The aperture on the lens has to be locked at F/22. That said, you will adjust the aperture to your liking in-camera, or the camera will do it for you. You should know, however, that this is not an AF-S lens. This means that your camera will not AF with it. It will be only manual focus. If you want a 50mm prime (non-zoom) lens to AF with your camera, you'd have to get the 50mm F/1.4.
The lens, however, is an incredibly sharp lens. I own one, it was my second lens. I absolutely adore it. The Aperture works great for blurring backgrounds, isolation, and low-light situations. Its an absolute
must-have for anyone with a Nikon. It's going to be hard to find someone who has never owned this lens, but has owned a Nikon body. Its a great lens. If you dont mind not having AF, get it for sure.
The Nikon 35mm F/1.8 is an AF-S lens, for not much more new than what you would pay for that lens. This is also an incredibly sharp lens, but is much different on a DX body. Since the Nikon 50mm 1.8 is designed to cover a full-frame body and the D3100 is a DX body, you will have a 1.5x crop factor involved, giving you a 75mm F/1.8 lens on your body. The 35mm F/1.8 is a DX lens, this means there is no crop factor involved. This will give you a true 35mm f/1.8 lens. So, here are your options with this:
50mm F/1.8D lens:
No AF
Incredibly sharp
1.5x crop gives you actually 75mm.
$120-140.
35mm F/1.8G AF-S lens:
AF fully supported
Incredibly Sharp
No crop factor, 35mm.
Less isolation due to wider angle.
$180-200.
50mm F/1.4G AF-S lens:
AF fully supported
Incredibly sharp above F/1.8 (I hear..)
1.5x crop factor gives you 75mm.
Better isolation and low-light work due to F/1.4
$450-500
All prices are given for NEW product..you can surely find them cheaper used from Amazon,
ebay, craigslist, etc, ect. Hope Ive helped a little.
Mark