This also raises another point: many Artist Alleys don't allow photography, and it's a good general rule to assume this is the case all of the time. People scalping artwork is a real problem, and even if that's not your goal, it kinda sucks for the tabler if you take a photo of their prints instead of buying one. If you really want to take a photo of something at a table, ask first, and if they say no, respect it.
I've also seen people rolling through Artist Alleys with low cameras angled to videotape entire tables. My best guess is that they're planning on reviewing the tape later in a less crowded space to see what they want to come back and buy, and I can respect it if that's the case. What you may not realize, however, is that the cosplayers in the area may assume you're trying to get inappropriate shots. And if that IS your goal, well, knock that crap off.
Rock on, guys, and never stop drawing.