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Back focus with Ai lens

Discussion in 'Lens Lust' started by Frits, Dec 12, 2005.

  1. Since a short while I have been playing with an old MF Ai 50mm f/1.4 (here's one recent post with it: https://www.nikoncafe.com/vforums/showthread.php?t=57900 ).
    Now I've been shooting some portraits from a much shorter distance and with large aperture.
    I notice that the lens back focuses, without question. Observations:

    - I have good eyes and I focus dead-on where I want the focus point.
    - back focus is quite a bit when I focus by eyesight: portraits are way off
    - when I focus and trust the "in-focus light" in the viewfinder, the portrait is perfect(!)
    - the (D1x) camera's focus system is dead-on with my AF lenses (some very demanding long focal fast lenses - no problem).

    So, in conclusion, if I were to focus with the image appearing OOF in the viewfinder, but with the "in-focus" light on, I get a perfect image. If I focus by eyesight in the viewfinder: no good, big time back focus.

    Last edited by a moderator: Mar 15, 2017
  2. see an optomitrist
  3. Here is what my line of thinking would be:

    1. Set up on a tripod and get to a known good focus on an object with lots of high contrast detail such as black print on white paper.

    2. With that accomplished, tweak the viewfinder adjustment to produce the sharpest image. I do not believe you can misfocus this way, but I have heard of it.

    3. Take another shot, but focus by eye. I usually test a camera this way using the fastest macro I have aimed at fine print at an angle so focus is painfully obvious. If the resulting image is different than the VF, then your main mirror needs to be adjusted.

    I will readily adjust my D50 or D70 (have never had to change the D50) because their adjuster screws are obvious. I have never attempted this with a D2X because the adjusters (same two of them) are different and are either adjusted behind the back cover or with special tool.

    I would suspect your main mirror needs a tweak.
  4. Thanks for your response. My thinking is in line with your suspicion. I believe that the adjustment screws on the D1x are in the obvious position in the mirror box, but if that is what it is, I'll just leave it as is.
    The question that remains is: Can the AF module focus correctly if the mirror needs adjustment? I guess I could imagine that the AF module in the bottom cavity (which gets its image from the secondary mirror) can be precise with the main mirror projecting an OOF image into the viewfinder.
    Makes sense???? If so, that would answer the question.
  5. Sure, the AF system can see an accurate image while the VF does not. The AF mirror hangs on a separate hinge and is adjusted by its own stop. Adjusting one can affect the other, for that matter, because the AF mirror hinge is usually mounted on the back of the larger mirror.
  6. Hey Frits, I know exactly what you mean!!!

    I studied all my mf lenses and now I know exactly how each lens behaves. I now know each of them by heart. Pretty easy...

    First, you will notice there is a 1-2 mm loose while the lens is in focus according to the camera (while the focus light is on).

    Secondly, you must do some tests shots starting from focusing close to infinity (right to left) and shoot a picture at the start of the blink. Then at the middle and then at the end of the blink (all within that 1-2mm focus range). Then, focus from infnity to close and do the same. Evaluate your test shots. You will notice you paticular lens is really in focus with one of the above method.

    For example, my 28PC lens was sharply in focus when I focused from close focus to infinity right on the "edge" when the focus light starts to blink and it was out of focus at the other "end" of the blink, even though the camera showed "in-focus" with both methods. Same goes for my Noct while my 35 1.4 behaves oppositely. When shooting at infinity with the noct, I have to keep the infinity focus about 1mm before the infinity mark, otherwise it backfocuses, even though it shows in focus within that 1-2mm range. It behaves like that at all focus settings. So I just understood I absolutely HAVE to focus from close to infinity and not the opposite way.

    This simple test won't take you long and you will start to have sharply focused pics all the time (I just hope you understood my explanations...)

  7. Frits, check the diptric corrrection on the camera. You probably learned to refocus your sight while the dioptric corrector is off from the start. Of course I'm not saying or implying you didn't know that, far from that (!) It's just that it happened to me. I was constantly refocusing my vision without really noticing...

    Happy Holidays Frits!
  8. Chris101


    Feb 2, 2005
    I know that one! Put the lens cap on and focus the LED annunciators to get the diopter correction dead on.
  9. heiko


    May 15, 2005
    Frits, thanks for bringing up this subject. I too have focusing problems on my D70 using a 50mm F1.4 AIS, but in my case I suspect that I have similar problems with other lenses too.

    I will have to shoot some focus chart, but haven't done it yet. While normal pictures can indicate a problem, the focus charts give much more precise results.

    Beezle, NB23 and Chris - this is some really valuable advise you gave. My suggestion is to have a sticky thread under cameras to address focusing issues and how to check and solve them. Your posts would give some excellent starting point.
  10. Thanks all for responding. There is a lot of good information in these answers. It also proves that I do not experience an isolated "problem". There really is a learning curve for using Ai lenses on autofocus bodies.
  11. lowlight_junkie


    Nov 28, 2005
  12. Good link, thanks! While we're at it, here is another one: http://md.co.za/d70/chart.html

    One thing though: my D1x does NOT backfocus with its AF system (see original post). It is just that the image in the VF is not sharp when the lens is in focus.
  13. Here is a quick example:

    Shot with Nikkor Ai 50mm f/1.4 from a short distance @ f/2.0:

    When I focus VISUALLY in the VF on the centre of the yellow flower - back focus:
    Subscribe to see EXIF info for this image (if available)

    When I focus using the AF light in the viewfinder (with the view in the VF NOT at its sharpest) - dead on!!:
    View attachment 20857
  14. Frits,
    I hope you realise I was poking fun, as is obvious i have no clue like these other good response. I was joking around.
  15. I never thought otherwise, Dave :biggrin: :biggrin:
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