Well, there's actually four spots for me.
1. You could go to the location that's right in front of the parking lot of the west hall as heekun already suggested. The lighting is really nice at night for people because it's dim setting off a grim or gloomy atmosphere.
2. If you walk about a couple steps over to the right of that area there's another place that's great for taking photos during the day. There's three or four metal abstract statues that align the entrance along the way towards the west halls parking lot. It's got green tealeaves and shrubbery so it's awesome for anyone looking for a natural background.
3. If you take the stair way to the side entrance of the west hall above the parking lot; usually it's the entrance to the masquerade. My friends and I like going there and standing on the landing that overlooks across the way towards the Nokia Theatre. Careful not to fall! Just because we have gotten away with it during the day when we can see doesn't mean I would even suggest climbing it at night.
4. Walk further down S Figueroa street until you pass Olympic Blvd. There is this really nice black granite waterfall with some beautiful greenery along the sides of it. I've seen many cosplayers take pictures there but I have never done it myself.
Hope this helped, I'll be attending AX 2015 so if you'd like to know of any other spots I happen to find out about or if you want to meet up so I can show you the spots personally I don't mind. My contact information is located in my all my cosplay photos. Best of luck to you, be safe!
♱⎨ғᴇᴛɪsʜғᴏʀғʟᴇsʜ: ᴛʜᴇ ᴅᴇᴠɪʟ ɪs ǫᴜɪᴛᴇ ʀᴇᴀʟ
ᴀɴᴅ ɪᴛs ɴᴏᴛ ᴊᴜsᴛ sᴏᴍᴇ ᴛʜɪɴɢ ᴡɪᴛʜ ʜᴏʀɴs ᴀɴᴅ ᴀ ᴛᴀɪʟ.
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