Originally posted by IMCorker2 Since the AV Setting can't be changed from f/4,
I think you mean f/8? But no, it cannot be changed. Realistically you probably have to shoot close to that to get enough depth of focus anyway.
There are a couple of mirror lenses (very expensive ones) that people seem to be OK with but in general the regular 500mm f/8 ones do not produce very good results. You can get better with practice but that is true of any long lens. They are fine for surveillance or just to prove you saw something but beyond that they are a challenge.
To get decent results you need a sturdy tripod, very bright light and a steady hand on the focus ring.
As far as exposure is concerned you will have to meter manually by adjusting the shutter speed and ISO since there are no electrical connections. Not familiar with the K100D but on newer cameras this is accomplished by using Manual mode and the green button to 'stop down' to get a meter reading. You might also have to change a setting in the menu to allow non-"A" setting lenses. Not sure about this on the K100D but you have to set it on newer cameras.
With the 2x doubler you are also losing another 2 stops of light so that hurts even more.