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  1. #21
    Diapositivo's Avatar
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    It should actually be easier to focus precisely a distant object than a near object. DoF decreases and the near object requires a more accurate focusing I would believe.

    I think the result might be due to the focus shift of the lens with the stopping down of the diaphragm.
    When testing the accuracy of the focusing system (alignment of focusing screen) I would only take pictures wide open: that way the focus shift with a different lens opening is taken out of the equation.
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  2. #22

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    Good point re: focus shift -- I did the test outdoors and had to stop down partially. I'll do it again in lower light tomorrow.

    I just looked through the viewfinder a few minutes ago, and when I focus and then press the depth of field button, the split prism (which works down to about f5.6) is way off. I say off, because it doesn't agree with the ground glass; and at long distances it would call for focus adjustments that are plainly absurd. I guess the plot thickens.

  3. #23
    Leigh B's Avatar
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    Quote Originally Posted by newcan1 View Post
    2. The point of focus in the split screen focusing aid in the center of the ground glass focusing screen must coincide with the best focus on the ground glass screen, absent manufacturing defect; the split screen prisms are embedded in the focusing screen.
    Yes, the point of optimal focus must agree between the GG image and the split prism. That's how they're designed and built.

    Quote Originally Posted by newcan1 View Post
    Curiously, I performed the same test on three different Nikkormats, with similar results. I also practiced with a D70 (sorry) with power off, then switched power on - each time the in-focus light was on, indicating that my focusing was accurate.
    The "in focus" light just means that the image was focused within the acceptance limits defined by the system,
    which means not terribly precise.

    Quote Originally Posted by newcan1 View Post
    But at this point I'm guessing that the problem is operator error.
    I'm afraid I must concur.

    I repaired Nikons for several years, and never encountered a focusing error on any that were not obviously damaged.
    In fact, I've never seen a focus problem on an undamaged Nikon in the 50+ years I've been shooting them.

    The probability of finding the same problem on three different cameras, all of which belong to the same shooter, are zero.

    - Leigh
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  4. #24

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    Leigh, it is good to hear with someone with Nikon repair experience. I can concede operator error, but only on the assumption that as operator I assumed the split screen would give a greater accuracy than it does. Others have suggested that the lens may change focus as it is stopped down; I have noted that the split screen only agrees with the ground glass at the widest apertures; and some have suggested I really get to know my lenses to compensate for system limitations. I am beginning to think that the last point is important - whether it is me or the lens, I am going to have to focus slightly beyond the subject at medium distance/aperture, for optimum results.

  5. #25
    Leigh B's Avatar
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    The split screen should be correct. Make sure you eye is centered on the viewfinder.

    The ground glass image on the other hand is subject to errors based on your eyesight.
    That's why they make diopters (corrective lenses) for the viewfinder. These only affect the GG image, not the split screen.

    The subject of focus shift is rather contentious, and has been discussed ad nauseum.
    Some people think it always happens; some think it never happens; many are in between or undecided.

    There's a very simple test for your lenses.
    Set the camera on a tripod and a good focus target at an angle across the field of view.
    Focus on the middle of the target using the split screen.
    Take a series of photos at all available apertures, controlling exposure with shutter speed.

    You could do this test initially at 5 (or 10) feet, then repeat at twice and four times that distance.

    Quote Originally Posted by newcan1 View Post
    I am going to have to focus slightly beyond the subject at medium distance/aperture, for optimum results.
    If that's true, there's a problem that needs to be corrected.

    I've never encountered a Nikon camera nor a user thereof that required that kind of correction.


    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  6. #26

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    Quote Originally Posted by newcan1 View Post
    I can concede operator error, but only on the assumption that as operator I assumed the split screen would give a greater accuracy than it does.

    I agree with you. It seems intuitive that a split-image focusing aid would be more precise (and accurate), but I think that this is wrong. My practical experience indicated that ground glass is more reliably accurate, as per a focus test using about a dozen different people.

    I won't go into details, other than they each focused a piece of test gear, one time using a split-image screen, then using only a ground glass. Everyone felt they had nailed it with the split image, whereas the ground glass seemed more uncertain. The results, based on marking the lens barrel position, showed the opposite to be true. The ground glass focus barrel marks were all tightly grouped, the split-image focus marks spread out more than twice as far.

    The results were so surprising that we repeated the test (an industrial decision was riding on this). The results were the same. Clearly, something odd was going on with the split-image system. Nothing changed except the person doing the focusing. We presumed that some difference in eye position was probably to blame, but did not investigate further. (The test was over, for our purposes.)

    This test was enough to convince me to use the ground glass portion, only, when best accuracy is needed. You never feel as certain that you have nailed the focus, but it seems that a split-image glass can give a false sense of accuracy. BTW, for practical purposes, only a couple of "outliers" would likely have shown up on film tests.

  7. #27
    Leigh B's Avatar
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    The only way you can accurately test focus on an SLR is with an auto-collimator. Eyeballs won't work.

    The auto-collimator will measure any error that exists between the optical path from subject to film and from subject to GG.

    The split-image rangefinder is coplanar with the GG surface by design and manufacturing process. It's correct by definition.

    If your eyes do not render the resulting GG image sharply, you need to add a diopter.

    - Leigh
    “Wise men talk because they have something to say; fools, because they have to say something.” - Plato

  8. #28

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    i've never been entirely sure of how a split image focusing dealey-bopper in an slr works -- is it dependent on separation like a rangefinder? Something else?

    I do know that there's no substitute for a good rangefinder, especially in dim light. Leica said its rangefinder was so accurate that if you put two pins, one inch in front of the other, a meter away and focused on the one in front you would notice a separation of the images on the one in back.

    With an slr both pins would look the same, but I've tried that with a rangefinder and it works. With a very narrow depth of field, that error can be critical.

    So, long way of saying -- if I were you I'd assume the ground glass is accurate, not the split image.

    ps...i see some people already tried this with an slr and say they can see the difference there, too -- ok fine but i bet its a lot quicker in dim light with a rangefinder

  9. #29

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    Quote Originally Posted by Leigh B View Post
    The only way you can accurately test focus on an SLR is with an auto-collimator. Eyeballs won't work.

    The auto-collimator will measure any error that exists between the optical path from subject to film and from subject to GG.

    The split-image rangefinder is coplanar with the GG surface by design and manufacturing process. It's correct by definition.

    If your eyes do not render the resulting GG image sharply, you need to add a diopter.

    - Leigh
    The Gokosha autocollimators we used project a fairly narrow beam, so only uses glass relatively near the axis of the lens. If a wider-aperture test lens had, for example, some spherical aberration, it seems to me that the best effective focus point won't necessarily agree with the autocollimator. So although an autocollimator (the ones I'm familiar with) is much more precise than anything else I have, I think the final arbitrator of image focus has to be an actual image on film or sensor.

    I can come up with possible explanations of how a split-image screen might have focus discrepancies compared with either an autocollimator or an actual image, I'm just not sure how to explain the significant variation in the test I mentioned, where everything but the viewer stayed the same.

    A ground glass seems to me to be the best way to mimic film (or sensor) response because it accepts image-forming light from all zones of the lens. Assuming that the ground glass position matches that of the film plane. This would certainly be a good test application for an autocollimator with focusing lens.

  10. #30
    Steve Smith's Avatar
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    Quote Originally Posted by Diapositivo View Post
    Focusing on the bare ground glass on the other hand is influenced by your sight. If your sight is not the one which is presumed to be by your viewfinder (normally 0, sometimes -1 dioptre, it's indicated in the camera specifications) the entire focusing system will act as a correcting lens for your eye, and the best focus for your eyes will not coincide with the best focus on the film plane.
    Yes it will. Best focus for the film plane will coincide with best focus for your eyes, it just won't be as good as it would be with perfect sight. It can only get more out of focus either side of the in focus point.


    Steve.

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