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I have a question regarding the SB on the Canon 50 mm 1.0 L that I have briefly tried.
At wide apertures, there is vignetting as expected. Stopped down to small apertures, the vignetting does not disappear, but becomes a hard limit on the image circle, at about 11-12 mm image height on the APS-C sensor. It is not an issue for me as I do not intend to use that lens with the SB. I am just curious to know what is causing this to happen at small apertures like from f/2.8 all the way to f/11. Thank you for...Show more →
The 50/1.0 Canon is a bit of an unusual lens in that it has an extremely long exit pupil distance. According to a patent example I've looked at (Japanese Patent 63-174,008 Example #1 of 2) the exit pupil is located more than 250mm from the image plane. In other words, its a nearly telecentric lens, and this is why there are issues with it on the Speed Booster at small apertures.
The Speed Booster works best with lenses having an exit pupil in the 50-80mm range, which is perfect for compact wide-angle, normal, and some short telephoto SLR lenses. Once the exit pupil distance exceeds ~110mm you'll start seeing "hard" vignetting in the extreme corners at small apertures. Paradoxically, this vignetting is reduced at large apertures because light from the lower portion of the oblique ray bundles strikes the Speed Booster optics at a better angle.
Note that exit pupil distance and focal length are entirely separate and unrelated things.