Sharpness/iso sanity check

[eviljunglenoodle]

I was shooting equestrian jumping last week, this is for a clothing catalog so clothing detail is super important- and all images shot in full sun or open shade when there was plenty of light were tack sharp, you could see threading and fabric textures zoomed in. These were shot around f/7 at minimum of 1/1000 and iso 100-400. In the late afternoon when we were losing light I bumped iso to 2000 to 4000 to maintain the 1/1000 and shot around f/5 but was losing the detail, they weren't tack sharp. If I shot a horse and rider just standing it was fine, but when jumping most were in focus but lacked detail.
Am I just up against low light and hi iso, and combining that with motion I'm losing detail? I was using 5DMKII with 70-200 2.8 IS at around 150mm.
I will post some examples tonight, don't have access to the pics atm- Thanks!

[DDCSD]

Yep, you loose a lot of sharpness at high ISO's. I counter-act the lack of sharpness in high ISO shots by not staring at my photos at 100%. ;)

I shoot at ISO 800 or higher about 80% of the time, so I'm always amazed at how sharp my ISO 100-400 shots are.