BG blur / subject isolation with different lenses/crop?

Discussion in 'Digital Photography' started by jdavtz, Jul 13, 2010.

  1. jdavtz macrumors 6502a

    Aug 22, 2005
    Quick question:

    In same scenario, fixed subject, fixed background.

    Which should give more subject 'isolation' (not sure of the best word - i mean a more blurred background):

    135mm f/2 and crop to give same size subject as...
    200mm f/2.8 and don't crop

    Actual lenses not really so important -- I'm interested in how you work it out.

    (I would test it myself but have lent the 200mm lens to my sister).

  2. Edge100 macrumors 68000

    May 14, 2002
    Where am I???

    On full-frame...

    135 f/2, with subject at 30ft - DoF = 1.78 ft
    200 f/2.8, with subject at 30ft - DoF = 1.14 ft

    Cropping after the exposure wont affect DoF...using a crop sensor camera to get the same subject size WILL (by moving you further away from the subject).
  3. toxic macrumors 68000

    Nov 9, 2008
    cropping does affect DoF since it enlarges the image more.

    by cropping the 135mm image to match the 200mm FoV, you're simulating a 1.48x sensor. going to a DoF calculator and comparing 135/2 on 1.5x to 200/2.8 on 35mm, the 200mm has 5mm less DoF at 5m and 17mm less at 10m...which is much less than 1/3 of a stop, which means the difference is negligible.

    the 200mm image will likely look better, though, since it isn't enlarged as much.

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