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Thread: Long focal-length lenses: less sharp ?

  1. #1

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    Long focal-length lenses: less sharp ?

    Are there reasons why the performance of long lenses is generally worse than that of shorter lenses ?

    I base this question on the lens tests I've seen on the internet, like those of Perez and Thalmann: we might not be shocked to see 60+ lpmm for an 80mm or even 150mm lens, but we never see that for a 300mm or longer lens.

    One would think that greater precision would be less important when dealing with comparatively longer distances.

  2. #2

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    Re: Long focal-length lenses: less sharp ?

    Quote Originally Posted by Ken Lee View Post
    Are there reasons why the performance of long lenses is generally worse than that of shorter lenses ?

    I base this question on the lens tests I've seen on the internet, like those of Perez and Thalmann: we might not be shocked to see 60+ lpmm for an 80mm or even 150mm lens, but we never see that for a 300mm or longer lens.

    One would think that greater precision would be less important when dealing with comparatively longer distances.
    With long focal length lenses even a slight breeze, or tripping the shutter, can cause camera vibrations that might not even be noticed, but they will cause serious degradation of the image that will show in testing resolution, especially at the level of 60 lpm.

    If you have digital camera that allows enlarging a small part of the image set it to the maximum, blow on the the camera with your breath, and watch how long it takes for the motion to stop.

    Sandy
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  3. #3

    Re: Long focal-length lenses: less sharp ?

    I think, as Sandy notes, that it is probably largely camera shake that causes this. When I did my own lens testing 10+ years ago I found that I had a very difficult time getting consistent results out of longer lenses, and I attributed this difficulty to camera shake. I think I took 15 different tests of a 24" Artar, and each test had a very different result.

  4. #4
    Dan Quan's Avatar
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    Re: Long focal-length lenses: less sharp ?

    After pulling the slide or putting the mirror up I learned to wait 10 sec to 1 minute depending on lens, camera and tripod set-up. Also the floor has an effect, concrete slab on earth is the most stable and suspended concrete or hardwood is very unstable. For instance no one can be walking around when everything is settling down or when the shutter is released. I learned to just relax and breathe and previsualize or imagine the subsequent series of events, and to make sure I didn't forget anything; just let everything settle down.
    “Life is short. Break the rules. Forgive quickly. Kiss slowly. Love truly. Laugh uncontrollably and never regret anything that made you smile!”
    ― Robert Doisneau”

  5. #5
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    Re: Long focal-length lenses: less sharp ?

    In the LF context, longer lenses of a given type are usually designed to cover larger image circles, with the tradeoffs that that entails.

    In small format, I don't think a generalization that longer lenses are less sharp would hold up. The hardest lenses to design seem to be the shortest FLs. Many of the high-end super-telephoto lenses are stellar performers on suitably-designed bench and field tests.

  6. #6

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    Re: Long focal-length lenses: less sharp ?

    My understanding is that this is due to chromatic aberrations.
    Many of the 35mm telephotos use low-dispersion glass just for this reason. -Thins Nikon ED, Canon L, Tamron LD, etc.

  7. #7
    fishbulb's Avatar
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    Re: Long focal-length lenses: less sharp ?

    There also appears to be a difference in performance between long focal length lenses and their 'telephoto' versions. For example a 360mm f/6.5 traditional lens vs. a 360mm f/8 telephoto design. Dunno if that's true or what but I thought I read that somewhere.
    -Adam

  8. #8
    Jac@stafford.net's Avatar
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    Re: Long focal-length lenses: less sharp ?

    In my early years I found vibration where I did not expect it and it puzzled me at first. Suffice to say, it can be subtle, not felt. A colleague who is an amateur astronomer pointed me to Celestron vibration suppression pads. We will see if they work when I have to use a long lens, probably a 360mm, on 4x5.

    The pads are three pieces - the casing, then a pliable gel (Sorbothane) surrounding the center where the tripod legs sits on a hard composite material.

  9. #9

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    Re: Long focal-length lenses: less sharp ?

    Also, any manufacturing imperfections of say a 150mm lens would be magnified 2x if it was a 300mm instead. Right? So a long focal length lens would have to be made to much closer tolerances to get the same acutance.

  10. #10
    retrogrouchy
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    Re: Long focal-length lenses: less sharp ?

    Some chromatic aberrations like secondary spectrum are proportional to focal length, which is the reason why you see expensive APO lenses that try to correct for that in the longer focal lengths. If you're using a long non-APO lens, it will have more fringing/softness than a shorter one of similar design, which means that (for example) a 300mm plasmat on 4x5 will look softer than a 150mm plasmat of identical design.

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