Recently I was having a conversation about D90 with a friend of mine who also owns one. It came as a surprise to me when I learned that he was missing out on some of the biggest features D90 offered. Here is a compilation of the things that we discussed.
The very first thing this body has in store for you is DOF Preview button on the front side of the body towards the right side of the lens.
Pressing this button will tell the camera to set the aperture at the selected level, allowing you to see what the shot will look like in the view finder – particularly the depth of field. This enables you to see how much of your image is in focus sharply.
source – dPs