Mitch -
You are correct that sensor/negative size has nothing to do with "magnification" as though a smaller sensor was a magic teleconverter. However, it does have everything to do with enlargement. The more you have to enlarge a negative to get to a standard print size, the more stringent a circle of confusion you have to use on the negative to have the standard print be acceptably sharp (acceptable DOF). In the context of this parameter, smaller formats actually have less depth of field, because they must be enlarged more.
However, this is overwhelmingly outweighed by the key issue (which is the answer to the original question). When you use a smaller format, you tend to use a shorter focal length lens and stand farther away with that lens than you would with a larger format. [{POST_SNAPBACK}][/a]
That's adding other factors besides the size of the sensor. That shifts the argument from a discussion of sensor size and DOF to sensor size, DOF, and other stuff.
The discussion was about whether an APS-C size sensor in a 35mm DSLR alters DOF or subject magnification. We both agree, it does not.
Alter anything else, of course things will change. But that misses the "key" issue in these DOF threads.
If you alter the distance to change the FOV, of course you will alter the DOF. But that has nothing to do with the sensor size. It has everything to do with repositioning the camera. Why confuse the issue?
You cannot get the same subject magnification and FOV with a full-frame sensor and a smaller sensor at the same subject distance. If you want the same subject size, you have to sacrifice FOV for the smaller sensor. Want the same FOV, you have to sacrifice subject magnification.
Same lens and same subject distance, full-frame or not, you get the same reproduction ratio. Use the same enlargement for both images, and nothing changes. Use different enlargements for your images and any differences in image appearance owe to those different enlargement ratios, not the differences in the size of the sensors.
I believe it is helpful, in discussions like this, to focus on what changes and what does not. If we hold everything else constant and change only the sensor size, the only thing that changes is FOV. Subject magnification and DOF are completely unaffected.
I do not tend to stand any further back with my 20D than I do with my 1Ds MkII. The relationship between subject and background and the overall effect I want the image to have on the viewer determine what focal length lens I use and where I stand.
Many people assume they get less DOF with a smaller sensor because of a telephoto multiplier effect. It is that widely held misunderstanding that I have been addressing.
The conversation about Circle of Confusion just adds to the confusion. That is a factor determined by the lens manufacturer for the expected enlargement factor. Medium format film cameras use a lower Circle of Confusion because the expected enlargement ratio is smaller. Same lens on two 35mm DSLRs with the same subject distance and aperture, one full-frame and one not, yields the same Circle of Confusion and the same DOF. You just see less of it with the samller sensor.
I see this argument all over the different forums. With a 10D or a 20D or whatever, you get a 1.6x multiplier effect, so your 300mm lens becomes the equivalent of a 480mm lens. (It does, but in FOV only.) The logic is then extended to DOF. Since the lens is the equivalent of a 480mm lens, DOF must be the same as a 480mm lens. Nope. For subject magnification and DOF, the lens is still the equivalent of 300mm.
Michael Reichmann has a really nice essay on DOF, Circle of confusion, etc.:
[a href=\"http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml]http://www.luminous-landscape.com/tutorial...eries/dof.shtml[/url]
Cheers,
Mitch