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Apply Crop Factor To Aperture ?

Discussion in 'General Equipment Chat & Advice' started by Bazarchie, Dec 1, 2018.

  1. Bazarchie

    Bazarchie Well-Known Member

    I have always considered that the crop factor should be applied to the focal length of a lens but not the aperture. However, the internet seems to be divided on this subject.

    Is there a right answer?
  2. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    F numbers are simply the focal length of a lens divided by the diameter of its aperture. If the focal lenth is 50mm and the aperture is 25mm the F stop is 2.0. It doesn't matter what the size of the the sensor is - the F number will still be the same. If anyone thinks otherwise find out what they're drinking! ;)
    Roger Hicks and RogerMac like this.
  3. Benchista

    Benchista Which Tyler

    In terms of exposure, makes no difference. In terms of DOF, then yes, it does.
    ChrisNewman and Roger Hicks like this.
  4. PeteRob

    PeteRob Well-Known Member

    The focal length of the lens is what it is, and the (max) aperture defines the F number. All the crop factor does is to account for the size of the sensor relative to 35 mm.

    There have been a couple of articles in AP discussing apparent depth of field and equivalence of effective focal length if that's what you are thinking of.
    ChrisNewman and Roger Hicks like this.
  5. daft_biker

    daft_biker Action Man!

    Absolutely, and another reason why I like to think in terms of magnification instead of focal length when considering Dof.

    ...for larger subjects it's fairly easy to guesstimate if the field of view at the plane of focus is tens, hundreds or thousands of times bigger than the sensor and get a feel for how much to stop down.
  6. Roger Hicks

    Roger Hicks Well-Known Member

    A "crop factor" also needs to be applied to the very rough-and-ready rule of "1/focal length". This is just about adequate for full frame 35mm but smaller formats require higher shutter speeds (because the magnification is higher for a given print/screen size) and bigger formats can be correspondingly relaxed (assuming the camera can be held steady).


    ChrisNewman likes this.
  7. Bazarchie

    Bazarchie Well-Known Member

    Thanks, that is all I need.

    This is what I meant by applying the crop factor to the focal length. No issues with this.

    This factor is something I try to get away with, or use a factor lower than the Canon 1.6. I also allow for IS. I cannot hold a long lens as steady as I used to so all this usually means having to use a high speed.

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