Focusing discordance

Discussion in 'Nikon' started by hique, Jun 10, 2005.

  1. My focusing aid acessories are discording about focus. Let me explain
    the story.

    I am trying to use a Peak 22x over the default 'B' screen of my F-3.
    I was able to "modify" the acrylic part under the loupe to achieve
    focus on the screen. Well, at least that is what I thought at first.

    The view I have using the 22x Loupe over the screen (without the
    prism, of course) is terrific: I can see all that small dirt over my
    screen. With an Ai-s 28mm lens focused at an printed ad, 20cm away, I
    can see the reticle of the offset print that is almost invisible at
    naked eye. It's very nice to have such a precise view of what I am
    trying to focus. That was my objective.

    The problem is between the screen Microprim and the Split Image. When
    I focus a straight vertical line, whenever the image seems perfectly
    in focus (and that is very obvious at 22x) the split image don't
    match. If I try to match the split image, the image is matched but
    seems blurried.

    Putting the prism back on, without any magnifier, the correct focused
    images seems to be those when the split image match.

    Using a DG-2 2x Magnifier attached to the Prism, the correct focused
    images also seems to be those where the split image match.

    It seems to be a conspiracy against the Peak Loupe that makes me
    think I did something wrong. But the Image through it is so nice, big
    and clear that it's hard to believe that it's wrong.

    It just seem that the Microprism and the Split Image can't get into
    an accordance.

    What is probably wrong? The screen? My "system" involving the Peak
    Loupe? Should I trust what I see with the prism and the prism plus
    the DG-2, even though it seems blurred when I see the screen at 22x?

    I hope somebody can help me with this one. I am sure that there is a
    simple optics thing I don't know so I am ingenuously ignoring.

    Cheers.
     
  2. jbq

    jbq

    With such a magnification I'd recommend focusing on a plain ground glass, and with the precision you'll achieve you might even need a very precise calibration of your mirror.
     
  3. “The view I have using the 22x Loupe over the screen (without the prism, of course) is terrific: I can see all that small dirt over my screen. With an Ai-s 28mm lens focused at an printed ad, 20cm away, I can see the reticle of the offset print that is almost invisible at naked eye.” --Marcio Santos

    Well what did you expect it’s an F3! If you want to see a mushy focus screen you need to try this on a D70 or D70s! Don’t tell me the prism doesn’t come off D70(s), you’ve got a pair of channel locks don’t you?

    Regards,

    Dave Hartman.

    BTW, the DW-4 is a lot more handy even if it’s only 6x.
     
  4. The accuracy of the split image rangefinder is limited by the
    distance between the two viewing points (can’t think of the
    correct term). It’s as narrow as the diameter of your lens’
    aperture. You mention a 28mm AIS Nikkor. I don’t know which
    one but even if it’s a 28/2.0 AIS that distance is rather
    small. I would not bet on the split image rangefinder at 22x.<br>
    <br>
    Why don&#146;t you run some test and see if the prime focus is
    where the loupe and focus screen&#146;s &#147;ground glass&#148;
    indicate. Better yet pickup a DW-4, 6x high magnification waist
    level finder. You can get one in &#147;Excellent&#148; condition
    from KEH.com for ninety four bucks. That&#146;s US bucks. Please
    don&#146;t shoot any harts (A male deer, especially a male red
    deer over five years old). Why didn&#146;t my family pick a cool
    name like Wolfmann? Why did they drop the double &#147;n&#148;?<br>
    <br>
    Regards,<br>
    <br>
    Dave Hartman.
     
  5. You are probably missing the subtle optical properties of the F/F2/F3 focus screen's which are plano convex and not flat like a ground glass on a large format camera. The curvature is designed to work with the finder optics not your 22x loupe. Your loupe is effectively changing the preceived imaging plane of everything but the split image prism.
     
  6. Lol. "don?t shoot any harts". That's ok David. Good one, by the way.

    Well, I would surely get the DW-4, but even though it seems wonderful it's a bit pricey for me.

    "I would not bet on the split image rangefinder at 22x"

    David, so do you think it's better to trust the the apparent focus at 22x? Shoot when it appears sharp?

    "Your loupe is effectively changing the preceived imaging plane of everything but the split image prism"

    Craig, are you suggesting that the apparent focus is an 'illusion' and that I should trust the split image? I understand that the lack of a prism makes the optical situation non-ideal, but I can't understand what optics fenomenon explains that and why it only happens with the 22x loupe (maybe it always happens, but at 0.8x magnification from the prism I can't notice it).

    Any ideas?

    By the way, I will conduct some experiment later and, who knows, maybe I have access to a 'D' clear screen to try that out also.

    Thanks for everyone so far.
     
  7. Just to keep the thread going...I discovered a reference that seems to be related to this:

    "A related issue is that many times, rangefinder split-image and microprism focusing aids may disagree with the ground glass focusing system. Unless your manual focusing camera is out of alignment, the ground glass focusing screen should be used as the correct focusing indicator."

    ---http://medfmt.8k.com/third/af.html

    So, does this seems correct? Does anybody ever notice this?
     
  8. Split image/microprism focusing aids vs. surrounding screen discrepancy has always been a pet peeve of mine as they usually exist on most Nikon screens from the F thru the F4 cameras. The only time I've seen all three agree was on an FE2 body. I also agree that based on critical focus tests, the surrounding matte screen matches film plane focus while the focusing aids focus slightly behind the subject(plus focus), never in front compared to the matte screen focus. I feel Nikon does this purposely to sacrifice a little center sharpness in order to compensate for blur caused by field curvature of high speed, wide angle and zoom lenses which greatly benefit from slight mis-focus in the plus direction. Lenses that don't benefit are macro and telephoto lenses, but these are easier to focus with the matte surround anyway because of their shallow depth of field. It's unfortunate that Nikon fails to explain the existence of the discrepancy in any of their literature but I suppose it's part trade secret and part a can of worms they don't want to open for fear of scaring away customers that feel their lenses optically beyond criticism.
     
  9. Very interesting Alan.

    So maybe for the tele or macro work I should trust the ground glass and for wide-angles I should agree to sacrifice some central sharpness in other to result in a sharper image all over the frame.

    It's a pity Nikon does not explain that, but as you pointed, a can of worms. True.

    Thank you everyone for the help.

    If someone find out some other resource or reference about this, I would be interested.

    Cheers.
     
  10. Well, the results were interesting. Actually the split-image was correct and the ground-glass wasn't.

    So, when I look at the screen at high magnification I can't trust my perceiving of the image in focus. I must trust the split-image. If this happens at high magnification, it happens at normal magnification also! So I can't trust my focus screen and my eyes! If the split-image is blacked-out, there is no way I can achieve correct focus.

    Is this strange?
     

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