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Depth of field question

Discussion in 'Sony Alpha E-Mount Lenses' started by adwb, Apr 15, 2016.

  1. adwb

    adwb TalkEmount Regular

    127
    Sep 30, 2015
    Bristol UK
    Alistair
    if I fit to a A7 body a Emount lens for a crop frame body for example a Sigma 60 2.8
    I understand why it with give a field of view and magnification equal to a 90mm (approx)

    What don't understand is if I fitted a full frame 90mm to any A7 body at f2.8 with the subject at a distance of 12 feet , the depth of field will be approx 10 inches .
    Would that also be correct if the Sigma 60 in crop mode is used with the same f2.8 / 12 feet = 10 inches?
     
  2. Nexnut

    Nexnut TalkEmount Top Veteran

    Nope. Whatever you do - the focal length doesn't change a bit*, only the field of view. Just think of mounting any lens on any sensor and then just look what happens in the central part of that sensor - exactly the same, only now you're looking at a smaller angle, equivalent to but not the same as a longer focal length.
    In other words: shooting at a fixed distance using the same f-stop your 60mm lens will produce a larger DOF than a 90mm lens - independent of the sensor size.
    If you compare images shot with a 90mm lens @f4 on FF and 60mm @~f2.8 on APS-C of the same subject from the same point and at the same distance they look about the same but you have to magnify the APS-C image 1.5 times that of the FF image.

    *apart from focus breathing - different story
     
    Last edited: Apr 15, 2016
  3. adwb

    adwb TalkEmount Regular

    127
    Sep 30, 2015
    Bristol UK
    Alistair
    Thanks Ken, that's what I thought.