This is kind of an extreem post. X2.
ISO 800 200mm f/5.6 K3, 2X teleconverter = 600mm equivalent f/11 Hand held.
As I was wrapping up I noticed this little fella (above) swimming past a fisherman. I just did this hand held and was amazed I got not just one acceptble image, but several.
I wanted to expitiment with my old 2X teleconverter and the 200mm f/2.5 lens. I'm not a fan of teleconverters but years ago I purchased one but had mixed results with an inferior lens. So I decided to test the teleconverter with both a full frame camera and a 1.5 crop sensor body
Camera: Pentax K1 FF
Camera: Pentax K3 1.5 Crop
2X Teleconverter like this one (Mine is not the same)
The image below is for perspective, shot on the K3 with my 24-70 at 39mm.
ISO 100 39mm f/6.3 1/320th
Compare these two images of fishermen across the pond from me.
K1, ISO 100 200mm f/5.6 1/125th 2X Teleconverter equivalent FL 400mm and f/11 Tripod mounted.
K3, ISO 400 200mm 1/125th 2X Teleconverter equivalent FL 600mm and f/11 Resting upon tripod without tripod mount.
I need to get another mounting plate for my tripod head! It is really difficult to nail focus at 600mm just resting on top of a tripod. But as a snooping tool...the 200mm plus 2X plus crop sensor does get you some reach.
K1, ISO 400 200 mm f/2.5 1/400th 2X = 300mm FL f/4.5
K3, ISO 800 200mm f/2.5 1/500th = 600mm FLE, f/4.5
The tree in the above two images is the same tree in the 2nd image shot with the 24-70.
It the 2X usefull? Yes. Can you make great images using it? Maybe but doubtful. If you really need the reach and you use a tripod you can get usable images, but there is image degredation. I read, after shooting these, that it is recommended to use fast lenses fairly wide open because with the 2 stop loss of light it gets hard to focus manually or auto focus if you stop down to much.
It would be fun to compare the K1 with the 200 mm and the K3 with the 200mm and the 2x. 200mm vs 600 EFL