One of the things I like about this forum is that I frequently read comments which challenge my assumptions and make me think “Hmmm...perhaps I should refresh my knowledge on this topic before I shoot my mouth off” ... er, I mean, before I add to the general knowledge.
Most of what Zipper has stated is true, but with qualification.
Depth of field (DoF) is a function of aperture and shooting distance and is essentially independent of focal length (except in true macro applications). DoF will not change when you change to a different lens as long as distance to the subject (and thus image size on film or sensor) remains constant.
So to say that a 100mm lens projects the same image on 35mm or on 1.5x sensor is partially true - it projects the same target size - but herein lies the problem. I don’t want the same size image on my sensor as I have on my film, I want the same relative size image.
I am in front of my model and doing a head shot (let’s say she takes up 75% of the frame image), if I take my 85mm lens from my 35mm camera and put it on my DSLR she now takes up 108% of the frame (my smaller DSLR sensor has cropped this for me). I have to move back to get the same target size as with my 35mm, and so my 85mm lens has now become the DSLR equivalent of 128mm and since my distance has changed (let’s assume aperture has not) this will increase my DoF.
If I have both cameras in hand with an 85mm on 35mm format and a 55mm on my DSLR, I am seeing the same image (in relative size), but in fact the ACTUAL image size on my sensor is smaller. Zipper is correct, the sensor, or film size, does impact the cropping of that image, so a 1.5x sensor will crop to a smaller portion of the image. Based on this then yes, my 55mm lens on my DSLR does indeed behave like the 85mm on my 35mm, and both will have the same DoF since DoF is irrespective of focal length.
Sharpness and bokeh, that’s another issue. The statement is only true if my magnification for my final print is correspondingly smaller that I would make if I were shooting on 35mm. You have to compare apples to apples here. Bokeh is a function of the Circle of Confusion a lens lays down - let’s say 1/250 inch or so. It will be identical if my final print is only, say, 66% of the size I would normally print. I doubt anyone will substitute prints that are 5 3/8” X 6 5/8” for 8” x 10” prints just to get the same bokeh and sharpness.
As sensors continue to increase in resolution the effect will become less noticeable, but I’d estimate we’d need about 30MP on an APS sized sensor to become equal, based on one report I’d read that the equivalent resolution of film is about 20MP, and larger to boot.
Just my 2 centavos worth.