Cosinon 55mm f2.1

MTHall · Joined: 07 Nov 2012 Posts: 120 Location: England UK

I have been given an old Cosinon lens, it had a loose focus grip that I have since fixed, it looks as though it should be okay as normal lens, but my curiosity has been aroused by its odd aperture at f2.1 Why not f2, or f2.8, or even f1.8. All standard sizes CZ did a 2.4/35, did Cosina try to go one better with a 2.1? Has anyone had any experience, or even a story to tell, with this lens? There is a picture of it on another thread where I inquired about fixing the rubber grips.

Mos6502 · Joined: 20 Jun 2011 Posts: 961 Location: Austin

Fuji made a 2.2/55, Ludwig made a 2.9/50, then you have all sorts of 1.8, 1.7, 1.2, etc. lenses which are all pretty weird stops even if they're more common.

MTHall · Joined: 07 Nov 2012 Posts: 120 Location: England UK

I suppose the main consideration is that the lens is up to the expected standard and that this is the maximum aperture at which the particular lens achieves within that standard. In the struggle to achieve the fastest possible lens, I suppose we have to expect odd F figures. I suppose my Cosinon design couldn't quite cut it at F2, like that Fuji and poor Ludwig expected a 2.8 but it fell short eh? Even CZ wasn't immune with their 35mm F2.4, brilliant lens as it is (one of my favorites.). Does this imply tho', that good designs with a nice rounded figure like F4, F2 and F1, were a perfect design?

In the mid sixties, there was a fad for fast lenses, While I was in the far east (RAF), you wouldn't be considered a `serious' photographic `bod', with anything less than a 1.8/50 prime. It was completely ignored that there were some very lovely 2.8's and 3.5's to be used (zooms hadn't yet `arrived' in those days). Wearing a Pentax with a Takky 1.8 WAS a main fashion accessory, just like your Rolex watch! With all that bright sunshine we HAD to have a !.8? It would seem that we were the `poseurs' just like today's young 'uns with their iphones? Life goes on eh?

Mos6502 · Joined: 20 Jun 2011 Posts: 961 Location: Austin

I'm sure there are a lot of different reasons. One might design a good lens that just happens to have an odd maximum aperture - like f6.3 for instance. Or one might need for instance to get a lens faster than f2, just for marketing purposes. It might not be that a lens just doesn't "cut it" at a full stop, but that the designers realized it's not possible to make a certain design any faster. So you'd never know unless you'd talked to those who'd designed them.