Next test is testing the tc. Is is better in the center or is it that it only helps lenses above a critical sharpness?
I used my sharpest lens, the Sigma 70 macro. Being a macro it is also sharp in the corners, more than the 55-300 plm in the center.
I found 20 feet or about 6 meters gave me the "just barley" zone.
You can see the tc+70 on the far left is the sharpest, this was focused in the center. The tc +70 on the far right is next sharpest. the 70 alone focused on the center (left center) and the 70 focused on the corner (right center) are about the same. Enlarged versions between.
This confirms my suspicion that a critical sharpness is needed for the tc to help with detail. It also shows that the tc is better in the corners than I suspected. This doesn't really help since most lenses are much weaker on the edges. It could be a tool to make blur even more pronounced in unfocused areas.
last the sigma 70 is sharp at the corners but it does have fall off. The fact that this test doesn't show much difference in the center to corner is probably because the area of the test is so small there simply are not enough pixels on it. This lens is so sharp that at 20 feet it resolved what the 55-300 plm was resolving at 40 feet. If you assume the focus breathing at 300mm bringing the plm down to 210mm, that is still 3x the 70 and the 70 could resolve this at only 2x.
This research is bringing me to another goldilocks zone. Where is it better to have one great lens and crop vs multiple cheap lenses. Will my sigma 18-35 at 20 and cropped beat my kit 18-50 at 50? Will my sigma 70 cropped, beat my takumar 135? This might be coming next.