Originally posted by Lowell Goudge My statement that your response was misleading apPlies directly to the first line of your post specifically that you apply crop FA for to both focal length and aperture
This is simply untrue
Lowell, please. It is true given the assumptions I'm making. You are making different assumptions so you arrive at different conclusions.
The assumption I'm making (which is shared by others in this thread) is that when comparing sensor formats it is difficult to make statements when you are comparing apples with oranges, e.g., prints of different size, images taken from different positions, etc.
Hence the assumption is that a photographer wants to capture a specific scene and print it to a specific output size. It simply doesn't make sense to change the size of prints just because you changed the sensor format.
This assumption gives rise to the notion of an "equivalent image", i.e., same content, same size.
Let's say you take an image with an FF camera with a 75mm lens at f/2.1. You can produce the equivalent image with an APS-C camera with a 50mm lens at f/1.4. Period. There is no debating about this. Use a DOF calculator or run the experiment in real life, if you are not convinced.
Originally posted by Lowell Goudge Depth of field is only a product of the lens itself and the print enlargement ratio
And since one shouldn't change output size when comparing sensor formats, the enlargement ratio changes. Also, one has to change the lens (and its settings) in order to achieve the same image. Comparing images with different content just isn't useful in a sensor format discussion. Of course you can look at all sorts of different images that can be created by various sensor formats, lens variations and changes in camera positions (Bob Atkins does it in his article) but I find these a bit academic. A photographer (rather than a gearist) is interested in a particular result and hence the notion of an equivalent image makes the most sense.
Originally posted by Lowell Goudge As I said, if you cut the middle 60% out of a print from film nothing in the remaining middle portion has changed, has it.
No one disputes that, it is just not a very helpful assumption that your APS-C prints are always 40% smaller than your FF prints and that you don't try to recreate the same FOV when changing the sensor format.
Everything you say makes sense with the assumption that one doesn't change the lens but changes the output size.
Everything others and I said makes sense when you are talking about equivalent images. I would appreciate if you stopped stating that my statements are "misleading" or "untrue".