#11 is the only photo that has a background that does not totally distract from the cosplayers...it's a fairly simple background, which makes HER the clear and obvious main focus of the shot. Some of the other background areas, like for example the really bright, sunny background in shots # 1,6,and 8, are just very distracting. In the half-body shot, photo #3, the background is a bit less in-focus because you used a longer lens focal length. Overall, the probklem I see is that the majority of these were shot with a small-sensor camera, and a short focal length lens, outdoors, with a small aperture...soooo...there is a ton of unwanted depth of field. That dreaded wide-angle look...just NOT good.
The whole idea is to make photographs that feature the cosplayer...and to minimize distracting elements like trees growing out of heads, and BRIGHT background color blocks, stairways, walking paths, and so on. The problem is how you set up. Needed fill flash on-camera? So that meant you needed a wide-angle lens from close-up. Better solution would be to have the flash set up on a light stand, and then move wayyyyyy back, and shoot with wireless flash, using a narrow-angle telephoto lens from 40 feet away.
You've just simply got to look through the viewfinder and get a clean background when you do shots like this. #8 is one of the best costumes of all, but paired with one of the worst backgrounds. At the very least, shoot with a longer focal length lens to minimize the angle of view behind the subject.
I feel some sympathy for you, trying to do event-type, on-the-go work without a mentor to help you along. One thought...look at shot #10, and consider that maybe you could have moved her back and had her pose with the tree, with the tree as a deliberate part of the composition, rather than as just a background element.
And again, try to get away from that 28mm focal length...it just shows too darned much background. You can get the same-sized person with a little longer lens, from a bit farther away. Choosing what background to include on these types of shots is very important. The angle of view **behind** the subject is what the different focal lengths help with!!! Longer lens = narrower view **behind** the subject. When the background is cluttery, the longer the lens needs to be to help eliminate as much as possible.
Move your feet. By that I mean move yourself and change your vantage point - I probably would have 'moved' those tree trunks by stepping one way or the other til I got the person so a tree was not directly behind them. You can also think about where you have the people stand, then if you need to, adjust your vantage point a step or two. Go out there and look thru the viewfinder and see what the background will look like before you have people there.
I agree, using a wide angle may not be the best option, I think it might accentuate how tall you are because I feel like as a viewer I'm towering over these people. I've done lots of sports and events and have usually used a 50mm or a short telephoto, depends on the event and how big the space is.
I like the way you got the people in many of the photos, it's the backgrounds on these that you need to think about.
I'm not a huge fan of shallow dof. The event was pretty crowded, so I tried figure out the good composition for the background. Since I received good advises from you guys, I know what to do next time.