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Thread: Unable to Focus Accurately with Wide Angle Lenses

  1. #1

    Unable to Focus Accurately with Wide Angle Lenses

    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?

  2. #2
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    Unable to Focus Accurately with Wide Angle Lenses

    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.

  3. #3
    Whatever David A. Goldfarb's Avatar
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    Unable to Focus Accurately with Wide Angle Lenses

    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.

  4. #4

    Unable to Focus Accurately with Wide Angle Lenses

    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.

  5. #5

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    Unable to Focus Accurately with Wide Angle Lenses

    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.

  6. #6

    Unable to Focus Accurately with Wide Angle Lenses

    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.

  7. #7
    Resident Heretic Bruce Watson's Avatar
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    Unable to Focus Accurately with Wide Angle Lenses

    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.

    Bruce Watson

  8. #8

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    Unable to Focus Accurately with Wide Angle Lenses

    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.

  9. #9
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    Unable to Focus Accurately with Wide Angle Lenses

    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.

  10. #10
    Moderator Ralph Barker's Avatar
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    Unable to Focus Accurately with Wide Angle Lenses

    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.

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