The Olympus S-20 flash was designed by Olympus and produced by Cosina (Apparently that may not be the case, the Flash may have been produced my Matsushita [Panasonic] http://www.ricohforum.com/viewtopic.php?f=34&t=1512, thank you for the update, Konrad) just like the Olympus 35-70 3.5-4.8 and OM2000. The S-20 is a very, very small flash measuring 56x28x82mm (~2wx1dx3.5h”) and weighing only 70g without batteries. It takes two AA batteries and has a guide number of 20 (ISO100). It has a rated power of 280 flashes on a set of AA alkaline batteries. It has 4 settings on the rear dial, off, Auto-1 (f/2.8, 1-7.2m ISO100), Auto-2 (f/5.6 .5-3.6m ISO100) and manual (full power). The Auto 1 and 2 settings use the photosensor on the front of the flash to control the autothyristor circuit to vary the flash output for a proper exposure of f/2.8 or f/5.6 (ISO100). The manual setting is full power and there is no real calculator panel on the flash, however a little quick mental math can figure out the F-stop needed. It is F-stop = GN/distance (in meters). So with ISO100 film the GN is 20, so at 4m the f-stop would be 5, at 1.8m it would be about f-11, etc.
The flash normally has a coverage of about 35mm (58 degrees), but the flash includes (or at least has the option of) a wide angle flash adaptor which gives 24mm coverage reducing the GN to 14. The flash duration ranges from 1/30,000s to 1/1,200s in auto mode and is 1/1,200s in manual mode. The color temperature is 5,900f and 5,800 with the wide angle adapter.
The S-20 is absolutely tiny, however it has enough height that it should avoid red eye. On my OM-1 it puts the flash about 5-5.5″ above the center of the lens, much higher then a pop-up flash on any camera I know of. The guide number is rather low, but it I don’t see it has being used for light huge groups of people or anything similar. Also no bounce flash option, but again I don’t see it used for anything like that even if you could (and it isn’t powerful enough for that). What it is great for is being really tiny to be able to be carried anywhere for when you need a flash in a pinch as well as having a couple of options for auto settings and a full power flash setting. The coverage with the wide angle adapter is also great. The S-20 is probably going to live in my camera bag. In my Lowepro Nova 2 AW it takes up less then half of the front pocket (about 1/3rd) and in my Nova Mini AW it still fits in the front pocket with enough room for a filter or two, remote shutter release and a roll or two of film. Its a great carry anywhere and travel flash. The S-20 is unfortunately pretty rare, it took me 3 months of looking to find one, but when you do find one they seem pretty cheap, I managed to get mine for $40 (Olympususa on Ebay). I haven’t used a T-20 before, but the S-20 seems a more versatile and definitely smaller and lighter flash then the T-20/32.
A good comparison of the size of the S-20 compared to my hand (medium/largish man hands) and a Vivitar 285hv