The reason most lenses are not at their sharpest after ƒ5.6 on APS-c is diffraction, based on how big the aperture is. The point at which diffraction sets in for both APS-c and full frame is between ƒ5.6 and ƒ8, but in most cases the difference between ƒ5.6 and ƒ8 is minimal. Ansel Adams belonged to the ƒ64 club. SO much smaller apertures are effective on later formats.
What I'm saying is that even with a TC on, ƒ8 should be sharper than anything smaller. Being wide open or not has not much to do with it. As you use a smaller and smaller aperture , the lens would become sharper and sharper were it no for diffraction. ƒ8 should still be the sharpest setting on a 150-450 @ 450mm. But it's all downhill after ƒ5.6, no matter what you do on APS-c. ƒ11 with a TC will suffer from diffraction just as much as ƒ11 without it. If the lens only goes to ƒ8, unless it;s a really crappy lens ƒ8 will be the sharpest setting, ultimately, although because of the narrow DoF at ƒ8 it may not appear to be if a lot of the subject is not on the focal plane.
If you understand how ƒ-stop is calculated you'll understand that the same aperture opening will lead to a different ƒ-stop when the focal length is increase. The same setting that gives you ƒ5.6 on the lens without a TC now gives you f-8 with the TC on, and it's still the sharpest part of the lens, and it behaves no differently than if it were a select on an ƒ2.8 or ƒ4 lens.
That's the theoretical truth. I'd be interested in any empirical test results.
Last edited by normhead; 06-02-2016 at 09:12 AM.