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bouch wrote:
Imagemaster wrote:
Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.
Isn't format size sort of tied to the camera?
Well Mr. Know-it-all, I guess you have not heard of multi-format cameras.
Why don't you just stay out of these discussions if you don't have clue?
I beg your pardon, I did not know that you had a monopoly on voicing opinions, or are you just a self-appointed moderator
Take a Mamiya 645 camera and take a photo. Now, without changing the lens, aperture, or lens-to-subject distance, put on a 35mm roll-film holder and take a second photo. Now take your loupe and see if you can find greater or less DOF on the 645 film. Surprise, it is the same regardless of the change in film size.
The DOF was determined by the focal length of the lens, the aperture, and the lens-to-subject distance. I could put a piece of lettuce on the film plane and it would not change the DOF just ahead of that plane. 
The original question: I have read a couple of posts here that indicate that DOF increases when sensor size decreases. If this is true, can someone please explain why?
"sensor size decreases" - PERIOD It is very convenient for all the vaiables others brought into this discussion, such as camera model, lens, printing, etc.
Try Mike Lepp's explanation: http://www.photozone.de/3Technology/depth2.htm
I have read a couple of posts here that indicate that DOF increases when sensor size decreases. If this is true, can someone please explain why?
I’ll explain what I call the physics by use of an example. Let’s say we take a picture of a point with a 4x5inch camera and crop out a 6x7cm, 6x4.5cm, and a 35mm frame sizes. (If you like, consider all these formats with the same focal length lens, at the same aperture, and the same distance to the point). According to the applets that take into account format size, the DOF changes for each crop. Does this make sense? Of course not! It is the same point on the same piece of film. As such, it can’t be magnified any more for each format and still look like a point and not a disk. Thus, we can conclude that COC is not a function of format. This leaves DOF as a property of the lens and focusing distance(review the above equations). Also note that the sharper the lens, the smaller the point is that can be produced on the film, the shallower the DOF (I’ll let you think about this), and the
larger a print can be made and still have the point look like a point.
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