Here are some lenses. Look at them:
From back to front, a Canon 24mm f/2.8, a Canon 50mm f/1.8 MkII, and right at the front a tiny tiny Olympus 21mm f/3.5. Like the rest of the OM range, the 21mm is very small and well-made. It is shorter and less girthsome than Canon's 50mm f/1.8, and as far as I can tell it is smaller than any other full-frame 21mm SLR lens before or since. Mine is number 116869. It has a production code of G16, which means that it was made in June 1981, according to a post by OM expert John Hermanson on this forum thread at Photo.net. I was intrigued to see whether it would be any good on my full-frame Canon 5D.
According to the ever-handy Mir.com, the 21mm f/3.5 is one of the oldest OM lenses, and dates from the mid-70s. There isn't much about it on the internet apart from this article in which it is used for night-time photography on a tripod. The lens is also overshadowed by the later Olympus 21mm f/2, which was for many years the fastest ultrawide SLR lens. Even today the f/2 model fetches a steep price on the used market. The modern Sigma 20mm f/1.8 is slightly wider and faster, but based on the tests I have seen on the internet it is unusably soft at wider apertures. Leica nowadays sells an extraordinary 21mm f/1.4 for a typically extraordinary Leica price tag of around $6,000, but it is Leica M-Mount only and thus belongs to another world.
The Olympus 21mm f/3.5 takes 49mm filters, which was the standard Olympus size. Unfortunately even the thinnest filter vignettes on the lens with a full-frame camera, and if you don't want to crop out the corners you have to use a stepping ring or a filter adapter. The vignetting isn't apparent through the viewfinder of my OM10, and perhaps in the days of machine-cut 6x4 prints it wasn't obvious. The lens focuses very closely, to whit:
The monkey was only a few centimeters from the lens. In practice this is the only way to get shallow depth of field effects; at f/8 and beyond pretty much everything is in focus.
On a conventional digital SLR it doesn't have much of a role, because it is no faster or wider or sharper than a typical kit lens. On a Four Thirds or Micro Four Thirds body it becomes a very slow normal lens that is rendered utterly obsolete by the Panasonic 20mm f/1.7. It is however interesting on a full-frame camera, which in practice means a Canon 1Ds or 5D (in common with most other vintage lenses, it will not focus to infinity on a Nikon body). Canon sells a 20mm autofocus lens, but no-one has a good word to say about it and it is bulky and expensive; the company also sells several zooms that cover the 20mm range, but they are much larger and more expensive.
Here are some tests. The lens has aperture stops for f/3.5, f/5.6, f/8, f/11, and f/16. Here's what the vignetting is like (using a large, Cokin filter mount that does not intrude on the image), at f/3.5 and then f/5.6:
Here's what the image looks like just to the left of centre, at f/3.5, f/5.6, and f/8. F/11 is just as sharp, f/16 is slightly softer due to diffraction. These are 100% crops from a 21mp original, with mild unsharp mask:
It has a slight glow at f/3.5, but is generally sharp, and then gets better one stop down and stays better. Here's a lengthy series of 50% crops from the bottom-left corner, through all the aperture stops:
In my opinion it seems to reach a plateau at f/8, beyond which it improves only slightly. In practice, at screen sizes and for A4 prints, nobody will care about the softness even at f/3.5, certainly not at f/8. There is a fly in the ointment, however: the lens has a problem with purple fringing and chromatic aberration. Here's an example, shot at f/11, with a crop of the top-left corner:
The lens is not great for shooting tree branches in the corner of the frame. To be fair, purple fringing is -as far as I can tell - a consequence more of the sensor design than the lens, and I can't fault the engineers at Olympus for designing a lens in the mid-70s that purply-fringes on a digital SLR in 2010. The CA is however visible through the viewfinder of my Olympus OM10, and so must surely be caused by the lens. Fortunately it can be corrected with software (or simply painted over, or even cropped out).
I have no problem with the colour rendition. It is not a particularly contrasty lens and does not cope well with flare, a problem exacerbated by the difficulty of finding a suitable lens hood. Olympus made a special 21mm lens hood. Good luck finding it in 2010. My copy of the lens has "MC" in the name, and so presumably is multi-coated, but I surmise that multi-coated ultrawide lenses from the early 1980s are never going to be ideal for shooting bright lights. So far I haven't mentioned distortion, the reason being that I haven't really noticed any. I'm sure it's there, it's just not particularly offensive.
It's useful for video. It's compact, and at f/8 it is pretty much focus-free. Indoors at f/3.5 it still has enough depth of field for comfort, although you'll have to bump the ISO up. Given that it is so small, it isn't a pain to carry in a camera bag. The lack of distortion is handy too; most wideangle attachments for video cameras, for example, make everything look like it was shot with a fisheye. Here's a short video shot at the Heygate Estate, south London, with the lens at infinity, f/8, Canon 5D MkII, music by myself:
There are other ultrawide lenses on the used market. Nikon seemed to have a thing for the 20mm focal range, and there was a late-70s Tokina 17mm that had a decent review at Photozone.de. Brand-new there aren't many 21mms - the Zeiss 21mm f/2.8 Distagon, which is apparently superb, and the intriguing but obscure Voigtlander 20mm f/3.5 for Nikon and Pentax and thus by adaption Canon, although I can't find any thorough reviews, and most Google returns seem to be people asking where they can find thorough reviews of the lens, or whether anybody has one.
EDIT: With the exception of Photozone.de, whose review was posted twenty-four hours after I wrote the above! The lens obviously wanted to be heard.