Originally posted by stevebrot I won't comment on your lengthy note, but I think you need to go back to the drawing board. DOF is truly based on three factors,* but not the ones you listed. They are:
- Absolute (not relative) aperture
- Magnification required for final image size**
- Viewing distance for final image
Focal length, taking distance, capture media frame size, and post-exposure crop all factor into magnification.
For a given composition and aperture, available DOF varies according to the media frame size. This behavior is readily demonstrable. Just ask anybody who shoots medium or large format. If you disagree, I suggest that you prepare a series of example images proving otherwise.
Steve
* There is also a fourth (assumed) factor, that being the notion of acceptable blur.
** Any doubts about magnification? Simply compare the DOF of my avatar image to the original:
https://flic.kr/p/5SzM3k
Post exposure crop? Really?
No need to go any further, other than to say you may wish to read up on how DOF works.
Example photos...
Take a photo, bring it into Photoshop. Crop the image.
Does DOF change? There's your answer about FF v.s CF sensors.
PERIOD.
True, DOF is somewhat used an a term of "acceptable sharpness" and it isn't 100% cut and dry.
But you need to throw out subjective issues up to and including how well a person sees... When you do and stay with consistent and definable terms, there is a specific amount of fall-off based on those three physical properties.
Other issues affecting "acceptable sharpness" include circles of confusion, refraction, signal-to-noise, high ISO noise and others...
BUT NONE OF THOSE are specific qualities to full frame v.s crop frame.
A crop frame may have smaller or larger circle of confusion based on the pixel density, among other factors.
A crop frame may have more or less high ISO noise than a full frame camera (depending on age, model, type of sensor)
A crop frame camera may have more refraction or glare based on a host of issues including lens, angle of light, etc...
All affect perceived sharpness, but none have anything to do with DOF.
If you want to check the math, go to any Depth Of Field calculator and see what happens. There are plenty of them out there.
CoC is included, but again does not apply strictly to FF vs CF sensors so its a moot point unless you give a specific brand and model as well.