Focusing problems on SLRs

Discussion in 'Film Discussion and Q & A' started by David R, May 4, 2009.

  1. David R

    David R TPF Noob!

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    This is a fairly lengthy intro to my question, please bare with me. (I'm also new to these forums)

    I have a total of 5 Nikon bodies. The two that I learned to shoot with, a Nikkormat FT, and a N2000. These two were for a long time fine for my kind photography. Recently I had the opportunity to pickup a couple of more bodies, a FE, a FE2 and N6006. Never use N6006, it was really cheap so doesn't matter. The FE2 became my favourite, love the metering.

    Anyways started to notice some focus issues. Images just didn't seem as shape as I thought they should be. Eventually isolated down to the FE and the FE2, problem seems to be around infinity focusing. Usually when I'm out Kayaking I just set the lens at infinity as most objects of interest are further than 50 feet, good enough for wide-angle lens. Well it turns out that on these bodies infinity comes early, depending on the focal length of the lens. By comparison the FT, N2000 and for that matter then N6006, hit infinity where the lens are marked as infinity. For those note familar with these bodies, the FEs have interchangable focusing screens, while the FT, N2000, and N6006 are factory fixed focusing screens.

    So after a lot of focus experiments I have something that doesn't really add up for me. I would expect that a check of the focal plane focus verses the view finder focus would agree. Well this is where I get confused.

    I've tried several techniques, a piece of frosted mylar as a focusing screen on the film plane, with x8 loop, a focusing screen from another camera refashioned to fit on the film plane with a x8 loop, a crude collminator setup using another camera with matching lens.

    The FE and FE2 both agree for view finder matching film plane focus, even though the focusing scale on the lens are off. The FT, N2000, and N6006 don't agree for film plane verse view finder, they're just slightly off with the film plane hitting focus before the view finder. The view finder agrees with the scale on the lens. While normally I use the split screen center for focusing, I also watched the matte part of the screens to be sure my eyes weren't the issue.

    So I'm a little lost as to what the problem is, or where to go from here. Is there something wrong with the FEs. Am I doing something wrong in my testing. I have tried using film tests, but it wasn't really conclusive as I was using slide film, then scanning, and my scanner isn't up to that level. I haven't tried specific tests with neg film and enlarging, which I could do. Its just a little time consuming and to costly right now.

    Any insights would be greatly appreciated.

    David R
     
  2. Battou

    Battou No longer a newbie, moving up!

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    Well, I'd first look at the mount on the body.

    Is there visable ware on the mount?

    If that is not the case I would check the mirror angle.

    Does the image in your view finder match up with the frame on the film?

    If the mirror is off it's going to effect the focusing screen by changing the distance of the mirror to the screen making the image in the screen closer or farther from the focusing aid than it would be to the film plain. Mirror angle is critical with any SLR, an exact 45 degrees is required, so much as half a degree can really tamper with things. This is more likely than mount ware but much harder to visually confirm. Slight mirror angle flaws are almost impossible to see, even under the most controlled testing, sadly I am not aware of many ways to detect it, muchless efficient ones.
     
  3. compur

    compur No longer a newbie, moving up!

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    If you want a "focus free" setting for your lenses do not focus them at infinity.
    What you want to do is set them to their hyper-focal distance. This is done
    by first deciding on an aperture, let's say it's f/11

    The lens below is set for its hyper focal distance for f/11

    [​IMG]

    Notice the lens aperture is set at f/11 and that it's not focused at
    infinity
    .

    Instead, the infinity mark on the focus scale is set for the upper limit of
    acceptable focus for f/11.

    This will produce acceptable focus for everything from about 5.5 feet to
    infinity. This gives the maximum range of acceptable focus at that aperture.

    Also, it's not unusual for lenses to focus a little past infinity. Many good
    quality lenses do. This is to allow for physical changes to the lens due
    to temperature, wear, etc.
     
  4. David R

    David R TPF Noob!

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    - battou, I have played with mirror settings on both of the FEs, but there isn't enough movement on the mirror stop, also there is no adjustment for the hinge point on the mirror. The only thing that resulted was being able to adjust the top to bottom focus in the view finder.

    - compur, I get what your saying about hyperfocal length, and I do frequently use the technique. As you say hyperfocal length is based on acceptable image softness. For me the problem I'm having is the peek sharpness isn't where it should be, where I'm focusing. Hope I made sense.

    Problem still remains, why do the FEs focus at infinity earlier than the other bodies. Why do the factory set bodies actually over focus, the film plane focus is past critical and going out of focus.

    I have been suspecting the mounts, but they don't have any significant ware showing. Only thing I can think of is that they have been modified, for which I have no comparison.

    I'm going to try to measure the mount to film plane distance o each of the cameras.

    David R
     
  5. David R

    David R TPF Noob!

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    - Compur, I reread my post and I don't think I said what I wanted to.

    Using your pictured lens scale. A range of acceptable focus for f11 is shown, ending at infinity. The problem I'm incountering is that the limit doesn't fall where the scale says it falls.

    I would think idealy that a very fast lens, say F1.2 or better, wide open, would still have to be able to focus at infinity when then scale says infinity.

    Did that make sense?

    David R
     
  6. Dwig

    Dwig TPF Noob!

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    I suggest you review your testing procedure. Be careful that your film plane focusing "screen" is in the right position. It must seat on the inner two polished film rails, not on the body casting and not on the outer two rails. It also must remain flat when you press on the lupe.

    If your tests were done correctly and the film plane and VF focus match the mirror, mirror box, focusing screen and film rails are properly align relative to each other. If infinity doesn't line up correctly then either the lens' focusing ring is mis-set or the body mount is in the wrong position. I good shop can confirm which.

    If the film plane and VF don't match, it points to either a testing error or a misaligned VF system (mirror, whole mirror box, or focusing screen).
     
  7. David R

    David R TPF Noob!

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    - Dwig, thanks for responding. I was very careful about my pressure points when working on the film plane. Fortunately my loop matches the inner rail for film suport. I don't think that's the issue. There could be a testing error as it is all make-shift materials. Still working on the measurements.

    I have contacted a few repair shops/people and they all advise just getting different bodies as their charges will be about the same. Which basically is why I'm doing all this myself.

    David R
     
  8. compur

    compur No longer a newbie, moving up!

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    If you mean that the distance scale on your lens is not accurate, then
    how far off is it? Can you post a photo illustrating the difference?
     
  9. David R

    David R TPF Noob!

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    - Compur, I'll try to get some shots tomorrow (better light and its late), showing the lens scale errors.

    I made some measurements this evening. These are the averages of four individual readings from each body. Using a digital caliper that can read depth (0.02 mm accuracy, 0.01 reaptability and resolution). I measured from the front of the center of lens mount (bridged) to the moment of contact with the film pressure plate. Interesting the FT had the greatest scatter, suggesting the springs for the pressure plate are weak. The N2000 faired the best with 3 of 4 readings being the same.

    FT – 46.665 mm
    N2000 – 46.635 mm
    N6006 – 46.725 mm
    FE – 46.663 mm
    FE2 – 46.665 mm

    David R
     
  10. David R

    David R TPF Noob!

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    OK here we go some pictures (no art awards here). They sure do make this a convoluted process. The target for these shots was the upper trunk of a tall tree several backyards away, 400ft give or take 50ft. An easy target to focus on because it was backlit at the same time. I have samples for the other bodies but these two should suffice. As you can see the shorter the focal length the worse the problem becomes.

    N2000 50mm
    [​IMG]

    FE2 50mm
    [​IMG]

    N2000 28mm
    [​IMG]

    FE2 28mm
    [​IMG]

    N2000 20mm
    [​IMG]

    FE2 20mm
    [​IMG]

    David R​
     
  11. compur

    compur No longer a newbie, moving up!

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    I see there is a difference. Is this a problem?
     
  12. David R

    David R TPF Noob!

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    Well...

    I guess if I put black tape over the focusing scale on the lens and do everything by eye then no.

    But, coming back to depth of field and hyperfocal length a problem arises. The last sample is the FE2 with a 20mm lens. The scale says that the tree is 5 feet away, when I know its more like 400 feet (the 28mm ~10ft, 50mm ~22ft). Where would I set my far point focus on the lens scale.

    And still there is the question, for me anyway, why is the scale so far off, the mount to film plane isn't significantly different from the other bodies. The focus in the view finder matches the focus on the film plane. So why the scale error

    I know this is a circular discussion, but its weird and it bugs me. Keeps me from being comfortable using FEs.

    David R
     

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