Originally Posted by Nerfherder
Conventions are often quite dark in-doors and objects you want to take photos of are often moving or far off (on-stage, posing several meters off, dancing, doing a skit). Groups you want to take photos of are often in a hurry to go somewhere else. Ocasionally you get a hold of a really good cosplayer and ask them for a longer shoot at a better looking/lit area.
Nikon AF-S 50/1,8G
Nikon AF-S 85/1,8G
Nikon AF-S 70-200/2,8G IF-ED VR II
It really depends on what you expect your style to be like. If you plan on photographing things like the masquerade, then a ranged lens like that 70-200mm f2.8 is a good idea.
If you plan on asking people in the hallways for photos, 50mm on a full-frame camera would be alright most of the time. I actually used a 50mm on an APS-C crop camera at my last convention
and that worked out too, but it is very tight and you need a lot of working room to get decent photos. Anyways, I'd probably consider a wider lens for photos like that.
There are lenses like the 24mm/28mm f2.8 that are lower cost, but there is also the 28mm f1.8. The classic option would be a FF capable 24-70mm f2.8 zoom if you prefer zooms and can afford it.