For example the Nikon Z 50mm f1.2 is 1090 grams, 150mm long, and has a 82mm filter size. The Canon RF 50mm f1.2 is 108mm long, but the other dimensions are similar.
Compare that to a Leica Noctilux 50mm f1.2 with a Techart, Megadap or similar adapter (available for Z and E mounts) for autofocus abilities: 405g lens +150g adapter = 655 grams, 52mm lens + ~11mm adapter = 63mm long and 49mm filter size. A little more than half the numbers in all dimensions.
This link approximately shows the size differece (the M to L mount is indeed smaller than the M to Z or M to E autofocus adapters, but the difference is small)
All of these have the same focal length (50mm), max aperture (1.2), and autofocus. So why do these newer mirrorless lens designs have to be so much bigger and heavier than using an old manual lens with an autofocus adapter? Sure the autofocus speed may not be as fast with an adapter but why can’t they design a native autofocus large aperture lens that is tiny like the Leica M lenses. Clearly it is possible to do so.
Carlinwasright has your answer!
Another example: The Nikon F 50mm /1.4 AF-D has 7 lenses in 6 groups. Thats 10 lenses less than the Nikon Z. BUT: Even in old times the step from 1.4 to 1.2 was relativily huge, the 1.4 weighs around 260g, the 1.2 around 380g…thats nearly a 50% increase
The modern prime lenses for digital have - in the lab! - much better optical qualities than the old primes like Leica or Zeiss that are around for literly decades. If anybody can see this differences in real life is a complete different discussion.
Yeah, even the Nikon 50mm 1.4D vs the 50mm 1.4G the difference in image quality is night and day. The D is also tiny compared to the G. Unfortunately, it’s a somewhat immutable fact of physics that good quality optics are big and heavy.
My Nikkor 50 f1.2 is many things. Sharp isn’t on the list at f1.2.