"dosferatu" wrote:
So DSLR's, I assume, don't have focusing screens?

They do have focussing screens. And there are even companies like e.g.
CatzEye Optics who make custom replacement focussing screens for special
requirement

Is that why looking thru
the viewfinder everything is in focus?

Well, it isn't. Quite the opposite actually. Under 'normal'
circumstances the apperture will be wide open, giving you a narrow DOF.

Do any DSLR's have Depth of field preview thru the viewfinder

You just close the apperture to the working size, typically by pushing a
small button near the lens mount.

or do you have to go to the lcd display?

Wouldn't do you any good because almost no DSLRs have what is called
'live preview'.

jue