Theoretically you wont see an improvement with the extender unless your subject is blue, your camera is <= 33MP and the TC has negligible image degradation.
The diffraction limit drawn assumes the best case scenario (perfect lens, perfect TC). In reality you may not see an improvement even if the TC point is below the line.
Steve Spencer wrote:
The diffraction limit is not an f/number that you should never shoot at or God forbid go shoot higher than.
This is obviously NOT the point I was making.
My point is that if the Airy disk diameter already spans two pixels with the lens wide open, using a teleconverter is worse than useless. Once you’re sampling at or beyond the Nyquist rate, increasing the sampling rate further with a TC yields no additional captured detail. Instead, the teleconverter adds more glass to the optical path, introducing further image degradation while also reducing autofocus performance.
Use whatever aperture you want, just don’t use the TC.
