With digital cams, well, many of them that even have a dof button, you set the aperture and hit the button, get a look. What was nice with some old film slrs is you could move the lens apeture and see the effect immediately, one stop to another. I did this at work today with a coworkers Minolta 35mm slr. I have not tried this with digital lenses that actually have aperture settings, using manual. I do not know if it will work. Many cpu digital lenses have no apetrure setting, using camera for that. Otherwise, you are advised to set the lens at max aperture and lock it there. Then the cam does the stuff. I do have a lens or two with manual aperture and will try to see if I can cycle through using the lens on manual and get a dof preview...I doubt it will work since the cam is wanting to do it's own stuff. But, we will see. With a film camera, the preview is very useful in macro and closeup shots. It is realtively easy to see the central area of focus and adjust as needed, to focus in a third or whatever. Well, you do need decent light and a good eye while doing it..at least I do. For macro, I see the feature very helpful even if I have neglected this feature.