Hi Bojan,
Yes I had always use it fully open at f/4 and full frame. I'm sure it could be a bit sharper at t/5.6 but I don't see any need for it now, better to collect more photons. My biggest problem now, I need more pixels in the camera.
I had the Canon 300mm f/4 earlier, a very good lens. But the vignetting at f/4 and full frame was a bit high for me. The built in focus motor is nice. I couldn't solve how to let the software to do auto focusing in that time, maybe now.
Built the motor focuser to the Pentax 645 300 mm lens was a lot of work, but it is done now and I'm very happy with it.
The second photo here of Andromeda is taken with the Canon 300mm at f/4:
http://www.astrofriend.eu/astronomy/...31-galaxy.html
/Lars