Here is a lens diagram comparison between the EF 50mm f/1.8 STM and the upcoming Canon RF 50mm f/1.8. The new RF 50mm f/1.8 STM definitely looks like a brand new lens design and not simply making the EF mount version work with RF, which likely would never be worth the time and effort anyway.
As pointed out by forum contributor privatebydesign, this is a “double-gauss” lens design.
I expect to see this lens announced next week, if not sooner.
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It does seem like it at least takes advantage of the new mount by enlarging the rear-most element, so hopefully that in itself can improve IQ a bit. I'll be interested to see how it turns out.
It's tough to tell, the diagrams aren't to scale, but looking at the rear mount bit, it does seem like the RF lens is slightly smaller, which is a big perk when you consider it's going on the R cameras without an adapter and will make a very small end result.
Double-Gauss lens - Wikipedia
Uncle Roger has a great series of articles on optical designs too.
Lens Genealogy Part 1
What I actually find interesting is the element offset, the RF lens elements are set further forwards than the EF version and are an effective 'adapter', they are not using the advantages of the shorter flange distance here one bit, merely rejigging the thing to do the same. I'd expect performance to be somewhere between the EF 1.8 and EF 1.4, the key will be the way they have used the special element in green.
The Nikon Z50 f/1.8 lens is some 600 euro... I agree, they market that one different, but still, it gives Canon an opportunity to ask 349,- for the RF50 f/1.8. Why not? I'll buy it.
And how does Nikon have to make theirs twice as big? I don't get it.
I would not rule out an f/1.8 bundle as a kit.
The RF 50 1.2 is the high quality 50mm rf prime. This will be the cheap & small one.
I am sure it is for a more cost effective production, maybe simpler mechanical parts and more automated assembly.
I hope the improvement is also for better IQ.
And I hope there will be a 50 mm lens with IS and better optics between this and the RF50L.
The basic double Gauss design stayed the same.
But I see different gaps between the elements.
And I see elements cut on both sides where one was only cut on one side (two working steps = more expensive).
And as the rear element seems to be bigger the exit pupil seems to be bigger, too.
= less refracting of the light rays in that last element = maybe less CA (but just guessing)
This is an at least much modifyed optical formula, if not totally redesigned (still based on the same double Gauss design).
Edit: and you don't see just from this drawing if they maybe added some new and better coatings.
RF 50mm f/1.2L USM is priced $2,300, about the same as the EF 70-200mm f/2.8L IS III USM @ $2,100
The RF 50mm f/1.8 STM will be priced say $300, 2.5x the price of the EF 50mm f/1.8 STM
Do you really think there's no place for an RF 50mm f/1.4 IS USM at $600 in between?
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Added in editing: Nikon, Sony, Sigma, and Zeiss all have a 50mm f/1.4 priced $1,000-$1,500. IIRC, someone wrote all those lenses are expensive pickle jars because the only way to make them fast & high IQ was a reverse telephoto design.
The RF mount cut the flange distance from 44mm to 20mm. Shouldn't that open the door for Canon to make a 50mm f/1.4 that combines the cheap double Gauss design with the high IQ of the above mentioned lenses? Is the RF 50mm f/1.8 it? Really no place for sub-$1K 50mm f/1.4 IS?