Should lenses be allowed to claim a specific f-stop speed if theyvignette, horribly?

Discussion in 'Digital Photography' started by RichA, Mar 23, 2013.

  1. RichA

    RichA Guest

    IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
    the corners, it should not be allowed to call itself what it claims
    its f-stop to be. It's like a lens that can only resolve to a claim
    in the very centre saying it is capable of "X" resolution.
     
    RichA, Mar 23, 2013
    #1
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  2. RichA

    Martin Brown Guest

    Re: Should lenses be allowed to claim a specific f-stop speed ifthey vignette, horribly?

    On 23/03/2013 08:28, RichA wrote:
    > IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
    > the corners, it should not be allowed to call itself what it claims
    > its f-stop to be. It's like a lens that can only resolve to a claim
    > in the very centre saying it is capable of "X" resolution.
    >

    Which is also what lens manufacturers do. It is a *very* unusual lens
    that can achieve the same high resolution at the corner of the frame.

    The on axis performance wide open is always better at the centre of the
    field on the optic axis than it is elsewhere.

    --
    Regards,
    Martin Brown
     
    Martin Brown, Mar 23, 2013
    #2
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  3. RichA

    RichA Guest

    On Mar 23, 4:13 am, Martin Brown <|||>
    wrote:
    > On 23/03/2013 08:28, RichA wrote:> IMO, if a lens wide open exhibits morethan 0.7 stops of vignetting at
    > > the corners, it should not be allowed to call itself what it claims
    > > its f-stop to be.  It's like a lens that can only resolve to a claim
    > > in the very centre saying it is capable of "X" resolution.

    >
    > Which is also what lens manufacturers do. It is a *very* unusual lens
    > that can achieve the same high resolution at the corner of the frame.
    >
    > The on axis performance wide open is always better at the centre of the
    > field on the optic axis than it is elsewhere.
    >
    > --
    > Regards,
    > Martin Brown


    But where do you draw the line? If the resolution falls by 50% to the
    edge? 80%?
    Canon's 17-40L on a FF sensor was like that, a disgrace for an L-lens.
     
    RichA, Mar 23, 2013
    #3
  4. RichA

    RichA Guest

    On Mar 23, 10:47 pm, Eric Stevens <> wrote:
    > On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA <>
    > wrote:
    >
    > >IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
    > >the corners, it should not be allowed to call itself what it claims
    > >its f-stop to be.  It's like a lens that can only resolve to a claim
    > >in the very centre saying it is capable of "X" resolution.

    >
    > It's got nothing to with speed, vignetting, image quality or anything
    > else. It's merely the diameter of the aperture expressed as a fraction
    > of the focal length. The expectation that this says something about
    > usable speed is a figment of your own expectation.


    Vignetting IS a reduction in lens speed at the edge, which is why you
    end up with it being darker than the rest of the image. The way to
    combat it is with better lens design (more expensive).
     
    RichA, Mar 24, 2013
    #4
  5. RichA

    Martin Brown Guest

    Re: Should lenses be allowed to claim a specific f-stop speed ifthey vignette, horribly?

    On 24/03/2013 20:05, Eric Stevens wrote:
    > On Sun, 24 Mar 2013 06:11:32 -0700 (PDT), RichA <>
    > wrote:
    >
    >> On Mar 23, 10:47 pm, Eric Stevens <> wrote:
    >>> On Sat, 23 Mar 2013 01:28:39 -0700 (PDT), RichA <>
    >>> wrote:
    >>>
    >>>> IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
    >>>> the corners, it should not be allowed to call itself what it claims
    >>>> its f-stop to be. It's like a lens that can only resolve to a claim
    >>>> in the very centre saying it is capable of "X" resolution.
    >>>
    >>> It's got nothing to with speed, vignetting, image quality or anything
    >>> else. It's merely the diameter of the aperture expressed as a fraction
    >>> of the focal length. The expectation that this says something about
    >>> usable speed is a figment of your own expectation.

    >>
    >> Vignetting IS a reduction in lens speed at the edge, which is why you
    >> end up with it being darker than the rest of the image. The way to
    >> combat it is with better lens design (more expensive).
    >>

    > It still doesn't affect the ratio of lens diameter to focal length.
    > The f number remains the same.


    But it is only accurate near the optic axis. Vignetting is in essence
    the change in geometric illumination factor as a function of angle off
    the optical axis. If you think about it then it is clear that it is very
    difficult to maintain uniform field illumination at large angles.

    --
    Regards,
    Martin Brown
     
    Martin Brown, Mar 24, 2013
    #5
  6. RichA

    John Turco Guest

    Re: Should lenses be allowed to claim a specific f-stop speed ifthey vignette, horribly?

    On 3/23/2013 3:28 AM, RichA wrote:
    > IMO, if a lens wide open exhibits more than 0.7 stops of vignetting at
    > the corners, it should not be allowed to call itself what it claims
    > its f-stop to be. It's like a lens that can only resolve to a claim
    > in the very centre saying it is capable of "X" resolution.



    Your lenses speak to you? Do they sing and dance, too?

    John
     
    John Turco, Mar 25, 2013
    #6
  7. Re: Should lenses be allowed to claim a specific f-stop speed ifthey vignette, horribly?

    Alfred Molon <> wrote:
    > In article <>, Eric Stevens


    >> While I agree with you, field illumination has nothing to do with f
    >> number. 'f' number is no more (and no less than) lens aperture divided
    >> into focal length. This is not altered by the desire of RichA (or
    >> anyone else) to read something else into it.


    > But people usually expect a certain level of illmúmination with a
    > certain aperture. And some lenses simply do not give it.


    http://store.sony.com/webapp/wcs/st...&storeId=10151&langId=-1&partNumber=SAL135F28
    aka http://preview.tinyurl.com/bsa5l57

    Do you consider the fact that f-stop != t-stop a defect in
    this lens?

    -Wolfgang
     
    Wolfgang Weisselberg, Mar 31, 2013
    #7
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