>>3414229>But why can't you just put 85mm f1.2 on speedbooster? Same field of view and same speed.Because that would neither be the same field of view nor the same speed.
It wouldn't be the same field of view because this is a 58mm full frame lens designed to be used on full frame. So you get a 58mm-equivalent field of view, because it's actually 58mm-equivalent. An 85mm on a speed booster would give you, at best, an 85mm-equivalent field of view. I'm not sure if you were comparing using this 58mm on a crop sensor (which isn't possible because Nikon does not yet make a crop sensor Z-mount body) or you were simply having a little bit of dyslexia (thinking it was 85mm f/0.95 instead of 58mm f/0.95), but either way, no.
And it wouldn't be the same effective speed because, again, at best a speedbooster just makes it so you don't lose anything by being on crop. I.e., the crop sensor will have about a stop worse noise performance, and about a stop deeper depth of field, for any given aperture. A speedbooster bumps you up about a stop, so it evens those out, but it doesn't get *better* than the lens would be on full frame. So if you had a speed booster to take an f/1.2 lens to f/0.95, you'd still get the depth of field of f/1.2 on the larger format, and the extra light from the f/0.95 hitting the sensor would be balanced out by the worse ISO performance on the sensor and it'd be a wash.
So, what you'd need to equal this lens' light gathering ability and field of view would be a MEDIUM FORMAT 85mm f/1.2 lens, which doesn't exist (and which would be way more than $6000 if it did) on a MEDIUM FORMAT speed booster to a full frame camera.
