DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
James O'Neill Veteran Member • Posts: 4,655
And here we see the misunderstanding.

MightyMike wrote:

awaldram wrote:

moving_comfort wrote:

Q: Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes? Because I think you used to hold the position that it does not.

You missed the point!

Does cropping FF (to ap-sc, for example) change DOF when displayed at the same sizes?

To crop and then display at the same size the magnification must change and therefore the appearance of DOF will change!

Simple question how can it.?

The lens projects an image onto the film plane that image is the same whatever sensor is behind it

look at it this way assume we have a camera with interchangeable backs.

We put a 50mm f4 lens on the camera on a tripod with a FF back on it and take a picture.

we then do nothing but change the back to a sensor 1/2 the size and take another picture.

if we superimpose the two images on-top of each other will the be different in any way shape or form where they overlap?

The overlapping parts are absolutely identical AND HAVE DIFFERENT DEPTH OF FIELD.

How so ? Because what I just typed sounds nuts.

Lets take your  50mm f/4 and focus it at infinity. We have a subject at 25m.

So the circle size is 50 x 50 / 4 * 25,000  =  0.025mm. Does that look in focus or out of focus ? 
The size at which what is not in perfect focus is noticeably out of focus depends on the size of the image.  That size circle looks very different indistinguishable from a point on medium format and an enormous smudge on a Q. If we zoomed in enough on the medium format we can see it is out of focus. D of F is the zone which is out of focus but not noticeably so.  You can notice a circle in a crop of an image which you don't notice  when looking at the whole image.

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