Originally posted by pschlute Steve
I agree with you that a lens cannot change its resolving properties (and thus the image it forms on the film or sensor plane) by moving it from one format to another.
But if you re-read my comments you cannot possibly say none of it is true. Please answer the following questions:
1. Is the APS-c sensor smaller than a 35mm film frame ?
2. To produce an 10x8 print, will the two formats have to be enlarged by the same amount, or by different amounts ?
3. DOF is measured on a given print size, NOT on the sensor or film frame. Yes or no ?
4. The more you enlarge any image, the larger the circle of confusion you create. Yes or no?
Once the above is understood, it is then easy to understand that DOF scales cannot relate to any lens in isolation, but were created by a technician using the lens, and the intended format magnification to a given print size to create those scales. When the 35mm film lens is then used on a different format, the scales are no longer accurate because the assumed of format magnification is not the same.
First let me answer your questions.
1. Yes, but it's not relevant.
2. I'll explain below why this is not a valid question.
3. No, no, and no. I'm not being redundant - none of these possibilities are correct.
4. See #2.
Your theory of "format magnification" is inherently flawed in the following ways:
1. Traditional
enlarging is an optical process. Does the DOF vary based on the enlarger? How about the brand of photo paper, or for that matter the brand of film?
2. Digtal
printing is just that, completely digital. Further more,
digital files have no physical size.
3. According to your logic, switching to a different type/speed/quality of film would change the apparent DOF of the same shot with the same lens. The same goes for switching from a 6 MP to a 14 MP sensor.
To thoroughly answer your third question, DOF is relative to the sharpest point in the image, within reason. Obviously if the whole image is out of focus, then there is no DOF.
Edit: Notice I never said anything about field of view, which is a completely separate subject.