This is very interesting, especially about how DOF depends on print size. No matter how large or small I print my images, I don't see any change to the DOF. Can you elaborate more on how you find DOF to depend on print size?
Also, I have taken identical image captures from a 36x48 CCD back with 9 micron pixels (Hasselblad CF22) and when compared to identical images taken with the same camera and lens but with a 36x48 CCD back having 7.2 micron pixels (Sinar e75LV), there is no difference in DOF either. You probably have more experience with many other digital backs. So, can you elaborate on your experience where you find the DOF to depend on the pixel size and not just the sensor size?
From what I can see in my own images, the DOF seems to depend only on the apparent object size relative to the sensor size. It would be great if you could explain better how these other print and sensor issues also affect the DOF.
I define Depth of Field as the inability to distinguish (in a meaningful/practical sense) between the sharpness of point A and the sharpness of point B.
It's easiest to see the effect of this with extreme examples.
Print a 60mp file as a postage stamp (say a 30mm square) and as a 3 meter print.
Even the very best (commercial photographic) printing technologies cannot get 60mp of detail onto the postage stamp. Look at a slightly out of focus area on the 3 meter print then look at the same subject area on the postage stamp. Even with a very close viewing distance (or a magnifying glass) you could not tell that this area of the subject was any less in focus than the sharpest part of the image.
Same goes with different micron sizes. Taking extremes again a 5.2 micron sensor (Aptus II 12) and a 12 micron sensor (H20): something that is slighlty out of focus on the 5.2 micron sensor cannot be shown as out of focus on the 12 micron sensor. So DOF will be moderately higher on the H20 (and the absolute level of detail in the in-focus areas will be higher in the Aptus II 12).
Said differently: DOF is infinite on a one pixel image. DOF is non-existent on a sensor with 1000 gigpixel resolution - the moment you look outside the plane of focus (at 100% on a monitor) you'll see the pixels are no longer as sharp as they were at the plane of focus.
Said differently again: If a camera/print is capable of resolving more detail then you'll more easily notice when it does not.
The difference between 7.2 and 9 microns will be subtle and will only appear if the 7.2 micron sensor is used with a lens fully capable of resolving on 7.2 micron pixels.
Doug Peterson (e-mail Me)
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