Last weekend I went to a horse show where I was shooting show jumping. It was all shot outside, where it was extremely sunny and hot. I shot at a shutter speed of 2000 with my aperture fully open at 4.5. My film was 400iso. Yet a lot of my shots ended up radically underexposed. How is this possible? As I understand the sunny 16 rule, with an iso of 400, you would set your aperture to 16 and your shutter speed to 400 (my camera only has 500). When you double the shutter speed, you would have to have the aperture (down to 8). Double the shutter speed up to 2000, and you would have to increase the aperture to 4. So, if my understanding is correct (and it may not be) why were my shots so drastically underexposed? They turned out very grainy and dark. I was also using a UV filter and Circular polarizer.