There is no one lens for outdoor portraits, but if there is one lens that is really hot right now for outdoor portraits, hands down it has to be the Canon 85mm f/1.8 or the Nikon 85mm f/1.4. These lenses have a nice focal length for portraits, but the real reason everybody loves them is for the insanely shallow depth of field they provide (if you get your subject so they pretty much fill the frame, the background goes so soft and out of focus that you’ll never want to shoot anything else). You hardly see a professional wedding or senior portrait photographer not shooting an 85mm to death right now, and the reason is it looks great and people (clients) love its almost cinematic look. The only downside is, of course, that since it’s the lens that everybody wants to shoot, and since it does all this magical stuff to the background, it isn’t cheap. It’s a business investment (the Nikon 85mm f/1.4 sells for around $1,600, but the Canon 85mm f/1.8 sells for only around $400, so that’s actually a pretty good deal). Also, don’t forget about 85mm lenses from Sigma (for both Canon and Nikon), which many photographers swear by, and they’re usually much less expensive than Nikon or Canon lenses.