Originally posted by nmcleod1993 How would you suggest I test the sharpness?
Shoot a fine detailed target in good light. Focus carefully, preferably in live view. Use a tripod and shoot with 2s delayed self timer. Shoot first at max aperture and then close one stop for each additional shot until f11. Examine the results at 100%. When I have some time today I can upload some 100% crops samples from my lens so you can see what kind of sharpness you could expect from the lens, at least from my copy.
Originally posted by nmcleod1993 So when you say OK, are you saying that my lens is not as sharp as this model should be?
On the contrary, what I meant was that your results with TC are on par with how this lens should perform with a TC.
This are quotes from Photozone site when they tested the lens in Nikon mount. Unfortunately they didn't do it in Pentax mount but the results should be the same.
" The Sigma AF 100-300mm f/4 EX DG HSM APO is one of the most impressive Sigma lenses tested to date. It is able to deliver a near-flawless performance with great resolution figures"
"The Sigma AF 100-300mm f/4 EX DG HSM APO delivered excellent resolution figures in the MTF lab matching the best zoom lenses in its range as well as many fix-focal length lenses"
"Combined with the Sigma AF 1.4x EX DG converter there's a drop in performance in the center but more so towards at the borders. However, the results remain generally on a very good level although it may be advisable to stop down a little here for an extra kick in terms of contrast."
Here the link for the complete test