DOF and Cropping take 2

Started 10 months ago | Discussions thread
James O'Neill
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Re: DOF and Cropping take 2
In reply to Ian Stuart Forsyth, 10 months ago

Ian Stuart Forsyth wrote:

moving_comfort 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?

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.

It did not have the same DOF until you changed the viewing size by cropping and enlarging! Which is what I and other have been trying, and trying, and trying to get across to you.

and trying

Here:

100mm f/2.8 FF @ 10 feet does not have the same DOF as 100mm f/2.8 aps-c @ 10 feet. Correct? Let's consult:

* Using the same lens on a APS-C crop sensor camera and a 35mm full frame body, the APS-C crop sensor camera image has 1.5x LESS depth of field than the 35mm image would have

So I assume you won't argue the point above, and agree that it's correct. Moving on...

However if you crop the FF shot to be the same FOV as the aps-c shot and display both at the same size (screen, print wherever) then they both do have the same DOF. Correct? Let's consult the bible:

* If you use the same lens on an APS-C crop sensor camera and a 35mm full frame body and crop the full frame 35mm image to give the same view as the APS-C crop image, the depth of field is IDENTICAL

100 f/2.8 @ 10 Feet has the same circle of confusion size regardless of the sensor. A 16x24mm image needs to keep the circle of confusion smaller than a 24x36mm image. Therefore the APS-C sensor has less Depth of field (and a narrower angle of view).

If you crop the FF image use a piece the same size as the APS-C then you have to adopt an APS-C size circle of confusion making the D.o.F the same.

And for completeness.  If you change the focal length to keep the angle of view the same the circle is 2.25 times the size (1.5^2) but you a circle 1.5x the size gives the same apparent-out-of-focusness.  Therefore a 150 f/2.8 on FF has the depth of field and angle of view of a 100 f/4.2 on APSC
For the same angle of view : a smaller image has MORE d.o.f, because the circle is smaller in absolute terms, and as a proportion of the image. 
For the same focal length a smaller image has LESS d.o.f  because the same size circle is a bigger proportion of the image.

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