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  1. #1
    gma
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    Do filters degrade images on wide lenses?

    It is a fact that filters change the focal length of a lens by some specific factor of the thickness of the filter. Logically that means that with a wide angle lens the light rays at the limits of coverage would pass through a much greater thickness of filter glass than the rays in the center, resulting in a more pronounced shift of focus at the edges. I wonder if any research is available that would show that this is visible. Probably any image degradation would be so slight as to be visible only with large apertures and at large magnification, but I think it would be of concern to manufacturers of high quality optics.
    [FONT=Century Gothic][/FONT][SIZE=7][/SIZE][COLOR=DarkOrange][/COLOR] I may be getting older, but I refuse to grow up!

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    Quote Originally Posted by gma
    It is a fact that filters change the focal length of a lens by some specific factor of the thickness of the filter. [...]
    Is it? What is the factor? No disrespect intended, but a citation would be helpful.

  3. #3
    David A. Goldfarb's Avatar
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    Glass causes refraction, otherwise we wouldn't be using it for lenses, so potentially it's an issue. My impression is that it's more of an issue behind the lens (hence the recommendation one sees sometimes to use thinner gel filters behind the lens).

    I think that one shouldn't use a filter unless one needs a filter. I only use protective filters when there is an obvious hazard like sand or sea spray or crowds. If one needs a filter, the advantage of the filter is going to outweigh the potential minute focus shift at the edge of the field.
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  4. #4

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    If you need a filter then use one. There is nothing that you can put between you lens and your subject that is going to improve the quality of your negative. The more plane parrllel and the higher the quality of the glass and coating the better the performance of the filter and the smaller the degradation. The only exception is the use of very high quality cemented 2 element close up lenses...some of these with the appropriate lens will outperform the lens when used with an extension tube. For example some Leica R lenses with the appropriate Elpro will out perform the same lens with an extension tube.

  5. #5

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    The Kiev fisheye requires a filter to be used. That's why it ships with four filters. UV,yellow,blue and green I think. The filter is actually part of the optical design of the lens.

  6. #6
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    I think you would be more prone to actually seeing some image degradation with cheap filters and longer lenses. tim

  7. #7

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    nobody has mentioned polyester filters yet. I use the mono set from Lee filters and think they are great. Tougher than gel, thin, extremely light and compact. After 4 years all are still perfectly fine, so no need to cough up on resin ones!

    I cannot tell when I have used a filter and have not in terms of image resolution, full stop. Cheapie resin filters might be different altogether.


    Tom

  8. #8
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    Do filters degrade images on wide lenses?

    I use filters with my wide lenses frequently, but still think the images are degradest.

    AFAIK, focus shift only occurs if the filter is placed behind the lens. Thus, if one isn't able to re-focus with the filter in place, I suppose the image quality would, indeed, be degraded. But, that's really only an issue with smaller formats and specialized lenses that have filter drawers behind the main objectives (usually, due to the size of the front element).

    Poorly made filters are an issue, of course. And, the addition of extra glass/air surfaces can introduce other problems (e.g. flare). But, those problems can usually be solved with proper shading.

    So, I say, filter away, McDuff, and darned be him or her who cries, "Hold enough (filtering)."
    [COLOR=SlateGray]"You can't depend on your eyes if your imagination is out of focus." -Mark Twain[/COLOR]

    Ralph Barker
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