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Thread: Same focal length but different apertures - which is best

  1. #1
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    Same focal length but different apertures - which is best

    Hello,

    If two lenses are of similar focal length but one has an aperture setting that goes to a smaller hole is this a better thing or does it not matter a whit ?

    I'm looking at the Fujinon W 300mm f5.6 and the Nikkor W 300mm f5.6 lenses. They are very similar in many respects but that the Fuji goes to f90 and the Nikon to f64.

    Would it really mean the Fuji might be sharper at the higher f-stops ? Perhaps it is a function of the way they are designed and it really makes little difference.

    I think I'd be right in saying the Fuji would be a little less contrasty which is good for portraits and less so for landscapes ?

    Cheers,

    Steve

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Same focal length but different apertures - which is best

    The optimum lens performance is generally achieved at two to three stops down from wide open.

    I expect the number of calibrated stops depends mostly on company design standards.

    So if you had f/2.8 leness, one would go down to f/45 and the other to f/32.

    - Leigh
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  3. #3
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    Re: Same focal length but different apertures - which is best

    Could I also suspect that one manufacturer might be simply wanting to create a point of difference between them and the opposition ?

    Why bother creating such a feature if there were no material benefit to it ? If the lenses are best between say f11 and f22 why go to f90 ??

    What's that f64 club anyways !! ...

  4. #4
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Same focal length but different apertures - which is best

    Quote Originally Posted by Steve McLevie View Post
    Could I also suspect that one manufacturer might be simply wanting to create a point of difference between them and the opposition ?
    Why bother creating such a feature if there were no material benefit to it ? If the lenses are best between say f11 and f22 why go to f90 ??
    The lens manufacturers aren't free to choose whatever numbers they want. They're constrained to use f-numbers corresponding to the maximum and minimum diameter of the shutter aperture.

    Those diameters are standard for any given shutter, and those are the numbers the lens maker uses since deviation would be a special production shutter at substantially higher cost.

    The aperture diameter is a function of the f-number for any given lens design. Specifically f/n gives the diameter of the entrance pupil as viewed through the objective, f being equal to the lens focal length.

    Given that f/64 and f/90 are adjacent values, it appears one design goes down to 90 and the other does not.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

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    Re: Same focal length but different apertures - which is best

    Quote Originally Posted by Steve McLevie View Post
    Why bother creating such a feature if there were no material benefit to it ? If the lenses are best between say f11 and f22 why go to f90 ??
    "Best" doesn't mean anything if you simply need the depth of field.

    My interpretation: One manufacturer restrains the use of the smaller apertures to protect the user from the effects of diffraction, while the other manufacturer lets the user choose to live with the diffraction if they need the depth of field. If the Nikon aperture dial doesn't physically stop at f64, you might still be able to use f90 even if it is not marked.

    Quote Originally Posted by Steve McLevie View Post
    What's that f64 club anyways !! ...
    It's not a club, it was a group. You might want to look them up, they had very interesting ideas (always using f64 wasn't one of them).

    Michael

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    Re: Same focal length but different apertures - which is best

    OK so I thought you could design a lens to gather heaps of light and send that light through a tiny aperture or you could design a lens and not bend the light quite as much (so to speak) and thereby send it through a larger aperture.

    The Super Angulon lenses get a lot of light (f5.6) through a Copal 0 and these lenses get the same amount of light (f5.6) through a larger aperture shutter - a Copal 3.

    Is it not possible to make a 72mm lens (with the same filter size) but use a Copal 3 instead of a Copal 0 ?? Its all about the type of glass and its curvature isnt it ?

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    Re: Same focal length but different apertures - which is best

    You are comparing a 72mm lens with 300mm lenses. Remember, the aperture is (focal length / entrance pupil), so if 72/x=5.6, the pupil comes to about 12.86mm, whereas for 300/x=5.6, the pupil is 53.57mm. That's over 4x larger, because the lens is about 4x longer in focal length.
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    Re: Same focal length but different apertures - which is best

    Hmmm. Something I never quite understood. I thought there was only a linear relationship with certain types of lenses. My erroneous thinking is that modern lens design got around this principle.

    Even so, why offer f90 when diffraction is a limiting factor beyond say f32 ?

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    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: Same focal length but different apertures - which is best

    Quote Originally Posted by Steve McLevie View Post
    Hmmm. Something I never quite understood. I thought there was only a linear relationship with certain types of lenses. My erroneous thinking is that modern lens design got around this principle.
    You need to elaborate significantly on that statement.

    Quote Originally Posted by Steve McLevie View Post
    Even so, why offer f90 when diffraction is a limiting factor beyond say f32 ?
    Diffraction is never a "limiting factor" for a lens.

    There are apertures below which diffraction may degrade the image.
    However, that degradation must be assessed against other factors such as out-of-focus conditions.

    In all cases it's up to the photographer to determine which characteristics and factors are most important.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

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