Photoclubalpha

http://egami.blog.so-net.ne.jp/2011-06-04#english

Sony Alpha Rumors reported on this but I wanted to skip the sensationalism and go straight to their source. I post it here because I wanted to share my analysis where there are significantly more capable minds than my own.

Sony applied for a patent about 50mm F1.2 and F1.4 hi-speed standard lens for full frame format. This is not a double gauss of 50mm. Sony put a converter lens together in a double gauss type of middle tele lens and realized a high-performance lens in the short distance.



Now, I may be a rank amateur, but the section labeled "GR2" looks like an 85mm design (consider the Carl Zeiss 85/1.4), a focal length consistent with what Sony calls "medium telephoto" on Sony Style. Hold that thought, and now let's do some math: the specifications describe a focal length of 51.55mm and an aperture of f/1.233, which would mean that the physical size of the aperture is about 41.81mm across. Now how many times does that number go into 85mm? About 2.03 times. So, is what we have here an 85mm f/2 which converts to a 50mm f/1.2 with a front-mounted 0.6x wide-angle attachment?

Nope. Look at DK's notes on the NEX WA converters. Front mounted WA converters don't effect the f value of a lens.

(I'm a rank amateur on this too!)

Dusty

That's an interesting new design. Modern 50 mm standard lenses (for 35-mm format) with moderate lens speeds, i. e. 1:2, 1:1.8, or 1:1.7, typically are so-called Modified Double-Gauss systems with six elements in four or five groups (a "group" being defined as two or more cemented elements, i. e. without air space between them). The section labelled "GR2" indeed looks very much like a conventional Modified Double-Gauss system with six elements in five groups. Modern 50 mm standard lenses with fast lens speeds, i. e. 1:1.4 or 1:1.2, typically add one element so they become seven elements in five or six groups. This was the rule across all lens makers since the 1970s. Back in the late '60s, a few 1:1.4 lenses were made with six elements (among them the famous Minolta Auto-Rokkor-PF 58 mm 1:1.4, later MC Rokkor-PF 58 mm 1:1.4 ... as old-hand Minolta afficionados know, PF stood for "six elements in five groups"). However it quickly became obvious that six elements are too strained at 1:1.4 so all manufacturers switched to seven-element designs by the early '70s. The added element always is another rear element—effectively, the (positive) rear element of a six-element design gets divided into two (positive) rear elements in a seven-element design. So there are always three elements before the diaphragm and four behind.

This was the state of the art until a few years ago when the first eight-element designs emerged. The most notable were the Leica Summilux-R 50 mm 1:1.4 (now discontinued), the Leica Summilux-M 50 mm 1:1.4 Asph, the Nikon AF-S Nikkor 50 mm 1:1.4 G, and the Sigma AF 50 mm 1:1.4 EX DG HSM. All of those used slightly different approaches—but all of them added the 8th element to the rear section, so they all have three elements before the diapragm and five behind.

Now the new Sony design is the world's first 50 mm standard lens design I am aware of that tries to walk a different route. Coming from the classic six-element Modified Double-Gauss design, it adds two additional elements to the front section, not the rear. That's genuinely new.

Now I wonder if they really are after something that's better than anything else, or are they just desperately trying to give way to other manufacturers' patents? Anyway, the design most definitely won't be "convertible"; the section labelled "GR1" won't be detachable. It surely will be an integral lens ... just like telephoto lenses that are physically shorter than their own focal length—these have, in a way, sort of a tele extender built in but you cannot simply take it out in order to reduce the focal length.