Originally posted by SeBEx When you say Higher ISO what are you suggesting?, with my Nikon gear I'm normally shooting at F5 -5.6 with everything apart from the 200mm F2 where I am usually at F2.2-3.2, and I never go over 3200 ISO - I know my cameras can go over this but its just personally I never go over that.
My experience with Pentax 645 lenses has been in order to obtain optimum sharpness and DOF, one must stop down to F8-F13 (depending on the lens). I've also found that even with wide angles, I get a larger number of keepers when I'm shooting handheld, if I keep the shutter speed upwards of 250th/sec or more. While I've been able to handhold and get acceptably sharp images as low as 1/15th, that usually requires multiple takes of stationary scenes. Throw moving horses and longer lenses into the equation, plus manual focus (unless you're planning or already own the AF versions), I think you will be frustrated with the results. Shooting, as an example, with the 80-160 lens (assuming AF version) you'll be at F11 to get adequate DOF, ISO 800-1600 to keep shutter speeds upwards of 1/1000th. If you're shooting in an arena with artificial lighting, the situation may become more severe.
In addition, at just over 2fps, you may find yourself wishing for a higher frame rate camera. Any of the higher megapixel FF bodies will be in the 4+-6fps range with faster focus. The Nikon 810 is a great camera but in my opinion, based on specs and Pentax reputation for ergonomics and build quality, the upcoming K-1 would be my choice. Dynamic range, color rendition, especially in the pixel shift mode shooting landscapes, may be as good as it gets with FF digital at this point in time. I will certainly be considering one as a second body to my Z.