It appears the results using a Sigma 2x TC on the Sigma 100-300/4 are not as good as simply cropping pics taken with the lens without a TC... Technically though pics shot at slightly different apertures and ISO's... I also may need to adjust the micro focus with the TC on as well and the camera doesn't recognize that the lens is now 600m and may affect the SSS... "appears" is not a scientific word lol