DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
awaldram Forum Pro • Posts: 12,736
Re: Confusion and mispeaking.

James O'Neill wrote:

awaldram wrote:

moving_comfort wrote:

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?

This is back to front. If you have a full frame image, and crop it (magnifying what is left by greater margin), smaller amounts of misfocus are visible, i.e. the depth of field is shallower.

If you use a smaller image area the circle of confusion where you can see something isn't in focus gets smaller. If you change the focal length of the lens to give the same angle of view, because circle size is proportional to square of focal length the change in focal length more than compensates for the change in circle size.

it never ever had less or more DoF it always had the same DoF if you don't understand that point I'm close to giving up.

The size of the circle of confusion remains constant. If you have an image the size of the side of house the c of c where you notice the image is out of focus is bigger than in the image is the size of a finger nail.

a lens of X length at aperture Y delivers DoF Z always it a characteristic of the lens nothing else it has to the entry pupils and hence magnification and hence f-stop remain constant so any image projected onto whatever target at the same distance has a constant CoC.!

Again , I think you mean "C of C, Z"

What you do with the resulting image affects CoC and so effects perceived DoF

It affects the C of C size which is visible as a circle rather than a point, not the size of the circle laid down by the lens. But I think that's what you meant.

Yeo apologise I'm intermingling C of C and DoF willy nilly with gay abandon probably time I went to bed

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