When I press the depth of field preview button the viewfinder gets a little dim, but I do not see a difference. Is there a subtle difference that I am missing? i also could not see a difference on my D90.
The viewfinder gets dim because when you use DOF preview you're viewing through the aperture diameter as it will be when the photo is taken. For example, if you use aperture priority and select f22 and use DOF preview, things will get REAL, REAL dark!
"Red is gray and yellow white, but we decide which is right ....and which is an illusion"
The most out-of-focus viewing will be seen with the lens wide open and bright. As you stop down the lens, along with getting darker, parts further in front and behind the focus plane will also get more in focus.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA
I have always felt that this was a useless feature - I cannot afford to depend on what I can bearly see to have elements important to the image to be focus- better to learn your lenses, hyperfocal distance etc to be sure about the sharpness of your image.
This feature works best when you are shooting with a very narrow DoF like in macro photography.
If you have a macro lens or a fast aperture prime, a 50mm f/1.8 or 85mm f/1.4 for example, and focus very close to your subject you will be able to see the increased DoF as the lens stops down when you press the button.