Originally posted by Ian Stuart Forsyth With the FF using a TC and you are not shutter speed limited you can use a longer shutter speed over the cropped camera body
Ah, but there's the rub. We frequently
are shutter speed limited. Regardless of the total number of photons shining up the wazoo, the intensity of that light is the same. So you if you shoot a FF camera with a teleconverter you have to sacrifice a stop of shutter speed or ISO to keep the exposure the same compared to a crop camera with the same lens and no teleconverter.
Yes, I grant you that a full frame sensor might give you more ISO latitude, but with the Pentax K-3 III vs the K-1 II I'm not sure there's not much in it.
Anyway, it's all a bit academic. There is no modern FF teleconverter for the K-1, and none on the horizon, so I would suggest the HD Pentax-DA 1.4x AW AF Rear Converter on the K-3 III is probably the best way to maximise reach in the Pentax ecosystem.