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Thread: Focal length / diameter of lens = f-stop?

  1. #1

    Default Focal length / diameter of lens = f-stop?

    Hi, does this eqn holds? http://www.kowa-usa.com/digiscope/gr...alculation.htm

    For example, I have a focal length of 1050mm and the diameter of lens is 70mm. So the f-stop will be approximately f15?

    Canon EF 400mm diameter is 90mm. 400/90=4.44. Why is it f5.6? Does it only hold for digiscoping set-up and not DSLR lens?

  2. #2
    Moderator catchlights's Avatar
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    Default Re: Focal length / diameter of lens = f-stop?

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    Moderator ziploc's Avatar
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    Default Re: Focal length / diameter of lens = f-stop?

    A better reference should be this one.

    To answer TS' question, yes the f-stop is = focal length / aperture diameter. That's why f-stop is expressed as f/2.8, f/16 etc.

    Note that the aperture diameter should be the diameter of the entrance pupil.

    Regarding the Canon EF400, where did you get the figure 90mm? As far as I know, the EF400/2.8's filter size is only 77mm, and the entrance pupil cannot be larger then that.

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    Default Re: Focal length / diameter of lens = f-stop?

    Quote Originally Posted by ziploc View Post

    Regarding the Canon EF400, where did you get the figure 90mm? As far as I know, the EF400/2.8's filter size is only 77mm, and the entrance pupil cannot be larger then that.
    maybe the barrel diameter is that, who knows.

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    Moderator ziploc's Avatar
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    Default Re: Focal length / diameter of lens = f-stop?

    Oops sorry, just realised I made a mistake. The 77mm filter is for the EF400/5.6. The EF400/2.8 is using a rear filter of 52mm. The max (outer) diameter of the lens is 6.4" which is ~162.5mm. This is about right, as 400/162.5 = 2.46, and 2.8 is smaller then that. If we're talking about the EF400/5.6, then 400/77 = 5.19, which is also about right as 5.6 is smaller that that.
    Last edited by ziploc; 28th September 2009 at 12:03 AM.

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