Cropping DOES change depth-of-field

There are many people who think cropping an image doesn't change the depth-of-field. I admit that it's not all that intuitive that removing a portion of an image would change the DOF, but it does. The reason it does is because we almost always compare two images at the same final viewing size (the angle-of-view the final image occupies in our vision) and that means once we crop, we have to enlarge the remaining portion. That enlargement enlarges the blur along with the image detail, thus making the blur more visible. That changes DOF through a change in the circle of confusion (CoC) parameter used in the DOF equations.

The DOF equations, and the calculators that use them, bear this out:

• Full-frame (1x), 100mm, f/2, 60 inches subject distance, DOF=1.02 inches
• 4/3rds (2x-crop), 100mm, f/2, 60 inches subject distance, DOF=0.51 inches
• 1/2.5" (6x-crop), 100mm, f/2, 60 inches subject distance, DOF=0.17 inches

Take a look for yourself. These two images are actually the same image, both shot at 100mm and f/2 on full-frame. The one on the left is a 100% (1:1) crop, the one on the right is the full frame downsampled to the same pixel dimensions as the 6x-crop.  The DOF is much shallower on the left image because of its much greater enlargement (to make the two final images the same viewing size), and thus much smaller CoC.

About 1/4" DOF on the left, slightly more than 1" DOF on the right.