Could someone please tell me what this does? (depth of field preview) I read in the manual that it says you can hold the button down and change the aperture and you will see the viewfinder darken or lighten. I see that and I understand that. I just dont know what this is "telling" me. So I guess my question is, what is the view finder showing me when its going darker or lighter. (if that makes sense) Thanks in advance!