DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
moving_comfort Veteran Member • Posts: 8,231
Re: DOF and Cropping take 2

awaldram wrote:

Ian Stuart Forsyth wrote:

Its kind of funny as Cambridge Tutorials has got this right and even their calculator is correct as this is what I used who would have thunk EH

Yep they seem to know what there talking about despite MC's bunk

The link Ian gave was for a different tutorial than what you linked (same site.) You missed his joke a bit I'm afraid.


as they say on the page you refer

"Be warned that both of these terms can be somewhat misleading. The lens focal length does not change just because a lens is used on a different sized sensor — just its angle of view. A 50 mm lens is always a 50 mm lens, regardless of the sensor type."

And just above they state exactly the point I;ve been making

Yes, just like a 5mm lens is a 5mm lens, regardless of sensor type - but they give radically different FOV when you move up in size from P&S. Not sure how this fits into your confusion or how it applies directly to things you've said so far.


"As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject."

i.e you alter the magnification to maintain size therefore altering CoC and intern reducing DoF.

as they state

"Note that the above calculator assumes that you have a lens on the new sensor (#2) which can reproduce the same angle of view as on the original sensor (#1). If you instead use the same lens, then the aperture requirements remain the same (but you will have to get closer to your subject)."

Again exactly what I've been saying

Actually it's not what you've been saying - you've been saying that cropping FF to aps-c of a shot taken from the same position and then displaying the same size does not alter DOF - and it does. It makes the FF shot equal the aps-c shot in DOF, when before (uncropped) it had less DOF. See how the DOF changed there, by cropping and enlarging to the same viewing size? I guess we can keep repeating it until it takes


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