there are sample variations and some tc's seem to work better with certain lenses.
here is a site that did testing on all the converters available and it seems the kenko mc4 is the best on average across the frame, that makes it a real bargain and i might pick one up and try it.
Am I reading the data in Traumflieger's site correctly? It seems to say the (presumably older) Kenko mc4 is not only better than the current Kenko version, but also that the mc4 is better (in the way it is being measured) than the Canon 1.4 Mark III? Please let me know if I am misreading the data or how this could be? Thanks!
yes, i found it interesting to see the results. what they are showing is the resolution of a lens and camera combo, a 5dii and 70-200 f4 at 200. the numbers below each tc are the drop in resolution center, mid, and edge, they also average the 3 and list them along with the resolution in lpm of all the tcs across the frame at the bottom.
others have pointed out that there are other attributes besides sharpness but according to the data on the site: the kenko mc4 is the sharpest tool in the shed
i also found it interesting that the series ii tc's are sharper in the center but not near as good in the mid frame and corners as the series iii which makes sense i guess if your lens is already a bit soft in the edges then losing another 500-600 lpm res is going to show pretty bad. it seems they choose to give up some center sharpness in order to get better mid frame and corner performance with the series iii