Do scratches matter on lens?

Discussion in '35mm Cameras' started by Guest, Aug 4, 2003.

  1. Guest

    Guest Guest

    Ok, I recently borrowed a Nikkor 105mm f/2.5 AI lens (~1980s). When I
    first got it, I looked at the front and rear elements, it looks pretty
    clean. I did not notice scratches of any kind.

    Shot a roll with it, the results are pretty damn amazing. Sharp and
    contrasty. What more could I ask for? :)

    Today, for no particular reason, I was looking at the lens straight on
    fron the front, naturally what I see is the back of the SLR through
    its viewfinder. What I did notice is there seems to be some

    Puzzled, I took off the lens, opened it up to the maximum aperture,
    and looked at it straight through the front against some light source.


    there were so many fine scratches (probably in the back of the lens)
    that I cannot even begin to describe it. I carefully inspected the
    front of the lens, I canot see any scratches. I looked at the back,
    there seems to be some very very subtle marks, but the pattern of the
    marks resembles what I saw when I looked at the lens straight on from
    the front.

    How come the pictures did not show any of the
    scratches/defects/anomilies? How bad does one have to scratch the lens
    for defects to show up on the image?

    When KEH says the glass is very clean, how careful do they inspect it? :)

    Guest, Aug 4, 2003
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  2. Guest

    Ben Abzug Guest

    Most fine scratches will not degrade image quality perceptibly (if at
    all) because the scratches will be *very* out of focus. For more info on
    this subject, try this thread...

    Good luck!
    -Ben Abzug
    Ben Abzug, Aug 4, 2003
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  3. Guest

    Mxsmanic Guest

    You've just answered your own question.
    See above.
    Because scratches usually don't matter. They interfere with only a very
    small number of light rays, and there are many, many light rays
    Probably bad enough that you can spot the scratches in any light from a
    food away. Even cracked elements may still produce clean images.
    Mxsmanic, Aug 4, 2003
  4. Guest

    Don Stauffer Guest

    The scratches do not show as scratches, because, as Ben says, they are
    out of focus. They will, however, scatter light. This causes mild
    flare with high contrast subject scenes, or, if the lens is illuminated
    by direct sunlight, can cause severe flare and ghosts. They are to be
    avoided. Take good care of optics.
    Don Stauffer, Aug 4, 2003
  5. Guest

    Guest Guest

    black ink?

    i don't understand, you are saying, if I use my ink pen and write
    something on the front of the lens (not that I really want to do it),
    it will not show up on the photo at all? :)

    Guest, Aug 4, 2003
  6. That's almost correct. If you have enough of it in a specific area of
    the lens (say 20% of the area) then you will start to see it. However
    scratches seldom come close to that and don't tend to all end up in the same
    area of the lens.
    Joseph Meehan, Aug 4, 2003
  7. Guest

    Mxsmanic Guest

    Correct. Scratches scatter light, causing flare (although it may be
    imperceptible for small scratches). If you cover the scratches with
    ink, they no longer transmit light, and so they no longer scatter light.
    The total amount of light passing through the lens is reduced by a very
    tiny amount, but the flare (if any) is eliminated. Obviously, you do
    this only if the scratch is severe and the flare is noticeable and
    unacceptable. It's better than junking the lens.
    Mxsmanic, Aug 5, 2003
  8. Guest

    Norm Fleming Guest

    This certainly seems to be the case with an elderly Canon QL17 rangefinder I have.
    The lens is very sharp despite what can only be described as a significant gouge on the rear
    element, near the centre of the lens. I've examined negatives and blow ups taken at all f
    stops carefully with a loup and can find no evidence of flare or any other defect, and all
    parts of the field look uniform.

    Although my experimental sample is small (n= 1 lens), this agrees with your notion above.

    Like others, I have read dire warnings about rear element scratches, but they don't seem to be
    accompanied by scientific support, explanation of the physics involved, etc.

    Maybe just another photo myth propagated in print and in discussion groups like this.


    Norm Fleming
    Norm Fleming, Aug 7, 2003
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