Originally posted by K2 to K50 Pretty good, especially considering your were using a 2x TC.
You need to consider the lens system here, in. Total.
The sigma 2x Tc is not like an ordinary TC, it is only designed to be used on a very limited number of lenses made by sigma. Specifically long and fast tele and tele-zoom lenses such as
70-200/2.8
300/2.8
500/4.5
Etc.
Sigma uses a design similar to the Pentax L rear converter where the front element of the TC protrudes forward into the lens it is connected to.
As a result, unless the lens barrel has a minimum of 35 mm in internal diameter and the rear elements are recessed about 15 mm or more from the mount, it simply won’t fit .
There have been published lists of the lenses the TC will work with, and since it shares some design features in common with Pentax it obviously will also work on the similar range of Pentax F and FA fast tele lenses.
The sigma lens I am using is a screw drive lens, and was produced before the advent of the sigma DG (full frame digital) series lenses, although except for the coatings is optically the same as the very first APO 70-200/2.8 EX DG lens. The screw drive Macro version was reportedly softer at the long end, as were several of the HSM versions. It has served me well for over 20 years.