Re: DSLR lenses not good wide open at wide angle?

Discussion in 'Digital Photography' started by Dauphin de Viennois, Jul 16, 2008.

  1. Alfred Molon <> wrote in
    news::

    > The (expensive) Carl Zeiss 16-80 for the Sony Alpha DSLRs is only good
    > from F5.6 onwards. With wider apertures at wide angle the corners become
    > soft to very soft, depending on aperture and distance from the image
    > centre. According to photozone the same holds for several other DLSR
    > zooms (the Sigma 17-70 being an example).
    >
    > However the lens of the Sony R1 is good wide open at F2.8, even in the
    > corners. Could this be because in the R1 the lens is very close to the
    > sensor (just 2mm distance vs. 2cm in an APS-C DSLR)?
    >
    > Also the Zuiko 14-54 is good wide open, which could be a consequence of
    > the 4/3 design.


    All the books on photography I have ever read say you should not use any
    lense at its largest aperature setting or smallest due to light difraction.
    Dauphin de Viennois, Jul 16, 2008
    #1
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  2. David J. Littleboy <> wrote:

    > "Alfred Molon" <> wrote:
    >> In article <8cgfk.74718$kx.60067@pd7urf3no>, Dauphin de Viennois says...
    >>
    >>> All the books on photography I have ever read say you should not use any
    >>> lense at its largest aperature setting or smallest due to light
    >>> difraction.

    >>
    >> Diffraction only occurs at small apertures


    > Right.


    >> and there are lenses which perform well even wide open.


    > But they tend to be slow primes, not fast 5x zooms.


    There are two different approaches to designing a zoom. The first
    approach is to design it to a minimum standard of performance
    throughout the zoom range. The second approach is to design it to a
    maximum performance standard at an important part of its range, and
    simply do the best you can, without disturbing the best performance, at
    more distant focal lengths than the best one.

    I get the impression that some of the latest best quality zooms have
    adopted the latter approach, so that what you get is reasonably good
    performance for a zoom at the extremes, allied with a part of the zoom
    range which approaches the quality of a prime. Of course in a lens
    with more glass elements (as zooms typically have) it will be very
    hard to avoid a concomitant drop in contrast.

    --
    Chris Malcolm DoD #205
    IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
    [http://www.dai.ed.ac.uk/homes/cam/]
    Chris Malcolm, Jul 16, 2008
    #2
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  3. Dauphin de Viennois

    Roy Smith Guest

    In article <>,
    Chris Malcolm <> wrote:

    > There are two different approaches to designing a zoom. The first
    > approach is to design it to a minimum standard of performance
    > throughout the zoom range. The second approach is to design it to a
    > maximum performance standard at an important part of its range, and
    > simply do the best you can, without disturbing the best performance, at
    > more distant focal lengths than the best one.


    This is true of designing anything, not just lenses.

    A device has some operating envelope. It will perform better in some parts
    of the envelope than in others. If I limit the allowable range of
    operation to just the part of the envelope where it works best, I get a
    device which is limited in functionality, but performs great over that
    limited range. If I allow it to be used over a larger range, I still have
    the good performance in the middle, but now you can complain, "But, it
    sucks near the edge!"

    So, which is the better design philosophy? Well, neither, really. They're
    just different.

    Why do we even have variable aperture, variable focus lenses at all? For
    any given image, I'm only using one setting. Let's say I take a picture
    with a fixed focal length lens (f = 85mm), focusing at 15.5 feet, and an
    aperture of f/8. The fact that the lens is capable of focusing closer or
    further away, or adjusting the aperture larger or smaller may make the lens
    useful in other situations, but for that specific image, it's all just a
    waste. If I told a lens designer to design me a fixed-everything lens for
    just that exact combination of settings, he or she could probably come up
    with a lens that produced a better image. The only problem is it would be
    such a specialized lens, nobody would buy it.

    Everything is a tradeoff.
    Roy Smith, Jul 16, 2008
    #3
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