Studio Tests (APS-C, with 1.4x Extender in place)

Snap the lens's built-in Extender into place, and the test results are still very good indeed. Sharpness isn't quite so high, but there's nothing at all to complain about in the other tests.

Sharpness Sharpness drops a little with the extender in place, but is still pretty high when shooting wide open at F5.6. The best results are obtained at F8; with only a little softening at F11. As expected diffraction takes a huge toll at F45, but this isn't exactly an everyday setting on a supertelephoto lens.
Chromatic Aberration Lateral chromatic aberration is slightly increased, but in practical terms still very low. There's a little red/cyan fringing towards the edge of the frame at the long end, but it's unlikely ever to be a problem.
Vignetting Vignetting is again negligible.
Distortion Distortion increases measurably with the Extender in place, but in practical terms this means going from nonexistent to insignificant.