Originally posted by Digitalis Have you ever seen what happens when you stop the pentax 50mm f/1.2 down to f/2? it is very sharp, noticeably sharper than a f/1.4 lens would be at f/2. And that is one of the many reasons why faster lenses like the leica noctilux 50mm f/0.95 exist so you can afford to stop the lens down to increase performance and DOF. but still retain high shutter speeds in dim lighting.
Yes, I have stopped a 1.2 down and a 1.4 and 1.7 to f/2 and frankly, no, I see little difference in sharpness. Hard to compare with Pentax as most lenses pre-date MTF, but Canon has par examples discussed extensively on their forums:
Canon EF 50mm f/1.4 USM - Review / Test Report - Analysis Canon EF 50mm f/1.2 USM L - Review / Test Report - Analysis
As one can see, the 1.4 is measured at 1.8, not 2.0, but if you follow the curve, by f/2.0 they are essentially equal. In fact, at 1.7 to 1.8 they are essentially equal. At 2.8 the 1.2 is actually inferior on the borders by a very small amount. Both are soft at 1.4.
The 1.2 has 5x the CA. Equal on the vignetting. The verdict says it better than I could.
The Pentax 50/1.4 seems to be a little behind the Canon, but then again, it's a much older lens. Maybe the 25 year-old A 1.2 is that much better, but we'd need an MTF comparison which is not available. I cannot see how it is much superior to the Canon 1.2; in fact, one could reasonably assume it is not, and I doubt Pentax could make a better 1.2 than Canon. The DA*55 fares a little better than the venerable FA 50/1.4. I do not see how the A 1.2 is "noticeably sharper" than a Pentax 50 1.4 by f/2.0.
Back to the Canon where there is objective data. Is the extra stop worth $1,150? Some of that price difference is other quality aspects to the 1.2 lens, but essentially that's gravy. It's a cool grand to get a stop softer meat.
Also, I would take issue with the "increase performance" aspect. You lose contrast wide open. That's lost colour and shadows data you never get back. Fast glass is a tradeoff. I agree you get headroom by starting lower, but the tradeoffs are still there until about f/2.0 to 2.8, even with the Leica. Look at the MTF's in this review:
diglloyd: Leica 50mm f/0.95 Noctilux-M ASPH
I like the line "hardly anything will be in focus". Again, a tradeoff. Same for the 6-stop ND filters! Read the other compromises inherent to the optics. For every increase in the aperture size, something has to give. The "too bright" comment is telling. You'd have to stop down if there's a single candle in the frame in a dark jazz club, by which point you're probably in the price range and quality of a lens worth $7,000 less, like the 1.4 Summilux where they are identical in performance. (In fact, it is highly recommended to have the Noctilux fixed with a factory 3-stop ND filter!). You don't just get a faster glass and everything else stays constant. Your wallet is certainly lighter. Don't get me wrong, fast glass is fun. But at a certain point it's like driving a car capable of going 600 MPH. Are you really going to push it that far often? No, and it makes a lousy vehicle for just going to the store for a jug of milk. The Noctilux largely about bragging rights. It's not about making someone a better photographer in a dimly lit, smoky blues bar. Sad for the guy whose Noctilux was mistaken for a beer bottle to be thrown at the chicken wire.