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Aperture numbers change if using a crop sensor?

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    Aperture numbers change if using a crop sensor?

    Someone on a Canon group on Facebook stated that on a crop sensor such as an 80D you multiplied not only the Focal Length by 1.6, but also multiplied the Aperture. He stated that this was because you're putting it a full frame lens on a crop sensor, you times the focal length and aperture by 1.6. A 50mm f1.8 on a crop sensor is actually equivalent to an 80mm f2.8.
    I have never heard of this before. Is it true?
    Janet

    Canon 90D, 77D, Plus a load of lenses, especially macro.
    http://www.leighcameraclub.co.uk/

    #2
    Re: Aperture numbers change if using a crop sensor?

    In the sense of exposure, no. In the sense of depth of field, yes.
    Please don't ask about my kit, it's embarrassing!

    Comment


      #3
      Re: Aperture numbers change if using a crop sensor?

      There is endless discussion (and inevitable heated argument) on this topic on the internet. I believe the basic idea is that when you fit a lens designed for full frame onto an APS-C sensor body only part of the light actually strikes the sensor and therefore the effective F-number is smaller. Certainly many reviews will refer to say an F2.8 200 mm lens as equivalent to a 320 mm F4 on the smaller sensor. The arguments become very technical if anybody can be bothered enough to follow them.

      One way in which effective F-number does vary is with distance from the subject. This doesn't matter for normal subjects but becomes significant for macro. The equation is : Effective F-number = F-number x (Magnification +1) So for a typical F2.8 macro lens used at life size this becomes:Effective F-number = 2.8 x (1+1) = 5.6. At higher magnifications this becomes very significant. The MPE-65 macro lens manual includes a table to save having to calculate these. So with the actual aperture set to F16 at the maximum magnification of 5, the effective F-number is 96!

      Of course, all of this is only relevant if you are using full manual.
      EOS 6D, 6D Mk II, 80D, 70D, 100D, 200D, M50, M100. Canon 10-18, 18 - 55, 55 - 250 IS STM lenses, Canon 16 - 35 mm F4L, 35 mm EF-S macro, 50 mm F1.8 STM, 60 mm EF-S macro, MPE-65 macro, 85 mm F1.8, 200 mm F2.8 L II, M 15 - 45 mm, M 22mm F2, M 32mm F1.4. Sigma 24 - 35 F2 Art, 135 mm F1.8 Art, 17 - 50 F2.8 DC, 105 mm OS macro, 100 - 400 C, 150 - 600 C.

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        #4
        Re: Aperture numbers change if using a crop sensor?

        Thanks for the replies. It’s funny how I accept what is said here, but not from Facebook!
        Janet

        Canon 90D, 77D, Plus a load of lenses, especially macro.
        http://www.leighcameraclub.co.uk/

        Comment


          #5
          Re: Aperture numbers change if using a crop sensor?

          Originally posted by Richard W View Post
          There is endless discussion (and inevitable heated argument) on this topic on the internet. I believe the basic idea is that when you fit a lens designed for full frame onto an APS-C sensor body only part of the light actually strikes the sensor and therefore the effective F-number is smaller. Certainly many reviews will refer to say an F2.8 200 mm lens as equivalent to a 320 mm F4 on the smaller sensor. The arguments become very technical if anybody can be bothered enough to follow them.
          Thing is that in terms of number of photons, they're correct - but exposure doesn't care about the absolute number of photons, but the number of photons per unit area - which (assuming uniform subject brightness) doesn't change according to sensor size. So exposure is not affected. An f2.8 lens will give the same exposure on any size sensor - that's the whole point of using relative aperture numbers.

          One way in which effective F-number does vary is with distance from the subject. This doesn't matter for normal subjects but becomes significant for macro. The equation is : Effective F-number = F-number x (Magnification +1) So for a typical F2.8 macro lens used at life size this becomes:Effective F-number = 2.8 x (1+1) = 5.6. At higher magnifications this becomes very significant. The MPE-65 macro lens manual includes a table to save having to calculate these. So with the actual aperture set to F16 at the maximum magnification of 5, the effective F-number is 96!

          Of course, all of this is only relevant if you are using full manual.
          Yes, in this case, the relative aperture IS changing.
          Please don't ask about my kit, it's embarrassing!

          Comment

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