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Michael Perlmutter
30-Nov-2004, 08:25
First-time visitor here, so I thought I would put out a problem that has been bugging me for a long time now. Sorry it is so lengthy, but a full picture of what I have gone through seems necessary . . .

I shoot architecture and interiors in Sweden, and shortly after I purchased a new 4x5 Cambo 45 SF about 7 years ago I ran some tests to get a handle on what my actual depth of fields would be when focused at different distances and with various apertures. To do this I set up a series of objects at 1 meter intervals, and took a series of photographs with the focus on different objects at several of the different distances. Each object I focused on was shot at several apertures, ranging from f/5.6 to f/45. I performed this test with 2 lenses, a Schneider 58mm XL and Schneider 72mm XL. Since I work a lot with a 6x9 back, the 58mm and 72mm are essential wide angle lenses for me, so I wanted to be confident about what to expect as different conditions and needs arise.

When I looked at the results of the tests, however, I was shocked to see that in all cases the object I focused on was not in focus at all on the film. And I mean it was often way off, not just a little bit. So for example when an object 12 meters in front of the lens was sharp on the ground glass, what showed sharp on the film with the aperture at f/5.6 was an object located 4 meters in front of the lens. 8 meters off! When focused on an object 6 meters in front of the lens, the only thing sharp on film when shot at f/5.6 was an object 4 meters in front – 2 meters off. Thinking I must have made some careless errors, I ran the test again. Same exact results. All tests were made with a good 4x loupe, using well-lit objects having good details (like texts, plants, etc.), with both the front and rear standards carefully levelled (no tilts, no swings) and all knobs securely tightened.

I went to the dealer who sold me the camera. He checked to see if the plane of the ground glass coincided exactly with the plane of the film in the holder when inserted in the camera. It did. We did the same test together in his store, where we both were in agreement about what was in focus on the ground glass. We tested a Cambo 6x9 back, a Linhof 6x9 back and some 4x5 sheets in Fidelity holders. Same results in all cases, though the 4x5 sheets were off to a slightly lesser degree. The dealer contacted Cambo in Holland, who requested that we send the camera and a sheet of test negs to their factory. A week after they received it they wrote back saying that they saw nothing wrong (!), but sent a new camera to me anyway (I guess to make me feel better). I tested it too. Same results.

Well, since I am nearly always shooting at f/22, I get by anyway with good sharp images, where I focus a little beyond the point where I otherwise would have if this problem didn’t exist. But it is really unmanageable when I need to open the aperture to reduce depth of field, or when for example there are people in a space who need to be sharp, which under lower lighting conditions requires faster shutter speeds.

I was prepared to agree with those who told me that Cambo is just an inferior product, until another dealer about a year later informed me he got in a used Arca-Swiss F-Line in good condition, which I could get for an affordable price. I told him about my focusing problems with my Cambo, so together we performed the same test in his store with my lenses mounted on the Arca. And guess what? Same results again!

Needless to say, I never did discover anything about depth of field. . . .

Can anyone enlighten me about what might be happening? I know wide angle lenses are supposed to be “difficult to focus,” but this is a bit extreme, isn’t it?

Thanks,

Michael Perlmutter



PS: I know all the literature says one should shoot 4x5 at f/22, that smaller apertures cause problems that degrade the image, but I must say that in my tests I saw no apparent degradation in the shots taken at f/32 and f/45. Just a much wider depth of field! Comments?

Gem Singer
30-Nov-2004, 08:53
Michael,

Are you focusing with your loupe while wearing corrective eye glasses? It doesn't appear to be an equipment problem. From all the equipment checking you have done, it sounds like an eyesight problem to me.

David A. Goldfarb
30-Nov-2004, 08:57
Sounds like you're checking all the right things. I would have suspected the registration of the back, but you say you've tested it, and if you have the same problem with two different cameras, it must be something you're doing.

First, ignore any Polaroid tests for sharpness, if you've tried that. Polaroid (except for Type 55 negs) won't be as sharp as film in general.

You're sure the loupe is focused on the ground side of the glass? Remove the lens from the camera to adjust the focus of the loupe and be sure the ground surface of the glass is sharp.

Also, when you remove the film holder, check the focus again and see if it's stayed in the same position it was in before inserting the filmholder. If not, then something is moving when you insert the filmholder.

As to shooting aperture--inadequate DOF is usually a bigger problem than diffraction at small apertures. Unless you plan to print really big, I'd go ahead and use f:32 or 45 if you need it.

Paul Fitzgerald
30-Nov-2004, 09:09
Hi there,

1) Do you use a fresnel lens?

2) Is it in the right place?

The difference in thickness of a fresnel will change your focus as you describe.

3) Have you had your film holders checked for depth with a dial indicator?

Factory tolerances are far from perfect, you may be in for a rude surprise.

4) Did you have the roll film backs checked the same way and matched to your cut film holders?

Roll film holders and polaroid backs almost never match exactly to cut film holders, sorry.

5) Are the lenses in the right shutters?

If they have been swapped, the location of the iris would be different causing a focus shift when stopped down.

Alan Davenport
30-Nov-2004, 10:15
The fresnel lens scenario seems like a good place to start. There's a lot of disagreement as to whether a fresnel should be on the front (lens side) of the GG or on the rear. Some manufacturers did put the fresnel on the front, but to achieve accurate focus they also made an adjustment in the placement of the GG. If you have a camera that came from the factory without a fresnel, and one has been added between the lens and the GG, that would change the plane of focus by about 1/3 the thickness of the fresnel. When you put the film in, it will be off by that much; this would also explain why you get acceptable results when stopped down.

Michael Perlmutter
30-Nov-2004, 11:06
To Eugene:

Yes I wear eyeglasses (am very nearsighted). But I need glasses to see clearly. If I remove them then virtually EVERYTHING is out of focus, both in reality and on the GG! Also, the 2 dealers who I performed the test together with do not wear glasses, and we were in agreement about what was in focus. How could wearing eyeglasses affect this problem?

To David:

All tests were performed on film (both pos and neg) and never on Polaroids. I can check the focus of the loupe on the ground surface, as you suggest. But if the loupe is not correctly in focus, shouldnt it mean that nothing at all will appear in focus when viewing an image on the ground glass through a lens? This is not the case with me: The image always appears crystal clear to me at the plane of focus. Finally, film holder-insertion does not cause a problem . . . I check this very often as a matter of habit.

To Paul:

1) & 2): Yes I use a fresnel lens. It is located on "my" side of the ground glass, i.e., the loupe rests on it when I do the focusing. The smooth side of the fresnel lens faces towards me. The matt side of the ground glass faces the lens, and this of course is the focusing plane, which must coincide with where the film sits when inserted in the camera. So the thickness of the fresnel in my setup shouldn't really affect the focus, as I understand it. To confirm this, I looked into my camera just now: when I focus on a specific point with the fresnel in place, and then remove the fresnel and place the loupe directly on the glass without adjusting anything else, I see no change at all in terms of what is in focus. I don't think the problem is here.

3) & 4): I don't know what a dial indicator is. My dealer (who also does service and repairs) measured the distances between the surface of the film holders and the plane of the film using a caliper. Is this the same thing? In any case, his measurements indicated that the plane of the film and the plane of the GG coincided ok. But perhaps more accurate measuring devices are needed? Do other 4x5 photographers do this kind of checking? If small deviancies are indeed found, what shall one do about it?

5): I am not sure what you mean here. The lens/shutter configurations are the same as when I purchased them new, nothing has been altered or swapped. But how could I check this to be sure?

Thanks to everyone for helping me to get to the bottom of this! I appreciate all of your input very much.

Bruce Watson
30-Nov-2004, 11:25
I use a Maxwell screen which is a fresnel lens. It's on the "other" side of the GG, inside the camera. Same side as the "ground" of a normal GG. Twelve to one, that's your problem.

James E Galvin
30-Nov-2004, 11:56
I calculated the distance to move the lens, for the 58 mm lens, to change the focus from 6 m to 4 m. It is only .3 mm. A misplaced fresnel would shift the focus more than that. Now you are getting a sharp image closer than you focussed on. This could be caused by the standard moving away from the lens after focussing, by .3 mm. Are you pressing the loup against the GG? Enough to spring the standard by .3 mm?
If we want a circle of confusion of say 1/50 mm, then the focus must be within about .1 mm at f/5.6. If we stop down to f/22, then we can be off by a more comfortable .4 mm, and 4 and 6 m are both in focus.

Gem Singer
30-Nov-2004, 12:33
Hello again Michael,

The combination of the correction in your eyeglasses and the correction (diopter/focus setting) of your loupe may be causing the shifting of the plane of focus. You do not need corrective eye glasses for critical focusing, only for composing the picture on the ground glass. The loupe serves as the corrective lens. Try this: compose the picture. Then, remove your eye glasses. Adjust the diopter setting of the focusing loupe until it is sharply focused on the ground surface of the groundglass. If necessary, tape the barrel of the loupe so that it cannot move acidentally after you have set it. Finally, focus the camera with the loupe but without your eye glasses.

Ralph Barker
30-Nov-2004, 13:09
It sounds to me like it might be a combination of the location of your fresnel and the focus point of your loupe, Michael. If your loupe is set to focus on the ground side of the GG, assuming it is on the back surface of the GG, adding the thickness of the fresnel would shift the focus point back by the thickness of the fresnel. As such, you might be focusing on the aerial image, rather than the image being formed on the ground side of the GG. If your loupe is fixed-focus, you may need to get a different, adjustable loupe to ensure that it is focused on the ground side of the GG.

If the loupe doesn't have diopter correction, and you wear bifocals be sure to use the close segment of your eyeglasses. Depending on the distance setting of the bifocal segment, you might still need to use the "least-blurry" focus point, as you're focusing an image that is a couple of inches away from your eye, not the actual distance to the object.

Michael Perlmutter
30-Nov-2004, 14:24
With my eyeglasses removed (they are not bifocals), I am unable to get the ground side of the GG in focus with the loupe I have. It feels like I would need to turn the focus ring another 180 to 275 degrees beyond its stopping point to get it sharp. Does it mean I need a better quality loupe, or is it that my eyesight is too far gone!?

Is my fresnel misplaced? If it is, then why, when using a loupe (with eyeglasses on), does an object appearing sharp when the fresnel is on also look equally sharp when I remove the fresnel and look again? Shouldn't I see a clear difference right away? Furthermore, if I placed the fresnel on the lens side of the GG (which in practice means the GG and the fresnel would switch places), then it means the plane of the ground side of the GG would be moved towards me by an amount equal to the thickness of the fresnel, and thus differ from where the factory placed it. This seems strange to me.

tim atherton
30-Nov-2004, 14:32
might want to read this as well (although note that a somemanufacturers do actually produce a GG setup with the fresnel on the inside of the GG - Toyo for example, I think, but that is basically a factory set-up)


http://www.wisner.com/viewing.htm (http://www.wisner.com/viewing.htm)

Gem Singer
30-Nov-2004, 15:19
Michael,

Judging from discrepency you are describing, yes, you probably do need a better quality loupe. You should be able to sharp focus the ground side of the groundglass, with your eyeglasses removed, using only your loupe, and adjusting the diopter setting on the loupe. If your eyeglasses are able to correct the discrepency, your focusing loupe should also be able to make the correction. The combination you have been using, your eyeglasses plus your loupe, is probably the cause of the focusing discrepency you have been experiencing, not your Fresnel.

Ralph Barker
30-Nov-2004, 15:37
It sounds like your eyes are unable to focus on objects that are very close, Michael. That's not uncommon, I believe. For example, without my glasses (bifocals), I can see quite well at a distance of about 10-12" or so. Any closer is all a blur. The focal distance of the bifocal segment of my glasses are set so I can focus on very small print at a distance of 5" or so.

Thus, you might need a loupe that provides both built-in diopter adjustment and adjustable focus. Or, you might find the inexpensive "reading" eyeglasses available at drug stores (and targeted toward the "chronologically-enriched" buyer) helpful. They are available in several diopters, so you should be able to find a pair that enables you to focus at 3" or so. (It's only your "loupe eye" that is important.)

Another approach that has been recommended in the past is the spectacle loupes (http://shorinternational.com/images/Images29/29164.gif) used by jewelers. These are set to work at very close distances, and clip onto your regular glasses.

Claire Curran
30-Nov-2004, 15:47
Michael says in his post that other people have also focussed his camera and had the same problem with the camera. I can't imagine they all have eye problems. Does this focussing problem happen only with the 58m and 72m lenses? Please let us know what other lenses you use and if the problem occurs with them.

Leonard Evens
30-Nov-2004, 15:51
This discussion has been very interesting. The others have suggested all sorts of possibilities. I focus regularly with a 75 mm lens at f/4.5 in 4 x 5 and a 65 mm at f/7 in 6 x 9. I haven't encountered what you describe, so it doesn't seem to me that this sort of thing is inevitable with wide angle lenses.. Let me suggest a couple of other things to think about.

First, don't trust measurements made with a caliper. Set up a target with lots of fine detail, like a page of newsprint, and photograph it at a 45 degree angle from about 15 times the focal length away. Use the highest power loupe you can find to focus carefully, and make an exposure. Repeat the process several times and then examine the negatives carefully. Do this first with a longer lens, and repeat with your wide angle angle lenses. You should see some shift but it should be quite small.

Second, as has already been pointed out, the shifts in the image plane for the lenses you are using at the distances you describe are pretty small, certainly less than 1 mm. The circle of confusion for someone with normal vision viewing an image from about 10-12 inches is supposed to be about 0.2 mm. With a 4 X loupe, it would be reduced to one fourth of that or 0.05 mm, but given that the surface of the gg is not perfectly clear and that you don't have the best vision, it might be more realistic to estimate the viewing circle of confusion as 0.1 mm. For sufficiently distant subjects (measured relative to the lens focal lenth), the depth of focus is twice the f-number times the circle of confusion. For f/5.6, that would come out to twice 0.56 mm or 1.12 mm. That means you might not be able to see any difference in the sharpness of the image over a range of 1.12 mm, or to be more precise within 0.56 mm on either side of the true position of the exact image plane. If we used .05 mm for the circle of confusion, that would reduce the depth of focus by half, but it would still be significant. Note that this is inevitable. By the laws of optics, there has to be a certain range in which the image looks equally in focus. Also, it doesn't really have anything to do with the focal length of the lens. The only way that enters is that the corresponding subject distances are much closer to the lens than for a longer lens.

So here are some suggestions for improving the situation.

First, consider using the highest powered loupe you can find. That would reduce the viewing circle of confussion and also the depth of focus for critical focusing. Unfortunately, that is likely to create problems with a normal gg because you will tend to focus on imperfections on the gg surface rather than on the image. so get the finest, brightest gg you can find.

Second, consider using the method of focusing described in the large format photography web page. In this method, you first focus on the nearest point you want in focus, note the position on the rail or bed, then on the furthest point, noting that position, and then place the standard halfway in between. Of course, you will also have the same depth of focus issues in focusing on the near and far points, but there are tricks for having the focusing errors cancel out. For example, you can approach the two points from opposite directions, so if you overshoot, you are likely to overshoot in compensating directions. You can also do the procedure several times, and take a rough average for the final position of the standard.

Paul Fitzgerald
30-Nov-2004, 16:02
Hi Michael,

As an aside check ebay item #3857124892, it may be useful to you.

"When focused on an object 6 meters in front of the lens, the only thing sharp on film when shot at f/5.6 was an object 4 meters in front – 2 meters off."

You just said that your ground glass is closer to the lens when focused than your film is. Your using an American camera with a Graflok back, the fresnel should be in front of the ground glass with the grooves towards the lens. The spacing difference would fix your problem.

A dial indicator is a type of caliper for measuring depth and run-out. I just measured a set of 12 new Fidelity Elite 4X5 holders. The max. difference between the shallowest & deepest was .035". The circle of confussion for 4X5 is .004" so tolerance should be .002". The only way to fix this that I have found is the shave down the deep side to match the shallowest side in the set and adjust the ground glass to match. A lot of fussy work BUT who wants to blow a once in a lifetime shot for a bad film holder.

The lenses you mentioned are new, high quality lenses design to fit Copal shutters that are now standard in size. There could be no problem with them being swapped, they would still work fine. This 'swapping' is only a problem with older lenses that were in barrels or re-shuttered. Lens spacing needs to be correct BUT the iris placement must be correct also or the lens will focus shift when stopped-down.

Happy holidays.

Michael Perlmutter
30-Nov-2004, 17:05
Claire: I also use 90mm, 110mm and 210mm lenses. As I recall, I only did a test with the 110mm (though for some reason I cannot find that test sheet just now ... it was all done 7 years ago, as I mentioned). As I remember it, I had the same type of problem with the 110mm, but it was not nearly as extreme as the wider lenses. Hence my concern that the problem gets worse as the lens gets wider.

I am nearsighted, not farsighted, which means I can see things up close quite clearly without glasses. With that in mind I shouldn't have trouble seeing the ground glass through a loupe with my glasses removed . . . that is, when I get ahold of a loupe that I can focus given my particular eyesight. I think this shall be my first area of investigation.

Many thanks.

Henry Ambrose
30-Nov-2004, 17:39
I ground the bottom of one of my loupes so that it would focus for my eyes. The normal screw adjustment was not enough. If you can see acutely at close range you should be able to focus these two lenses adequately without a loupe or get very very close. But the difference in visual focus on the GG and actual focus on film you write of seems huge.

David Karp
30-Nov-2004, 17:57
Hi,

I think that Eugene is on to something here. I am nearsighted and must remove my glasses to use the loupe, and to frame and rough focus. My loupe allows me to focus, and for me, focus is at the extreme of my loupe's ability to adjust. (Unfortunately, I can't give you a brand to try since it is an old loupe that I found somewhere, and the brand name and power information have long been rubbed off.)

I do not think the problem is with your Fresnel lens. If you are using the Cambo 45SF with a Cambo Fresnel, it is designed as the snap on, snap off type, and properly sits on the photographer's side of the groundglass when it is in place. This is to allow its use while composing, while giving the photographer the option of removing it for fine focusing, just as you described above.

I have a Cambo 45SF and a Calumet 45NX (both with similar backs - one is reversible, the other revolves), and have not experienced this problem with my 75mm and 90mm lenses. I do not think it is an inferior product, as others suggested.

A while ago people reported having problems rough focusing 80mm XLs wide open. Was this a problem with the 58mm and 72mm also? I know you photograph interiors, but perhaps rough focusing stopped down a bit might make a difference. Or perhaps try focusing on a small flashlight pointing back at the camera. (Along these lines, do you have the same problem with photographs taken outside in bright sunlight?) I don't know if any of these suggestions are useful, I am just grabbing at straws to find something that might help.

Eric Woodbury
30-Nov-2004, 19:15
You sure got a lot of good feedback. I don't know the answer, but I would leave the fresnel off until you figure this out. Eliminate variables.

Bob._3483
30-Nov-2004, 19:57
OK - we can rule out the camera because you got the same results with two different ones.

We can rule out the lenses because you get similar results regardless of lens.

We can rule out the film holder/roll film backs because you get the same with whatever type you use.

We can rule out your eyesight/other human error because other people have used your camera and get the same result (assuming these people were familiar with view cameras).

Ermm... something wrong here - there is nothing left. That's all the variables covered. As these tests were done 7 years ago, I suggest you repeat them now, with all your lenses, because something does not add up.

Cheer, Bob.

Armin Seeholzer
1-Dec-2004, 03:49
Hi

Did you use differnet loupes or always the same? Did you maybe use a to stron loup for a fresnell screen?
You should not use a loupe with a frensel stronger then 4 times it gets really difficult to focus with a loupe for example 7x on a fresnell. I can do it but for critical focussing then I put the fresnell away on my Sinar and it is much easier to see if it is in focus.
This was a reason for me to switch from an Arca F-Line to a Sinar because on the Sinar I just take the fresnell away because it is behind the groundglass and it solved my focusing problems with the wider lenses, which I had with my Arca!
Good luck!

Michael S. Briggs
1-Dec-2004, 04:08
You've received a lof of advice, most excellent, some off the mark. Let me try to add a few ideas.

LF cameras are as WYSIWYG (What You See Is What You Get) at its gets. All that has to be done by the camera manufacturer to get this to work is to place the ground glass in the same optical position as the film and to have the visible edge of the ground glass coincide withe the edges of the film in the film holder. This really isn't that hard. Without a Fresnel lens, or with the Fresnel lens on the photographer's side of the ground glass, the correct optical position is the same as the mechanical postion, i.e., the ground side of the ground glass should be in the same plane as the film. Since Cambo has checked the camera, it seems likely that this is correct on your camera.

The filmholder manufacturer must make their holders within the depth spec. Are you using modern Fidelty/Lisco holders?

The 72 mm Super-Angulon XL lens is the shortest lens that I have. I am an experienced LF photographer and it is the most difficult to use lens that I have. Your focus problems may be due to the difficult of using these lenses -- I expect your 58 mm to be even harder to use. The difficulty is that, except on the optical axis, the light rays arrive at an extreme angle to the ground glass.

Finally, the lens manufacturer must design and produce a lens that doesn't shift focus as you stop the lens down -- this occurs in a lens with signifcant spherical abberation. I know of no modern lens with this problem. Certainly I don't have this problem with my 72 mm SA-XL. With my 110 mm Super-Symmar-XL, I find that focusing is difficult off-axis when the lens is wide-open because the image is not sharp enough off-axis & wide-open. The 110 SS-XL can be focused with the on-axis image wide-open, or off-axis by stopping down about a stop. This procedure might help when you use your 110 mm SS-XL.

Since your tests were done 7 years ago, I second the idea to repeat the tests from scratch. I further suggest that you start with by testing focus with your 210 mm lens as the easiest to use. I second the idea to make photographic tests. I suggest carefully using the lens on a bright sunlight day and focusing on a slant to a newspaper (as suggested by Leonard) or to a fence, or setting up a scence with objects just in front of and just behind the object that you focus on.

Some photographers think that depth-of-focus (the required accuracy in positioning the focus) is smaller for short focal length lenses. This is not true for photographying distant objects -- the equations (with reasonable, small approximations for distant objects) show that depth of focus depends only on f-number. So you can use your 210 mm lens to test the accuracy with which your camera can be focused, e.g., the accuracy of the position of the ground glass.

Incidentallly, I have measured with a depth micrometer and photographically tested the focusing of several modern LF camera and modern Fidelity/Lisco film holders. All have measured to be within spec and tested to have accurate focus. I think most of the focus problems from incorrect equipment are for old cameras and holders, or from a Fresnel lens or screen that has been incorrectly installed, removed or adjusted by an owner of the camera.

The correct adjustment of your loupe is to focus it on the ground surface of the ground glass. Without a lens on the camera, point the camera at a bright light and focus the loupe on the grain on the ground glass. Poor eyesight shouldn't cause a _consistent_ focus shift. It might make it difficult to judge the sharpness position and thus random shifts about the best focus. I suggest that you try the photographic tests several times and see if the focus shift, if any, is always the same. Even use the same test setup -- after taking a test photo, move the standard off focus and start focusing over again. Do you repeatedly get the same focus results on the film, or is the item in best focus different from attempt to attempt?

My preferred loupe powe is about 4x to 6x. Higher power than 6x usually doesn't help because of the grain of the ground glass and the dimmer image.

Re your final question: I have tested several modern LF lenses for image quality vs aperture and found their best performance, considering the entire 4x5 film, to be at about f22. The image was still excellent at f32, but was slightly softer. The image was significantly less sharp at f45. While the results will depend on the lens, no lens can avoid diffration effects. You might not notice the difference between f22 and f45 if you don't photograph the same object or if you aren't checking detail in an object at the distance of best focus. If you need the depth of field, f45 is usable, but I wouldn't try to solve focus problems by always stopping down to f45.

Leonard Evens
1-Dec-2004, 07:04
Let me expand a bit on one point Michael made.

The formula for depth of focus is

2 N c (1 + M)

where N is the f-number, and c the circle of confusion. For sufficiently distant objects, the magnification is so small that 1 + M is so close to 1, that you may as well drop the term M. In any case the result is not directly dependent on the focal length f.
The way the focal length comes into it is as follows. If u is the subject distance, the magnification is given by

M = 1/(u/f - 1)

So when the ratio of the subject distance to the focal length u/f is small enough, the magnification is large and the depth of focus is larger. Usually, u/f is considered sufficiently small for this to matter when it is less than 10, which by convention is called the close up range.

Since depth of focus is not directly dependent on focal length, it is puzzling why people often complain that it is more difficult to focus wide angle lenses. One reason, metnioned by Michael, is that off axis the rays are striking the gg obliquely. And, of course, at the same relative aperture, the image is dimmer. For relatively close subjects, the above analysis may also provide a partial explanation. For a subject 1 meter away from the lens, you are well within the close up range for a 250 mm lens but clearly outside it for a 75 mm lens, so you actually do have visibly more depth of focus in the former case than the latter. I think another reason is one's subjective expectations based on one's off camera perception of the subject space. One tends to see things in normal perspective just looking at the scene, so in some sense there is a predisposition to a "normal" focal length lens. What you see through a wide angle lens distorts that perspective, putting everythings further away, so to speak, and thereby subjectively expanding all distances, at least in the final print. So the same error, looked at purely in terms of the optics, looms larger in terms of one's expectations. With a longer lens, you would not be surprised that you were not able to focus to within a inch or two for a distant subject, but you are when using a wide angle lens and the subject is closer. But your defintion of "close" and "far" has to be modified with considerations of viewing perspective in mind.

Michael Perlmutter
1-Dec-2004, 10:35
Lots of responses, difficult to sift through what seems best. One thing I note is that nobody really said they have had the same problem, at least not to the degree I have experienced it, which tells me that my problem seems to be an individual problem, not a common one.

I have started with an investigation of the loupe question. After looking at several loupes from 2 different LF dealers here in Stockholm I discovered that there are no loupes that allow me to focus on the GG when not wearing my eyeglasses! I can never turn the focus ring far enough to get the grain on the GG sharp. I can of course keep looking for other models . . . but it appears to me that this is not going to be a solution, or even something I could test. And just generally speaking, I have to wonder if my focusing through the loupe wearing eyeglasses would really lead to an object 12 meters away appearing sharp, when in fact (according to what’s on the film) I am really focusing on an object 4 meters away. That’s very extreme. There must be thousands of more-than-slightly-nearsighted LF photographers out there who are managing alright keeping their glasses on . . . or?

If I can accept as givens that 1) I must wear eyeglasses while focusing, 2) my GG/fresnel setup is ok, since it is what the factory ships, and 3) All of my equipment is state-of-the-art (and I therefore shouldn’t really need to be fussing with grinding down film holders and loupes), then among all the suggestions that have come in, the only thing I can see to do at this point is to test again by photographing a newspaper at 45 degrees in the manner described, and see what happens with that. I am a little uncertain, however, about what I will learn that is new. . . . I will return about this later.

A question for Michael: What do you mean by focusing on-axis and off-axis? I don’t quite get this. Do you mean looking into the loupe straight on rather than slightly from the side?

Alan Davenport
1-Dec-2004, 10:43
Wow, lots of great ideas here. One thing to check, that I didn't see, is which way your groundglass is turned? The frosted side must be toward the lens for it to be in the same place as the film emulsion. Then the fresnel is behind, so the photographer looks through the fresnel, then through the clear side of the GG, to see the image on the frosted side of the GG.

Problems such as your own eyesight are not relevant with a frosted GG, because you are viewing a true image on the GG, not a virtual image in midair; poor vision won't affect what you are focusing on, only lessen the acuity with which you see it. (IOW, you'll see the best image through the loupe only when it is actually in focus; your bad eyes won't focus it any differently.)

Also, you've already confirmed that the fresnel is NOT on the lens side of the GG. That leaves (to my mind) only two things: either the GG is in backward, or it isn't located in the same plane as the film emulsion. The former requires you to flip the GG over, the latter either shims or removing some material where the GG seats until it IS in the right plane.

David A. Goldfarb
1-Dec-2004, 11:06
I'm pretty nearsighted, and I focus with my glasses on. I use either a Schneider 4x loupe or the Toyo 3.3x loupe.

"On axis" means "close to the center of the image circle" (i.e., the lens axis) and "off axis" means "out at the edge of the image circle." If you use a normal loupe off axis with a wide lens you won't see very much, because the light from the lens is striking the glass at an angle. If this is the problem, you might try a Silvestri tilting loupe, which is designed to address precisely this issue.

Leonard Evens
1-Dec-2004, 12:45
"On axis" refers to the axis of the lens. Imagine a line through the center of the lens and perpendicular to the lens board. That is the axis. If you could see the whole image circle, the center of that circle would be where the axis intersects it. If the camera is set up with no rise/fall or shifts, that will also be the center of the frame.

It is easy to correct a lens for optimal performance on or near the lens axis. As you depart from the lens axis, lens defects become prominent. Also, illumination drops off.

Michael Perlmutter
1-Dec-2004, 12:57
Michael S. Biggs: You mention that you also have difficulty in using your 72 mm Super Angulon XL. What exactly is the difficulty you are experiencing, and how do you deal with it?

Armin Seeholzer
1-Dec-2004, 15:20
Michael

There are maybe not one photographer wich has no problem with very wide lenses like 47mm to75mm.
I for example use always the longest possible lens for a job espescially inside and in low light conditions!
But in my next 3 jobs I know I use only the 47, 55, 75, 90mm combo inside but I will take a very strong light with me for the focussing and framing part of it!
So it can be done, but it isn't easy at all!

Michael S. Briggs
1-Dec-2004, 16:38
Michael P., as I stated above and Armin seconds, even experienced LF photographers can have problems with very wide-angle lenses. This is why I suggested that you resume your focusing experiments with your 210 mm lens. I suggest taking the problems one step at time: figure out whether your system (camera, film holders, etc.) is correctly configured, and if you are focusing correctly, with an easier-to-use lens, then move on to lenses that are more difficult to use.





As the others have answered, on and off-axis refers to the optical axis of the lens -- the line that goes through the center of the lens. Without movements, the optical axis will normally be aimed at the center of the ground glass / film. At this point the light rays fron the lens are mostly going straight back and are easy to view.





The difficulties with using a wide-angle lens that I was referring to arise because the off-axis rays from a short focal length lens strike the ground glass at an oblique angle and thus are harder for the photographer to see. I deal with this by viewing the ground glass at an angle, depending on the spot that I am viewing, so that more of the rays reach my eye. I use a handheld magnifier, perhaps a better way would be to use a tilting loupe.





Yes, you should use your eyeglasses with the loupe. If you vision isn't close to "normal", then loupes will not have sufficient adjustment range to be used without your eyeglasses. You are focusing on the ground surface of the ground glass. Poor vision shouldn't cause a systematic and consistent focus shift -- for example, it won't cause you to consistently and repeatedly to focus on 1.5 m when the object is at 2 m. What you are doing is judging the point that the image formed on the ground surface of the ground glass displays maximum sharpness or detail. (See the second paragraph of Alan's answer above.) Poor vision might cause you to have difficulties judging sharpness and thus to inconsistently make focus errors, so that you might focus at 1.5 m on attempt for an object at 2 m, and then at 2.5 m on a second attempt. This is why I suggested you repeat your focusing experiments several times, in order to try to distinguish between systematic and consistent biases in focusing that might be caused by defective equipment versus random and inconsistent focusing problems from the operator having difficutlies, e.g., for a too dim image or poor eye sight.





Some past answers that might help: Ground glass focusing plane at
http://www.largeformatphotography.info/lfforum/topic/497868.html (http://www.largeformatphotography.info/lfforum/topic/497868.html),
and 4x5 Focusing Screens: THE FINAL WORD at http://www.largeformatphotography.info/lfforum/topic/500118.html#531356 (http://www.largeformatphotography.info/lfforum/topic/500118.html#531356)

Rob Hale
1-Dec-2004, 19:21
Hi Michael

As you have just said a great deal of information.

May I suggest that Jim’s calculation showing that .3 mm variation of the focused plane could produce similar results to your findings could be a clue to your problem.

Not having the formular I accept Jim’s calcs.

On this basis I did a quick check. 5 inch (127 mm ) engineers small striaght edge laid diagonailly across a Linhof GG, press gently in the center of GG and what do I see a 10 to 12 thou. gap (.25 to .35 mm) or bow between the glass and the striaght edge.

So – how use this information well I would not at all surprised if holding a loupe against the GG whilst “doing ones best “ to get the focus spot on did not cause some form of deflection and .3mm is a very small amount in every day terms. Now while considering this remember the back stand may well all so be deflecting, again not much in everyday terms.

How would I check this :-

First use only one rollback/film holder(and only one sideof the film holder)for all tests.

Borrow or find a hand held magnifing glass ( I would use my clip on lenses which give me about 5-6 inch focal length with my reading glasses, these are ESCHENBACH OPTIK Lupe N ).

Choose any two or three of your test distances and make two runs.

First run using your lupe, defocussing and refocussing the back stand between shots.

Second run using your hand held glass or clip ons, defocussing and refocussing the back stand between shots.

This should indicate whether the GG and or the back stand is springing back after focusing.

Regards Rob Hale

Michael Perlmutter
2-Dec-2004, 03:10
I am very grateful for the suggestions I am getting, and first want to thank everyone for their contributions. I am amazed actually at the responses that you all have put considered effort into. Clearly it's a sensitive issue: exceptionally sharp pictures are what we are after (there are exceptions, I know), which is one of the main draws of large format photography.

One thing I might add about myself: even if I sound like a beginner, I am actually an established architectural photographer where I live, and am able to make an ok living through my work. I guess it's just that I am in the category of a "non-technically oriented" photographer, who has learned on my own from reading, talking with knowledgeable people, and making my own mistakes and discoveries as I go along. I have managed thus far by using small apertures, which works for me 95% of the time. Now, however, I have chosen to check into the problem with the other 5% . . . it's more satisfying knowing you have complete control, always.

Anyway, after digesting everything I have read here, I think I am beginning to understand better how to proceed. If I first test the 210mm, which is the easiest of my lenses to focus, and discover that what I focused on at f/5.6 is also what is sharp on film, then I can right away eliminate that I have an equipment failure somewhere. The film plane would be shown to match the plane of the ground glass, my film holders would be ok, my loupe would be ok, my fresnel/gg setup would be ok, and my focusing with eyeglasses on would be ok. Afterwards, all testing of my wider lenses would only indicate possible errors in my focusing methods, and have nothing to do with failures in my equipment or failures in my eyesight. It will be interesting to see what I discover.

One thing though: during the dark and usually cloudy month of December here in Sweden, a bright sunny day is a rare commodity indeed! I will therefore have to test indoors, blasting a lot of light with my redheads. This should still work, even if less than ideal.

Thanks again!

Paul Fitzgerald
2-Dec-2004, 09:42
Hi Michael,

"One thing though: during the dark and usually cloudy month of December here in Sweden, a bright sunny day is a rare commodity indeed! I will therefore have to test indoors, blasting a lot of light with my redheads. This should still work, even if less than ideal."

That would work better for this because tabletop / macro range has no depth of field, it's in focus or it's not. I use a deck of playing cards to do a focus test, just cut 7 slots in foam-core, wooden molding, an egg container, whatever is handy.

Place the ace of spades vertically facing you in the center and the other cards horizontially with the back facing you and slightly staggered. Focus the best you can on the edge of the ace, lens wide open, and take the picture with flash or hot lights. Don't worry about bellows extention or developement, this isn't fine art just a focus test.

If everything is correct you will see fine hairs on the very edge of the ace and be happy. If sharpest focus is closer then the ace your glass is closer than the film and will need to be shimmed. If sharpest focus is farther then the ace the glass is too far back and the view screen face will need to be shaved down. Remember 'I cut it twice and it's still too short' applies, make small changes and re-test.

A little trick for interior work is to use 4 Mini-Mag lights. These are small flash lights, 2 AA size, that can focus. The head turns to turn it on and focus BUT unscrews to become a base to hold the light, an electric candle. The bulb is 2mm and extremely bright, you can focus it at f/90. Just set them up for near, far, left and right extremes of your shot to focus and compose.

Have fun with it and good luck.

Kirk Keyes
2-Dec-2004, 11:00
Just a thought about why you can't focus the loupe without glasses is not that you need more correction, but perhaps you have a high astigmatism and at no setting your eye will be in focus.

Anyway, definitely verify that the camera system can be focused with longer lenses.

kirk - www.keyesphoto.com