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  1. #1

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    Help - baffling depth of field test results!

    I've been doing tests on the relative effects of diffraction and depth of field with various focal lengths in 35mm. The latest was with a 21mm Zeiss ZF. I photographed a long hallway at apertures ranging from f11-f22, focusing at ~infinity, and then repeating the exposure series with focus set to hyperfocal distance. The hallway had detail from near to essentially infinity which I could use to evaluate definition in the resulting prints.

    As expected, once you get to f22 diffraction effects on both near and far detail, regardless of point of focus, are quite noticeable. What I did not expect, and do not understand, is the following. The effect was observed at all tested apertures but was most visible at f11 due to the lower diffraction:

    At all apertures, the shots focused at ~infinity showed visibly better focus not only on far objects as expected, but also on very near detail, when compared to the shots focused at the hyperfocal distance (ie a point of focus closer to the film plane)!

    I am at a loss to explain this. The only issue in the test is since the subject was an empty hallway, there is nothing close to the film plane in the center of the field of view. The near details used for evaluation were anywhere from about mid way off center, to the edges of the frame.

    What is going on here? Is this due to field curvature? Or is it that lenses are optimized for infinity focus and will show better definition at that setting for subjects at all distanced even if depth of field is reduced?

    I don't get it.

  2. #2

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    Well a 21mm lens focused to 10 ft has a dof from 4.6 feet to infinity. At f11 and 6 ft, dof is 2.5 ft to infinity.

    Play around with this for a while.

    http://www.dofmaster.com/dofjs.html

    Diffraction is probably rearing it's ugly head around f5.6.
    Bob

  3. #3
    ic-racer's Avatar
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    In terms of 'hyperfocal' focusing for things like landscape, it always baffles me why people would focus on something that is not the point of interest. For example, a 'landscape' scene with plenty of 'infinity' detail and some closer objects looks better to me (and perhaps others) when the small trees in the distance (infinity) are resolved very well; they need the extra resolution otherwise you won't be able to tell they are tiny trees. As objects get closer and bigger they can tolerate less resolution. In fact a very close tree can be pretty blurry and you can still tell it is a tree. This progression of a similar resolution for all objects in the scene works out just right when focused at infinity, not at the hyperfocal distance.

  4. #4

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    Quote Originally Posted by ic-racer View Post
    In terms of 'hyperfocal' focusing for things like landscape, it always baffles me why people would focus on something that is not the point of interest. For example, a 'landscape' scene with plenty of 'infinity' detail and some closer objects looks better to me (and perhaps others) when the small trees in the distance (infinity) are resolved very well; they need the extra resolution otherwise you won't be able to tell they are tiny trees. As objects get closer and bigger they can tolerate less resolution. In fact a very close tree can be pretty blurry and you can still tell it is a tree. This progression of a similar resolution for all objects in the scene works out just right when focused at infinity, not at the hyperfocal distance.
    I agree with you on this. I find in most cases with near-far detail, you are better off being infinity-biased with your focusing, even though technically depth of field is not maximized.

    Bob, diffraction or not, one would assume near objects would be sharper when the point of focus is nearer to them vs focusing at infinity. Yet that is not what I am observing. It seems completely counterintuitive.

  5. #5

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    Aha! just like my eye sight which can't see close subject clearly.

  6. #6
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    Your hyperfocus may be off. At 21mm, only the tiniest focus adjustment is required and my guess is that you're focusing too close. Or you may have field curvature; I don't know about that lens. Are objects in the centre of the field at the focus distance sharper than those at the edge?

  7. #7
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    I think the problem lies here: The Circle of Confusion assumed by most DOF calculators match the lens manufacturers; and the CoC used by them is actually based upon human visual acuity levels which are LESS THAN the 20/20 vision standard!

    In the DOF scales on fixed FL lenses, many of us knew to use the DOF lines for f/8 even though we were shooting at f/11, to compensate for the poor human vision limits assumed by lens manufacturers!

    So, as a result, when you put focus at Infinity, you get sharp detail. But when you use DOF scales to focus upon the Hyperfocal distance, your eye is able to perceive BLURRED infinity detail! My suggestion is to try using DOF calculators assuming a 1EV larger aperture than you are shooting at, to find Hyperfocal distance!!!

  8. #8

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    Hi Polyglot: yes the focus only had to be adjusted slightly for the hyperfocal distance (vs infinity). The weak point in the test was the lack of something close in the very center of the frame, although the detail evaluated was not too far from the center. Field curvature was one of my guesses too.

    Hi wiltw: This is what I always do, sometimes more than one stop too. I use CoCs quite a bit smaller than the usual 0.03 the manufacturers use. And one of the main reasons for this kind of testing was for me to find the proper balance between depth of field and CoC because while stopping further down is always good for depth of field, eventually diffraction makes things more fuzzy than if they were slightly less in focus. The issue you raise could explain why people get fuzzier infinity detail than they expect when they use hyperfocal focusing, but it still doesn't explain what I'm seeing, which is that the near objects are sharper when the lens is focused at infinity vs the hyperfocal distance.

  9. #9

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    Well there is always the possibility of mechanical problems with the lens or camera. Which camera are you using for testing?
    Bob

  10. #10

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    Nikon F3 in perfect condition. I made some tests with other lenses but hadn't printed them yet. Tomorrow I will repeat the test with the 21mm and also print the other tests and see what I come up with. My best guess at this point is field curvature.

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