First, we'll state the obvious: Keeping the format constant, a shorter focal length will give a wider AOV, and a longer focal length gives a narrower FOV.
Second, again holding format constant, a shorter focal length will yield smaller circles of confusion than a longer focal length. Thus, a shorter focal length will have greater DOF at a given f-stop than a longer focal length.
Longer focal lengths are associated with stacking up an image or compressing perspective. Shorter focal lengths are associated with elongating or decompressing perspective. This is one reason that short teles and mid-range teles are preferred for portraits; they tend to yield just a slight amount of flattening of the subject's features, rather than elongating the nose, for example.
A smaller format will give the impression of larger circles of confusion as its results are enlarged to some typical print size. Larger formats will offer the impression of smaller circles of confusion, because less enlarging is necessary to get the same sized print. This means that smaller formats (holding everything else equal) will appear to have narrower depth of field, and larger formats will appear to have greater depth of field.
Now here's where I get a little foggy.
Let's take a 50mm lens, and put it on an APS sized format camera. Let's take a 75mm lens and put it on a 35mm sized format camera. Let's assume they both share equal optical and mechanical characteristics except for focal lengths. And lets assume we're making two prints, one from each lens, both enlarged to 8x10. The 50mm lens on the APS format camera (or DSLR) is going to have its depth of field made to appear narrower due to the "crop" factor (or more accurately, as its image is enlarged to some print size). But the 75mm lens on a 35mm camera will show narrower depth of field due to the increased focal length. Will the results in this respect be identical?
What other differences will be detectable, using those same two lenses, same subject distance?
