T-stop measurements "out" certain lenses and prove they are not whatthey claim

Discussion in 'Digital Photography' started by RichA, Aug 24, 2013.

  1. RichA

    RichA Guest

    RichA, Aug 24, 2013
    #1
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  2. RichA

    Me Guest

    Re: T-stop measurements "out" certain lenses and prove they are notwhat they claim

    On 25/08/2013 1:36 p.m., Floyd L. Davidson wrote:
    > RichA <> wrote:
    >> The Sigma is likely a very sharp lens, but it isn't
    >> f/1.4. This is just one example. It's important
    >> because the claim of f/1.4 when you deliver f/1.7 is
    >> worth hundreds of extra sales dollars.
    >>
    >> http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Sigma/Sigma-35mm-F14-DG-HSM-A-Pentax

    >
    > Tstops are not fstops.
    >

    True.
    And while RichA was visiting DXO, perhaps he should have read this
    article from 2010:
    http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues

    So even if the T stop of the lens was closer to the F stop, the light
    actually reaching the sensels at wide apertures can be significantly
    less than optimal - something the camera makers have known about - and
    "compensate" for in a slightly sneaky way IMO.
    Me, Aug 25, 2013
    #2
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  3. RichA

    Mort Guest

    Re: T-stop measurements "out" certain lenses and prove they are notwhat they claim

    Me wrote:
    > On 25/08/2013 1:36 p.m., Floyd L. Davidson wrote:
    >> RichA <> wrote:
    >>> The Sigma is likely a very sharp lens, but it isn't
    >>> f/1.4. This is just one example. It's important
    >>> because the claim of f/1.4 when you deliver f/1.7 is
    >>> worth hundreds of extra sales dollars.
    >>>
    >>> http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Sigma/Sigma-35mm-F14-DG-HSM-A-Pentax
    >>>

    >>
    >> Tstops are not fstops.
    >>

    > True.
    > And while RichA was visiting DXO, perhaps he should have read this
    > article from 2010:
    > http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues
    >
    > So even if the T stop of the lens was closer to the F stop, the light
    > actually reaching the sensels at wide apertures can be significantly
    > less than optimal - something the camera makers have known about - and
    > "compensate" for in a slightly sneaky way IMO.


    Hi,

    I agree that raising the ISO setting in a hidden fashion is really
    sneaky. I suppose that it is meant to compensate for the various light
    losses inside the cameras involved, and that for the PHD crowd (Push
    Here Dummy) it makes no difference.

    Mort Linder
    Mort, Aug 25, 2013
    #3
  4. RichA

    RichA Guest

    Re: T-stop measurements "out" certain lenses and prove they are notwhat they claim

    On Saturday, August 24, 2013 9:36:18 PM UTC-4, Floyd L. Davidson wrote:
    > RichA <> wrote:
    >
    > >The Sigma is likely a very sharp lens, but it isn't

    >
    > >f/1.4. This is just one example. It's important

    >
    > >because the claim of f/1.4 when you deliver f/1.7 is

    >
    > >worth hundreds of extra sales dollars.

    >
    > >

    >
    > >http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Sigma/Sigma-35mm-F14-DG-HSM-A-Pentax

    >
    >
    >
    > Tstops are not fstops.
    >


    Ya think? Of course they aren't, if they were, we wouldn't need them!
    RichA, Aug 25, 2013
    #4
  5. RichA

    RichA Guest

    Re: T-stop measurements "out" certain lenses and prove they are notwhat they claim

    On Sunday, August 25, 2013 5:51:31 PM UTC-4, Floyd L. Davidson wrote:
    > RichA <> wrote:
    >
    > >On Saturday, August 24, 2013 9:36:18 PM UTC-4, Floyd L. Davidson wrote:

    >
    > >> RichA <> wrote:

    >
    > >>

    >
    > >> >The Sigma is likely a very sharp lens, but it isn't

    >
    > >>

    >
    > >> >f/1.4. This is just one example. It's important

    >
    > >>

    >
    > >> >because the claim of f/1.4 when you deliver f/1.7 is

    >
    > >>

    >
    > >> >worth hundreds of extra sales dollars.

    >
    > >>

    >
    > >> >

    >
    > >>

    >
    > >> >http://www.dxomark.com/index.php/Lenses/Camera-Lens-Database/Sigma/Sigma-35mm-F14-DG-HSM-A-Pentax

    >
    > >>

    >
    > >>

    >
    > >>

    >
    > >> Tstops are not fstops.

    >
    > >>

    >
    > >

    >
    > >Ya think? Of course they aren't, if they were, we wouldn't need them!

    >
    >
    >
    > You are comparing them, which clearly indicates ya didnt
    >
    > think.
    >
    >
    >
    > The fstop of a lens is based on a physical measurement,
    >
    > and has only slight relevance to how much light gets
    >
    > through the lens.
    >
    >
    >
    > Think of it like this: Adding a neutral density filter
    >
    > does not change the fstop, but it does change the tstop.
    >


    F-stops are used in the commercial still photography community for two things: To understand depth of field and (rightly or wrongly) light throughput.T-stops never came into real use for still photography until recently, when cine lens makers (Canon, Zeiss, Samyang) began adapting their products for still camera use. F-stop really is only good for establishing DOF whereas T-stops are needed to gauge how much light actually reaches the sensor.
    RichA, Aug 27, 2013
    #5
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