With APS size DSLR sensors the 35mm lenses are effectively half again as long because the angle of view respective of the image frame diagonal is smaller. This does not change the behavior of the lens optics however, unless you select a focal length that provide the same angle of view.

In other words if you shot with a 24mm with a 35mm SLR you would need an 18mm with an APS DSLR to obtain the same angle of view, which would provide approximately the same depth of field as the 24mm at a stop wider aperture.

If you use a 50mm normal 35mm SLR lens on an APS DSLR the effective focal length becomes 75mm, but the depth of field remains the same as it would be used on the 35mm SLR if the camera to subject distance is the same.

David... thanks for your reply... I still do not understand the Pentax istDS manual's statement "but compared to a 35mm camera, the value is roughly one aperature setting lower( the focused range becomes narrower). Page 128 of the manual. However, this camera has sold me on digital... I can use my old Pentax M and KA lens with ease.

Very often the language in camera guides gets a bit mangled in translation from Japanese engineer's explanations into American english. If you understood what I described, there really isn't anything more to it.

Sadly the fact the camera companies chose to describe the effect of a reduced size imaging area in APS DSLR's as a 1.5X effective focal length factor, has caused more confusion than understanding.

Again...thanks David... the bottom line is that I will get the same depth of field from a 24mm lens whether it is on a 35mm film camera or an aps digital camera... or for that matter a medium format camera (if they made such a lens for medium format)

Marty,

Yes, essentially depth of field performance is a function of the optical factors of a given lens. The confusing aspect with APS DSLR's comes from the fact the smaller imaging area changes the effective angle of view, so at a given distance from a subject a particular lens on a 35mm SLR will cover a larger subject area than it will on an APS DSLR. If you move back with the APS DSLR to obtain the same area of subject coverage, that of course is a difference, but one that does not appreciably affect depth of field in practical terms.