10D Depth of Field

[Kaptekarev]

Just got my 10D and am having lots of fun. I'm still trying to get a handle on all the functionality and can't seem to understand how to use the "Depth of Field Preview." I've red the instructions, pressed the button and the view finder gets dark. What does that tell me? What info am I supposed to get from the depth of field preview?

[RichardtheSane]

It shows you how much more of the frame will become sharp when you stop the aperture down.

To see what I mean focus on a flat surface (a lawn is good for this) at about a 30 degree down angle. You will see that the area just a few inches before and after the point you focused on will become more blurred. Set the aperture to F16 or F22 then press the DOF preview button after focusing. You should see much more of the grass become sharp.

Hope I explained it well.

[hmhm]

Ordinarily, when you're looking through the viewfinder, the "diaphragm" that sets the aperture is wide-open. When you press the shutter button, the diaphragm closes down to whatever aperture is set, then the shutter triggers, then the diaphragm opens back up again.

So when looking through the viewfinder, you're seeing something like what the shot would look like if you used the widest aperture (i.e. the smaller f-number). To give you an idea of what depth of field you'd get from the aperture set on the camera, the DoF preview button gives you a way to close down the diaphragm to the chosen aperture while looking through the viewfinder. This smaller aperture setting lets through less light, so of course the view is darker, but it also shows you greater depth of field.

Try putting the camera in Av mode, hold down the DoF preview button, and dial through a variety of different aperture settings, and see what happens.
-harry