Please explain the no depth of field preview for the Nikon D3100?

I was reading specs on this camera, and came across this. What exactly does this mean?

This gives you a preview of what effect your chosen aperture will have on depth of field.

It's useful up to a point, but on an APS-C penta-mirror viewfinder (not all that sharp, not all that bright) I'm not sure how much benefit it will be.

The lens has an aperture which affects the brightness and the focus of the lens, for the last 40 years most cameras let you shoot with the lens wide open, as this keeps the viewfinder nice and bright and clear, which is helpful for focusing and composition. The shooting aperture may be different, and may have a pronounced effect on background blur or how much detail of your shot will be sharp, if you need to check this a depth of field preview button was the way.

90% of photographers didn't bother with them, and 99.9% of beginners or budget users didn't bother with them.

If you are a student, or do extreme macro or want to do shallow depth of field portraiture it would be useful, if not, don't worry about it, or buy canon instead.

I have that function in my D90, but I rarely use it, so you are not missing out on much.

If you want an estimate of the depth of field for a specific lens focal length, aperture, and focal point, and if you have an ipod touch/ipad/iphone, download the "DOF Calculator" from iTunes. Its free.