How can I use aouto-crop effectively?

[mbetrafa]

Hello,
I have pictures whose backgrounds are almost black, but the background color changes a bit by position since it is not a computer painting. I tried to auto-crop this picture, but I couldn't crop the right side of the background although I have increased the tolerance. When I increase tolerance more,it cuts the picture's itself too. If I continue to increase again, it does not crop,it does not work. I dont understand. To auto-crop this picture effectively, what should I do?