DOF and Cropping take 2

Started 8 months ago | Discussions thread
James O'Neill
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Re: Cropping from within a format (101)
In reply to Ian Stuart Forsyth, 8 months ago

Ian Stuart Forsyth wrote:

As you can see that they share the same FOV and DOF now why is this? (bear with me if you Know)

They are all the same because you have not achieved the same FoV by changing focal length but by cropping, In one you have cropped once and the rest you have cropped twice.

The smaller the fragment of the image you zoom in to, the easier it is to see what is out of focus. The cereal is just as out of focus in the whole frame as it is in the cropped-down-to-Q frame, but at such you can't see it in the whole frame.

This does not prove F4 on any of the 4 format sizes is the same but rather that if we crop an image from format and compare it to another format at the same FOV that the DOF will be the same only because the share the size portion of the project image circle from the 55mm F4 lens (same format size). So at the end of the day we still have F2.8 is not equal to F2.8 across formats.

That's not the clearest phrasing in the history of English.  A small image from the camera - whether it started small or was cropped from a bigger one - has to be magnified more than large one to fill the same size sheet of photographic paper or computer screen. As you magnify what was in the D.o.F zone because you couldn't see it was slightly out of focus is revealed to be blurred.

D.o.F isn't some magical "What's in is perfectly focused" zone , but "What's in is so slightly out of focus you don't notice". The same focus "error" with the same lens and same aperture will produce the same distance of blurring on the film or sensor: 0.02 mm of blur is harder to see on a 33x44mm image than a 3x4 mm one. Hence all d.o.f. calculations use a circle of confusion size which depends on image size : and that is image size after all cropping has been done.

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