Depth of Field, Depth of Focus, and Film Flatness

[Kirk Gittings]

"is a kind of silly point of view in my opinion."

Can we try and stay away from this kind of language. Calling someone's opinion silly is where the flame wars begin. We don't have to put each other down to have an intelligent discussion.

[Kirk Keyes]

Sorry, it was just my opinion...

[bglick]

Steve.... In the cancelled post, I tried to be a peace maker. In that thread (which was temp. removed by the moderator), I was amused and confused how one person could possibly have so many enemies. This post, which is not a question, but rather your version of a continuation of the cancelled thread, clearly demonstrates your struggles with 1) admitting being wrong, 2) offering credit to posters trying to help you.

In all fairness Steve, you could have at least credited several of the posters who "beat their brains out" in the cancelled post to first, try to convince you the concept of "depth of focus" even existed. Then after you blasted them for such, they still took the time and energy to educate you on this subject. At first, you did not acknowledge the topic of "depth of focus", next you took the position, it only exist for anal photographers, and now in this post, it exists...... but you explain why it's not signficant. As an author in this field, it's my opinion you should at least give credit to those who educated enough to write this post. Gestures like this may reduce the enemy count and possibly increase the number of buyers of your book and magazine..... it's just a suggestion, it's obvious that, me and you have two different mindsets on how to treat others.

Now, based on what I learned about your "mode of operation" in the last cancelled post, I do not expect a response regarding any subject matter that does not present you, or your opinions in a favorable manner. In addition, this post was not a question like most, it was an attempted tutorial, so I am only making comments on your tutorial.

Although I do applaud your recent R&D on this subject matter.... it appears you did more then make phone calls to a few friends who knew nothing about "depth of focus" subject, and therefore dismissed the subject as being virtualy useless. This post is simply trying to further that position. Although your understading of "depth of focus" has increased, your tutorial, in my opinion, is too much of an oversimplification and in some areas the vagueness makes it border on being wrong. You continue to refuse to express the relative nature of your desciptions in numbers. I understand your approach, reduce all math and any detail down to a few vague descriptions. So, I will do my best to stick to your premise....

> What appears to be in sharp focus in front of and behind the plane of sharp focus is really a function of the size of the circles of confusion.

"appears" is rediculously vague for a writer of technical books, but it would require a tiny bit of simple math, which you do not include, so this describes nothing. Now, I would state this as, the size of the Circle of Confusion (CoC) is often a function of the "defocus" principle, i.e. relative position of the near or far subject distance compared to the point of exact focus. Of course, the CoC can be effected more, or less, by lens MTF performance, aperture diffraction and depth of focus. The point being, Depth of Field alone, is rarely EVER the only determing factor of the CoC diam. This is NO small detail.



> If these are small enough then those parts of the image will appear to be sharp when the photograph is viewed from a normal viewing distance.

"normal" Now, I know you don't like using numbers, but how can you define normal for us? "appear sharp" what is sharp? as compared to what?



> What is depth of focus? Depth of focus is the space behind the lens where the film plane position can vary slightly and still not cause any loss of sharpness.

Your statement is not completly wrong, just too vague to be clear. More accurately, Depth of Focus, is the the region in front of and behind the focal plane where the diameter of the light cone is smaller than the permissible circle of confusion.

> However, if the subject area completely fills the depth of field area the depth of focus becomes so small that there is no tolerance for any variation in the film’s position

First, subject area always fills the depth of field area, otherwise it would not be considered depth of field area, as depth of field area never includes "air", as lenses do not focus on air. Point is here, the only thing that effects Depth of Focus (film play as you call it), is the f stop, or effective f stop for close up photography. The film plane doesn't have a clue what is happening on the other side of the lens, or depth of field.



> thus depth of focus can not really be used to make up for an incorrectly aligned plane of focus (mis-aligned film and lens board planes) or a poorly positioned piece of film.

Again, not sure if your wrong or right, as I read it 5x but still don't understand what you wrote..... I will offer, Depth of focus is not a tool to correct for any physical shortcomings in the camera system, instead, Depth of Focus simply defines the region in front of, and behind the focal plane, where the diameter of the light cone is smaller than the permissible circle of confusion. The definition can not be shortened and remain clear. I still have not used one single number, just short sentences as you like.



> In most situations the depth of focus is so small as not to be an effective aid to us.

What? This is contradictory. The fact the Depth of Focus is "so small" explains the potential it has for being the bottleneck in the entire optical chain! The fact it is so small is what makes it so dangerous, vs. "not to be an effective aid" ??? Steve, I truly beleive you have grasped part of depth of focus, but are are not applying it properly. You have the tail wagging the dog here!



> However, depth of focus really becomes an academic concept that is not of much use to use when we are making a photograph IMHO.

Well, here is how I would have worded this...... Depth of focus becomes an academic concept that is not very useful when using a box type roll film camera (MF or 35mm) and shooting at f stops above f 4.0 (based on Zeiss study), this assumes not using a roll film back with reverse rollers which can alter the film buckle dramaticaly when the film is left in the back more then 5 minutes and film is advanced.

However, when using a view camera, Depth of Focus should be something to consider as the cameras size and floating standards can put more demands on depth of focus requirement. Also, the film holder and gg alignment positioning must be considered as unlike box cameras, the film holders and cameras are rarely made by the same company and often not of the same vintage before all cameras / film holders used the same set of standards. Depth of Focus shortcomings with view cameras can surface under these scenarios.....



1. the front and rear standards are not well aligned and square at all corners. The lens board is not square to the front standard. Once again, older cameras can easily become out of alignment, as well as those knocked around in backpacks, and those, such as Benders (and others) which simply have poor manufacturing tolerances to begin with.

2. The gg/film plane alignment error is close, or outside the depth of focus region. (to allow for other shortcomings) Often an issue with mixing different roll film backs of different vintages on view cameras of different vintages, as well as using older film holders before that maker complied with ANSI standards (which assumes the camera complies also) Steve you forget how many people use vintage gear, specially hardcore hobbiest.

3. You shoot at low f stops, which decreases the depth of focus region. Stopping down, increases the depth of focus, but at the expense of diffraction and shutter speed. So not always such a wise choice. This is especially true of people using the newer digital lenses with film, whereas their MTF's are optimised at f8 - f11.

4. Focussing at infinity and using lower f stops then one is used to, suddenly, depth of focus shortcomings can appear.

It should be noted, that Depth of Focus shortcomings when photographing scenes with a large amount of depth of field will not always be apparent, as sometimes the alignment errors simply move the point of exact focus in front of, and behind the focal plane. However, when photographing a scene with very small Depth of Field, or none, such as a graffiti on a brick wall, the same camera / lens set up that seemed flawless, can suddenly produce images that are shockingly out of focus. (Rob tried to explain this several times in the cancelled post)



> One thought about the importance of depth of focus is that of the four primary books on large format technique, only one of the books even mentions depth of focus.

As Rob, Brian and many others pointed out in the cancelled post, the fact a given subject matter is not covered in 3 books you chose to select, should not dismiss it as a valuable piece of information to photographers in the modern era, armed with spreadsheets, programmabe calculators, higher education, etc. etc. For example, in your book, you couldn't have mentioned it, since you did not know it existed. Maybe this is the case with the other 2 authors you mentioned? I personaly would never dismiss something as "not useful" if myself and 2 other people were not aware of its existence? What about all the other photography books which are much more thorough then the books you mentioned? Are those 3 books the Golden standard of photographic knowledge? What about literature that goes back to the late 1800's? All useless, since a few short, low production books don't mention it? I just don't grasp your reasoning, although I keep trying.

> and in my knowledge of hundreds or working large format photographers doing the same, an unsharp image due to poor film flatness in a holder is an extremely rare commodity.

Well, in my less then 30 years of doing this, and many LF photographers I know, I have seen the Depth of Focus issue ruin a lot of images, waste a lot of money on film and travels and upset several people. Are the photographs usable? Sure, but they fell way short of the expectations of the photographer. In addition, there is many photographers that may not know they even have the problem, instead, it just limits the enlargement potential of some of their work, sort of a "reverse engineering" solution to a problem they were unaware of.



> Many studies have been done looking at the effective film flatness of various holders, whether or not they have a pressure plate, etc.

Yes, one in paticular was done by Zeiss in the mid 80's. They concluded film flatness is a major issue, and the sharpness of lenses was now being limited by film flatness in MF cameras. This was less of an issue in the earlier days in photography, as the diam of Coc was more relaxed vs. today. The study also reviewed film buckle on reverse rollers which became a severe issue at f 8 or lower and overall film flatness was becoming a major issue at around f 4.0 - 5.6 (from memory). Now, does this dismiss the problem for LF shooters since we shoot at f 16+? I am sure in to defend you position, you would think it is a reason to dismiss such. But in my opinion, NO! Why? Because a view camera has two major shortcomings vs. the MF box camera when it comes to film alignment / flatness. MF cameras are of box design, wheras the lens is always fixed at ONE position, eliminating loose, floating, front and rear standards. Sheet film can buckle as it's not held tight at both ends like roll film, it also can be prone to gravity when pointing the camera downwards.

To benefit some readers Steve, I am sorry, but I feel compelled to introduce a few numbers.... What is the difference in the Depth of Focus at f 5.6 vs. f 16 for non-close up photography? I will use the same CoC for the same enlargement factor of each....

f16, .033 CoC = .50 mm in either direction of focal plane

f5.6, .033 Coc = .19 mm in either direction of focal plane

.31 mm difference in allowable depth of focus. This is equal to the thickness of 3 pieces of standard 20lb bond copy paper. Now, hold 3 pieces of paper between your fingers and try to consider if you feel a view camera, with all it's physical slopiness, film buckle, focussing errors, etc, (vs. solid Hasselblad type design) can be off by this amount. If so, then you have reached the same conclusion that Ziess had, film flatness / alignment at the focal plane is an issue. Now on the other hand, one might consider the fact Steve and his 100's of friends never experienced this in 30 years as more compelling evidence. Everyone can judge on their own.



> But the key question is do these minute variations make any difference to a photographer making a photograph. Rarely and only in the most extreme conditions is the answer.

And what tests are these based on? Are these tests superior to the Zeiss tests? The LF community never had the benefit of the MF community whereas larger makers such as Ziess, hassy, etc, once had huge revenues to justify all this R&D. The LF camera makers were always spread out over many vendors, making each of one of them very small on a relative basis vs. MF camera makers. So most all tests are often of the home brew variety. But in this case, just applying some common sense can go a long way, even if one has to use a few numbers to grasp the relative nature of the subject.



> We can debate these minute differences

Steve, really, how do you define "minute" when you refuse to apply numbers to anything? I offered you a slew of examples in the cancelled post, which you chose to completely ignore, which is fine by me, but as an authority in this field, how can you continue to write with such vagueness when most of your customers are willing to arm you with the right information and make you look like a hero? I just don't get it. I hope, I have provided some "relative feel" to this problem for those intersted. For those not interested, as Steve states, relax, go photograph. Maybe these issues are of no concern today, but what happens when someone changes to close up photograpy or uses sharper lenses, slopier cameras, ULF, etc. etc.

As Brian and Jorge pointed out so clearly in the cancelled post, as an author, why don't you arm your readers with accurate information, and allow them to make their own decisions whether a paticular subject should concern them. Why write vague descriptions, with no references, nothing relative to what is known, and then conclude, if it's not an issue with you and associates, and it's not covered in a few books, then, everyone can relax, and go photograph. ???

I feel the entire DoF, Dof, gg/film alignment issue, are all equally important regarding on-film resolution, which is one of the reason many people migrate to LF photography. There is many very knowledgeable posters on this list. I would suggest to the moderator to have one of these posters prepare a tutorial that explains these relationships in detail and have it posted on the front page of this forum, just as other important topics have been covered. I would hate to have future users of this forum research this subject matter by referencing threads such as this, as the information is too valuable to be dismissed so quickly by someone who just discovered it a few days ago. And unlike digital photography, this issue will remain with us for as long as film exist.

[bglick]

When writing my lengthy post, many others responded in that time, so I apologize if some of my post was redundant of others above.

Oren, I find your post so entertaining, how can this be? Was Steve the author of the magazine back then? If so, this needs to be entered into the post which was cancelled....

[Jerry Fusselman]

Well, it is obvious to me that depth of focus is a useful concept. It led me to fix my roll-film holder rather than assuming I made some sort of error in technique. After fuzzy results, I wasted a roll of film and looked at it in my roll-film holder---I could see that it was clearly outside of my depth of focus, and therefore needed to be fixed.

[William Mortensen]

"Mark, I too like to think of the depth of focus at the film when enlarging, and depth of field at the paper. I don't know if there is an actual convention to the nomenclature for it." --Kirk Keyes

Kirk- Actually, I think I'm leaning towards the negative side being on the depth-of-field side, as that is the subject the lens is focused *on*. The projected image would be at the depth-of-focus location, as it is with the camera film plane. Not that the nomenclature matters that much...

BTW, I enjoyed your website, but it needs more b&w work!

[neil poulsen]

One can try to think in broader, more general terms, and it's also possible to speak in specifics by citing numerical examples.

Both add value. On the one hand, it helps to understand something by making a few calculations. But I think that in the application of these moderately complex principles, we must ultimately arrive at some generalizations to guide us in the field. If we were out in the field with computers, we probably wouldn't be shooting LF, because we'd be tethered to a digital camera.

As I said, I think that both approaches add value.

[Henry Ambrose]

It appears to me that when dealing with Depth of Focus a photographer has the option of acknowledging Depth of Focus and conducting a complete audit and adjustment of his equipment or simply ignoring it and using his camera gear as-is, assuming that the makers have dealt with the issue. Most seem to take the second approach and assume that their gear is properly adjusted and go forth to make pictures. Both approaches work but I can see that folks on either side who have from practical experience concluded that their way works might think that others who express a different view are nuts.

QT brought up the "good enough" concept. While I agree that "good enough" for professionals -can- be low it can also be quite high. Certainly for the bulk of pictures we see everyday, optical excellence requirements are very low compared to the near perfection needed to make huge enlargements. The point I make is that for paying work a good craftsman delivers work up to the standard called for or maybe just a bit higher. Using a 4x5 camera to photograph a small object that will be reproduced in a catalog at 3 inches wide is sorta like buying a Ferrari to make grocery runs. And of course sometimes "excellence" at performing a job is defined by something other than optical perfection.

A particular "look", high production rates and meeting deadlines are other possibilities (and not all) for judging "excellence" in photographic craft. To this point I have performed with "excellence" (in my mind) when the client needed all 106 shoes photographed with cmyk files delivered in one week - at three inches wide in the catalog they looked great and the job was on-time and on-budget. I have also performed with "excellence" by delivering a single image file for a large poster that took days to perfect. Neither performance made me a better person or proved me to be a great photographer. I really only did what the client wanted. So I might have been "good enough" or I might have been "excellent". Who can say?

So the question comes to "how good is good enough?" I'd guess that carrying a 35mm camera out in the world is not good enough for some folks on this forum. But for some things 35mm is the right tool for the job at hand and other times 4x5 might be just "good enough". When I, with no thought to Depth of Focus, use fine modern lenses on a properly aligned and adjusted view camera with film holders that hold the excellent modern film in the proper flat place, and then scan that film with a cheap flatbed scanner do I committ heresy? Certainly not if my goal is to make excellent 8x10 or 11x14 prints. I've proven that for myself and it is simply beyond arguement to me. Are drum scanners better than my flatbed? Of course they are, but my practical experience tells me that I am producing the desired result and the "betterness" of a drum scanner would be overkill.

In a way I think that a floppy old wooden field camera would be incapable of making a photograph of any perfection. (most of us know better from having seen with our own eyes) But how would it be possible if Depth of Focus is so critical? For that matter how is it possible to properly use the finest, tight, new monorail with a binocular viewer when I know that simply removing the heavy viewer produces some movement in the camera standards? I've just blown any chance I had at making a perfect picture! These two extremes clash and the heat generated is considerable.

There is no shame in either course, (or anywhere in-between) just go out and make the results you desire.

[Ted Harris]

Bill, Oren and Mark make an excellent point as it relates specifically to the use of roll film holders. Many users of older Graphic roll film holders, for example, are well aware of problems they can encounter as a result of improper tensioning/placement of the film in the holder. I understand it can be wholly or partially corrected but it is an issue nonetheless. I was the victem of this problem about 15 years ago when working with a Graflex XL system. The first test shots I took with the beast wre all 'softer' than they had any right to be or were partially soft in some places. I knew the rangefinder was right on and given the rigidity of the system knew it had to be related to the holders. I couldn't find anyone to fix them for me in a timely fashion and that coupled with the focusing nipple issue with that camera made me give the system up after only a few months of use. I go intothis detail simply because it can also be a problem with other roll holders as pointed out above. Usually you can solve the problem simplyby tensioning the roll before you close the back but not if the internal tolerences of the gears and rollers are sloppy.

I never thought of the problem in terms of Depth of Focus, and to this day don't tend to think of film flatness or gg placement issues in terms of Depthof Focus but they are, in reality, all a subset of this issue.

[Oren Grad]

A postscript to Ted's point, I was amazed to learn that there's no standard for the film-to-ground glass register for roll holders. Look at the table on this page:

medfmt.8k.com/mf/rollfilmback.html

The difference between the extremes in that table (Linhof vs Wista) is a full 0.25mm. Maybe the manufacturers are intentionally building in a fudge factor reflecting their measurements of typical film bending with their respective film paths, but in the absence of a specific explanation backed up by empirical data this is pretty disconcerting.