I have a Nikon D3100. I'm trying to buy a lens that will mainly take outside pictures of kids with great bokeh. I can't decide between 35, 50, or 85mm.
I'm worried about autofocus as well. With kids, I like a quick auto-focus. What will work with my camera body?
For the D3100, which doesn't have an in-built autofocus motor, you'll need an AF-S lens. That rules out the f/1.8 "D" lenses (35, 50 and 85mm), leaving the f/1.4 lenses and the newer f/1.8 "G" lenses.
Outdoors you don't need f/1.4, and those are expensive lenses.
35mm is often a good focal length indoors where space is limited, but outdoors 50mm and 85mm are probably better choices. If you want out of focus backgrounds, a longer focal length helps. The 85mm produces gorgeous images - nice bokeh and a good length for portraits, as long as you can keep the kids in the frame!
I find the 35mm and 50mm are more versatile overall than the 85mm. I take 10x more shots with my 50mm than my 85mm, but on the occasions when the 85mm is used, it's stellar. You won't be disappointed with the 85mm, but you may get more use out of the 50mm. Also depends on what other lenses you have.
I would recommend the new AF-S 50mm f/1.8 G lens for you. It has the in-built motor, and the f/1.8 will allow you to achieve lovely creamy bokeh.
You only mention primes in your question but if you want a good zoom too, the 70-200 f/2.8 VR (and VR II) would be great for following them round a soccer field or other outdoor places. Just make sure you keep it trained on your own kids - lest some over-zealous parent decide to make a scene!
Enjoy :)
