment.

Perhaps it's not your fault that you are rude. Maybe you just don't know any better. But your contempt for me or others will come back to bite you in the butt. (this post as small proof .. arf, arf)

But Happy Chinese New Year, for gosh sake.

Grrrrrrrrrrrr
.