Originally posted by *isteve APS has more DOF than FF
I read this many times now and I feel tempted to clarify a little bit. Again, its pure optics.
The Depth of Field (DoF) depends on the circle of confusion, focal length, aperture f-stop, and the subject distance. You may find a calculator for this at
Depth of Field Calculator - Mark Roberts Photography.
Now assume that you have two cameras A, B, where B has a sensor of twice the size of A, a focal length twice as large to compensate, an aperture f-stop again twice as large to make both lenses having the same absolute diameter (capturing the same amount of light onto the sensor, i.e., making f/sensor-size equal); and a circle of confusion in B again twice as large as in A because the sensor is twice as large.
Then, cameras A and B have the exact same DoF. You may cross check this if you like.
The images of A and B will almost look identical out of focus. The image of B will potentially be sharper
in the focus plane but with higher ISO noise because there will be the same light per sensor but less light per sensor cell. Therefore, taken the sharpness in the focus plane, one would easily have demanded more DoF for B. And, taken the ISO noise, one would easily have demanded more aperture for B. This is why most people perceive the DoF to be an issue with larger sensors.
In summary: For a given absolute (in mm) aperture and a given image composition, the DoF and image quality does not depend on sensor size. The point is just that with increasing sensor size, the DoF decreases as much as image quality shall be improved.