Picked up a Tamron-f 2x TC cheaply so was interested to see how it might work on the 31A. M42 fit, so the inconvenience of switching mounts and fitting an M42 adapter as well.
The moorhen came out ok. And the canada goose.
Crop shows the softening and increased aberrations of the full sized image.
My inclination these days is to avoid the use of a tc, mainly because I don't think the 31A is a particularly good lens to use with them due to its f5.6 max aperture and its proneness to purple fringing and coma. But this one seemed to be doing quite well. However the snipe came out better without tc (pics resized to 800px).
With tc
However when the red breasted mergansers got into a wee fracas with the great crested grebes in the middle of the lagoon it was because I had the tc on that I was able to get these pics.
Attack!
"That showed em!" "Right on bro'"
"Just ignore them" " I quite agree"