As per title
Just curious to know if anyone has tried using a mirror lens with a speedbooster?
Guessing the results would be dire.. but a 500/56 with peaking … I’m a little curious.
"Mirrorless" or "mirror" (i.e. reflex / catadioptric) lens? I've tried a bunch of "speedbooster" focal reducers. The only one that wasn't optically terrible was metabones. Reflex lenses usually have really weak contrast. Speedboosters just amplify problems. They don't improve them. Temper your expectations when combining both together.
Consider using an older SLR 300mm f/4 or 400mm f/5.6 refractive lens with a TC and a dumb (hollow) mount adapter. Even the worst refractor is going to be better than the all but the absolute best reflex lenses. Reflex lenses that are actually sharp like the Zeiss Mirotar will cost much than old manual focus SLR refractors like a Nikon Ai-S 400 f/5.6.
