Well the first thing I have to say is it looks like she's having a good time, and that's the key ingredient great portraits (imho). I like the Sepia tone, it seems to work well with her skin tone and hair color.
The one thing that seems to be off a bit is the background. While this is a great background and works quite well for portraits, her hair and the bark of the tree almost blend together in a tonal sense. Of course you can make out her hair fairly well, but I spent the same amount of time looking at the background as I did at her.
Maybe moving her forward a bit more and having less light on the tree directly behind her would give her head/hair a darker figure to be placed in front of?
Just my two cents, I hope it helped!
Quadrajet |