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Thread: Aperture(F-number) for peak performance on Pentax lenses.

  1. #1

    Default Aperture(F-number) for peak performance on Pentax lenses.

    As the optical characteristics on lenses vary, there are different optimal aperture/f-number of each lens for obtaining peak performance(both contrast and resolution) on center, corner and extreme corner.

    For eg, a lens with fast f/1.4 might not give peak performance at f/1.4, but stopping down to f/5.6 may give excellent center, corner and extreme corner contrast and resolution. Then further stopping down will just gradually go downhill due to diffraction.

    Usually i refer to reviews on lenses, MTF charts etc. But i think it's better to get real world sample images and real world experience as taking photos isn't just about comparing charts and doing carefully-controlled tests.


    In my opinion, with this knowledge we will be able to manipulate the characteristics of each lens to obtain the exact effect we want in our photographs.

    So lets share our experiences on the lenses we have so that we have a simple database which both new and old pentaxians can refer to as a guide to get the maximum out of the lenses.

  2. #2
    Member scorpioh's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    I have a strong tendency to use all my lenses wide open. Reason being I usually prefer a shallow dof for my subjects and I seem to be always in need of more light. Less than tack-sharp photos are sometimes the best imho.
    I say the lens is the soul of a camera. What about the body? Well, it's just the body...

  3. #3
    Senior Member darrrrrrrrrr's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    In the real world,

    1)I shoot wide-open in low light and and for subject isolation
    2) I stop down to f/8-f/11 for greater depth of field in landscapes and when I have my camera on a tripod
    3) I stop down to f/11-f/16 when shooting macros

    I think that kinda applies to all lenses.. Anyway I know my lenses are sharp enough so I don't think too much about squeezing out the best performance

  4. #4
    Senior Member oceanpriest's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    Unfortunately K-x dun have MTF program line like in K20D.

    http://k20dblog.blogspot.com/2009/02...-priority.html

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    Senior Member creampuff's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    As a rough guide towards the determining the aperture that gives the optimal sharpness, just shoot at an aperture that is roughly 2 stops smaller than the maximum aperture of the lens.

  6. #6

    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    Quote Originally Posted by creampuff View Post
    As a rough guide towards the determining the aperture that gives the optimal sharpness, just shoot at an aperture that is roughly 2 stops smaller than the maximum aperture of the lens.
    Ditto that. All lenses would perform better when stop down, and normally they perform the best 2 stops down from the widest aperture.

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    Member scorpioh's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    Quote Originally Posted by fengwei View Post
    Ditto that. All lenses would perform better when stop down, and normally they perform the best 2 stops down from the widest aperture.
    I have heard claims that many leica lenses perform best wide open with no improvements stopped down.
    I say the lens is the soul of a camera. What about the body? Well, it's just the body...

  8. #8
    Senior Member creampuff's Avatar
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    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    Quote Originally Posted by scorpioh View Post
    I have heard claims that many leica lenses perform best wide open with no improvements stopped down.
    I have owned a few Leica lenses (R series) in the past and I can most definitely say there is no basis for that sort of claim. All lenses are subject to different design, cost and production constraints, but none to my knowledge have performed sharpest at the widest aperture, even those fast f/1.2 or Noct or specialist large aperture lenses. Such lenses may offer unique photographic images from narrow DOF, subject isolation and soft bokeh but they do often suffer from aberrations like coma and flare as a consequence of the optical design. There is a trade-off in everything.

  9. #9

    Default Re: Aperture(F-number) for peak performance on Pentax lenses.

    Quote Originally Posted by scorpioh View Post
    I have heard claims that many leica lenses perform best wide open with no improvements stopped down.
    AFAIK the only lenses w/o improvement when stopped down are those fixed aperture lenses

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