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  1. #1

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    SLR Focusing Accuracy

    I have a bunch of Nikkormat Ftns and a couple of EL's, an EL2, and an FE, among others. Recently I have been wondering if I can just assume that the cameras focus accurately. I noticed in some instances that where the camera has a split screen focusing circle in the middle of the screen, that it will show the image to be in focus at a point where, looking at the surrounding ground glass, the focus point is slightly different on the ground glass. Sometimes I feel that my images could be a bit sharper. My lenses aren't the greatest, but include things like a Nikkor 35-135 and a Nikkor 50mm f1.8 -- not too shabby.

    I suppose I could do some critical focusing tests. But if any of the cameras are off, what would be the reason, and is corrective action possible? I guess I just don't know how the focusing mechanism works on an SLR. The lenses are all manual focus.

  2. #2
    BobD's Avatar
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    Your camera can only focus to one particular distance at a time. Different areas of the scene may appear somewhat out of focus because they are at difference distances from the film plane than the point you've focused on.

    Also, with most SLRs, you are focusing at the widest lens aperture regardless of the aperture setting on the lens. The widest aperture has the least amount of depth of field (focus) so some objects may appear somewhat out of focus in the viewfinder even though they may be in focus in the final image.

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    What Bob said.

    One quick thing you can try is using the split prism to focus on something at infinity. If the lens and the split prism agree, you're probably good. If you notice a discrepancy, be aware that there are many possible causes, including the lens, mirror, focusing screen, diopter, and your own eyes!

    Personally, when I get a picture that’s out of focus or otherwise unsharp, it’s almost always because of an error on my part.

  4. #4
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    Quote Originally Posted by LJSLATER View Post
    Personally, when I get a picture thatís out of focus or otherwise unsharp, itís almost always because of an error on my part.
    Me too.
    Mark Barendt, Beaverton, OR

    "We do not see things the way they are. We see things the way we are." AnaÔs Nin

  5. #5
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    Quote Originally Posted by LJSLATER View Post
    What Bob said.

    One quick thing you can try is using the split prism to focus on something at infinity. If the lens and the split prism agree, you're probably good. If you notice a discrepancy, be aware that there are many possible causes, including the lens, mirror, focusing screen, diopter, and your own eyes!

    Personally, when I get a picture thatís out of focus or otherwise unsharp, itís almost always because of an error on my part.
    In mirror reflex cameras focusing is dependant on the eyesight of person using the viewfinder because the eye becomes part of the optical system even split image prism screens in perfectly adjusted cameras can be apparently in focus to the user if their eyesight is defective when the image at the film plane is not in focus which is not the case with optical range finders.
    Ben

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    Quote Originally Posted by markbarendt View Post
    Me too.
    Me three.

  7. #7

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    Quote Originally Posted by BobD View Post
    Your camera can only focus to one particular distance at a time. Different areas of the scene may appear somewhat out of focus because they are at difference distances from the film plane than the point you've focused on.
    Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.

  8. #8
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    Quote Originally Posted by benjiboy View Post
    In mirror reflex cameras focusing is dependant on the eyesight of person using the viewfinder because the eye becomes part of the optical system even split image prism screens in perfectly adjusted cameras can be apparently in focus to the user if their eyesight is defective when the image at the film plane is not in focus which is not the case with optical range finders.
    I did not know that. You are saying that aligning the split image in my SLR prism is somehow less accurate than aligning the images in a rangefinder because of the acuity of my eyesight? Somehow that does not really make sense to me. In both cases my eyesight is involved. I should be able to align to blurry halves of an image in my SLR viewfinder as well, or not as well as the case may be, as I can two blurry ghost images in a rangefinder spot. Hmmm.

  9. #9

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    Quote Originally Posted by Pioneer View Post
    I did not know that. You are saying that aligning the split image in my SLR prism is somehow less accurate than aligning the images in a rangefinder because of the acuity of my eyesight? Somehow that does not really make sense to me. In both cases my eyesight is involved. I should be able to align to blurry halves of an image in my SLR viewfinder as well, or not as well as the case may be, as I can two blurry ghost images in a rangefinder spot. Hmmm.
    If your eyesight is poor, you cannot focus properly with either system. My limit of sharp vision is about 10" from my eyes, so I use my glasses or a corrective diopter on the cameras. The only time I remove my glasses is to compose on the groundglass of a view camera; fcussing on the GG is done with a loupe.

  10. #10

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    Quote Originally Posted by newcan1 View Post
    Understood. But the phenomenon I was referring to is this: I focus on an object using the split screen, then I move the camera slightly so I can see the same object on the ground glass, and then I find that the sharpest image of the same object on the ground glass is at a slightly different focusing point.
    That's probably beacuse by moving the camera you have placed the object a different distance from the filmplane. A few inches will make a differnce if the object is nearby and the lens is wide open.

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