Since this lens doesn't have an iris and is therefore always "wide open", just use Av mode - no need to green button or anything. Just chimp a pic or two to get the exposure compensation you will probably need.
600mm is a long telephoto, the challenge with lenses this long is:
1. nailing focus, note that focusing by eye on a standard focus screen with no aids at f8 (actually more like T10) is next to hopeless. And your model pentax doesn't have live view? Lean on focus confirm and try some judicious focus bracketing. Practice will improve results as you get a sense of what good focus looks like and how results relate to the focus confirm signals. Good luck.....
2. reducing shake and movement. Shutter speeds 1/1000 + for any sort of hand holding - up the asa. Better to use tripods/support etc. I never leave home without my bean bag with a lens like this, and a remote shutter release.
Last edited by marcusBMG; 07-18-2018 at 10:48 AM.