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Thread: Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses

  1. #1
    Member konmin's Avatar
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    Default Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses

    Just sharing with everyone. When we 'calculate' the 35mm-equivalent focal length for APSC cameras, we usually multiply the focal length by it's crop factor. But we often forget about the equivalent aperture value.



    Example:
    50mm F1.8 lens on a Canon APSC camera is actually:
    1.6 (crop factor) x 50mm = 80mm, 1.6 (crop factor) x F1.8 = F2.88 [on a full frame camera].

    Hope everyone find this useful.

  2. #2
    Senior Member edutilos-'s Avatar
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    Default Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

    I am pretty sure you don't just multiply the F stop value by crop factor, based on an app I downloaded.

    It is true that you will have greater DOF (and reduced DOF control) with smaller sensor size of course, that is a well known fact.

  3. #3

    Default Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

    if my exposure on crop f8 and 1/100
    will I have same shutter on FF using f8
    shoot same things and same lighting conditions
    宁愿遇见丢失幼崽的母熊,也不愿碰上做蠢事的愚人

  4. #4

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    If one use a 50mm on FF to shoot a scene, and then use a crop with the same lens to shoot the same scene, he will need to move backwards in order to get approximate the same field of view. Because u move back, the distance between camera n point of focus is increased, hence less DOF.

    Exposure will still be the same, assuming the scene is uniformly illuminated.

    To get the same dof as the 50mm (e.g at f/1.8) on the crop camera, the 50mm on the FF will have to move forward (to get same dof) and stop down the aperture by a factor of the crop factor, to get same dof (but reduced exposure). Everything is approximate values, but works well most of the times.

    Hope this helps.
    hi

  5. #5
    Senior Member edutilos-'s Avatar
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    Default Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

    Quote Originally Posted by Shizuma View Post
    if my exposure on crop f8 and 1/100
    will I have same shutter on FF using f8
    shoot same things and same lighting conditions
    Yes with same ISO.

  6. #6

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    U never heard before full frame can achieve shallower depth of field? How come shallower? Because the aperture is bigger
    D7100,SB910,17-50/2.8OS,105/2.8VR,85/1.8D,2xE-M1,O60/2.8,12-40/2.8,35-100/2.8,14-42,LX100

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    Senior Member edutilos-'s Avatar
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    Default Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

    Quote Originally Posted by sin77 View Post
    U never heard before full frame can achieve shallower depth of field? How come shallower? Because the aperture is bigger
    Erm, you seem to be saying that:

    Full frame (sensor size) can achieve shallower depth of field because aperture (lens characteristic) is bigger.

    It's more of crop frame sensors taking a "crop" from the centre of the image circle.. Nothing to do with aperture size. FF and CF, same aperture, FF still have shallower DOF.

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    Default Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

    Quote Originally Posted by konmin View Post
    Just sharing with everyone. When we 'calculate' the 35mm-equivalent focal length for APSC cameras, we usually multiply the focal length by it's crop factor. But we often forget about the equivalent aperture value.



    Example:
    50mm F1.8 lens on a Canon APSC camera is actually:
    1.6 (crop factor) x 50mm = 80mm, 1.6 (crop factor) x F1.8 = F2.88 [on a full frame camera].

    Hope everyone find this useful.
    quite illuminating for a newbie like me. never knew about this

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