Light Falloff and Teleconverters
Prepared 2006-02-06 (146/49906#4) by Bill Claff
I took my Sigma 100-300mm lens
and with step down rings and a set of extension tubes I created very
significant light falloff in the corners.
The business end of the Sigma lens with this contraption attached is shown on the right.
I took two shots at 100mm wide open.
The upper shot is without the 1.4x TC and the lower is with the 1.4x TC.
Clearly the TC cropped the image circle and eliminated the light fall-off.
I initially thought that stepping down from 82mm to 52mm would introduce visible light falloff but it did not.
(That's why I also had to also use the tubes.)
I did expect, and notice, an overall falloff of about 1 stop.
So although every ray entering the lens contributes to the final image regardless of sensor size, the contribution is uneven and the outer rays contribute less to the smaller DX image circle.
So yes, using the DX sensor also can improve the edge preforance of any full frame lens.
My conclusion; whether you crop by reducing the sensor/film size or by using a TC you eliminate light rays from the edge of the lens thereby reducing and light falloff and edge aberrations.
Of course, the TC may introduce it's own new aberrations.
However, using similar reasoning; these new aberrations are likely to be closer to the edge of a full image circle rather than a DX image circle.