I thought this mildly interesting. I've been playing around with an old Vivitar 2x teleconverter, trying it with various vintage Nikkor primes. I had heard that a teleconverter will exacerbate any aberrations of the lens you put on it. So I was surprised to find that the TC actually improved the image quality of one lens, my Nikkor-S.C 50mm f/1.4, in the center of the frame at f/1.4.
I have attached two photos of the same boring scene: the first, taken with the unaugmented 50mm f/1.4; the second, with the same lens on the 2x TC, with aperture still wide open.
Hopefully you can see how, in the center of the frame where the outhouse is, the TC shot has better contrast and less spherical aberration (glow), and is perhaps sharper, than the lens-only shot.
I attribute this effect to the fact that the teleconverter only uses the light from the central part of the lens, where aberrations are at a minimum. As everyone knows, the reduction in aperture imposed by a 2x TC is two stops. Indeed, the image characteristics at the center of the TC shot are not dissimilar from those delivered by the lens alone when it is set to f/2.8.
