Canon AE-1 PROGRAM Instructions Manual page 53

6. Now you can take your shot.
Hey! There must be
something wrong . The
viewfinder is getting darker
and it's difficult to focus .
Try turning the aperture ring to the
smallest number for viewing. Then, while
looking at objects in the foreground and
aperture you'll be using.
background, turn the aperture ring to the ,
I
Oh, I see, you're previewing the depth of
field. As you turn the aperture ring to a
higher number, the diaphragm closes
down and lets less light in.
Oh, I see now. There's more in focus!