Here are some shots I took at my granddaughter's last competition. The field was soaked due to a very heavy overnight rain and a lot of the horses were baulking at the jumps since they couldn't get solid footing.
My granddaughter's horse had just changed medicine and hadn't been acting right, so she was riding some of the stable's horses. The brown horse that she often rides wouldn't jump because of the wet field, but she rode it for the non-jumping events and rode the black horse for the jumping events.
All of these were taken with a 30D and a 135mm f/2L lens.
One of her friends - 1/1600s f/2.0 at 135.0mm iso100
1/2000s f/2.2 at 135.0mm iso100
1/60s f/8.0 at 135.0mm iso100
One of the horses refusing to jump. 1/2500s f/4.0 at 135.0mm iso400
Hope you get to see the race and I wouldn't bet against "Big Brown".
Bob R
