Kind of what it says on the tin. There's plenty of questions on Photography.SE about the fastest lens out there, but let's go the other way - what's the highest (non-infinite) f-number on a production lens? One-offs and weird stuff like this are fair game.
Specifically, I'm curious about two things:
I'm stretching the definition of "lens" a bit, but certainly the slowest imaging device you can buy is a 1-micron precision pinhole. See for example the products from Thorlabs.
Any smaller than that (and practically speaking, any smaller than about 5 microns) and diffraction effects will ruin your image. :-) .
That depends if by production you mean in production now, or has been in production in the past and I will generally reference lenses for typical photography applications.
The Hasselblad 500mm Appo-Tesar made for the V system could be stopped to f/45 (and is the smallest aperture in my collection and the smallest I have come across). The 350mm Tele-Tessar, 250mm Sonnor, 120mm and 135mm S-Planar could also be stopped to F/45. However all of these lenses are currently out of production. With that in mind a lot of their more modern H-System lenses have similar specs.
The Cooke PS945 can stop down to a whopping F/90 and appears to still be in production.
Keep in mind that since F-Stop is calculated in comparison to the front (entrance) element as well as the focal length. Thus a quite sizable front element is needed for say an F/1.2 50mm lens while an F/45 500mm lens can be achieved with a relatively easy to machine hole size and modest diameter front element.
Also for the record, that Zeiss F/0.33 lens never actually worked.
On a similar note, generally speaking, photographers have always wanted faster lenses to allow the use of higher shutter speeds (and possibly slower film/ISO speeds) in low light. Practically speaking, ambient sunlight never really gets bright enough to warrant some extremely small F/Stop for a somewhat "standard" exposure. There may be some desire for motion blur (slow shutter speed, small aperture setting) but any kind of extra reduction can be compensated for with an ND filter. Technically we may even be better at making tiny apertures than huge fast lenses there is just no practical need for them.
