So I've recently watched a video about how Tony Nortrup is referencing that you should buy the lens that perfectly fits your image circle, to get the most sharpness out of it, or so I'm understand. I've looked into focal reducers which seem to shrink the image circle so that it "fits" perfectly on a smaller sensor, yielding the correct image circle size.
A post here states that:
There are some other bold claims made by the manufacturers of the speed booster. In addition to increasing speed by a stop they also claim the resultant image is sharper, which goes against conventional wisdom.
My question here is, what is the conventional wisdom stated above (my guess is putting anything between the sensor and lens degrades quality?) and in practice does the sharpness of the lens, get closer to something you would get from a lens on a "correct" body, or is the gains marginal and the only real benefit is shallower DoF, like an FF?
I realize again this is highly suggestive depending on the adapter in question and lens but to bring it into context I'm curious if it's worth using a focal reducer on a Nikon AF 300mm f4 on my Sony a6000 is worth it, or is it better spent saving the money to get a FF body to "match" the lens? I realize the AF performance might not be the best so I'll ignore that, for this discussion.