Okay, correction -- looking up guide numbers, and discovered that #11 was the smallest M synch screw base bulb, not a focal plane as previously suggested.
It depends on the flash -- I have one here, a Gold Crest BC-7 (a near-clone of a Honeywell Tilt-a-Mite) that accepts bayonet, midget, and AG bulbs all in the same socket, but I'm certainly glad I didn't work on the line that assembled that socket! The only Kodak flash I have is one I bought for my Pony 135 (uses a bracket that goes on the tripod mount, and has an ASA sync cord), and I don't recall if it can use a bulb smaller than a bayonet, but there *are* adapters. I've got one flash (from an Ansco Pioneer 620) that has a screw base to bayonet adapter; there were also adapters from bayonet to midget and AG, from screw to midget, and from bayonet to the Phillips PF series baseless bulbs that were sold in Europe contemporary with the AG bulbs here. I'd love to get a bayonet to midget adapter, since I have a couple flashes that use only bayonet bulbs, but while I have a bunch of M2 and M3, I don't have a single #5 in the house.Originally Posted by mhv
Some of the numbers are related to the total light output (area under the light curve) of the bulb -- an AG-3 is similar power to an M3, though the AG-3 usually has a lower guide number because it's normally used in tiny reflectors (like the one I have for my Minolta 16 MG) instead of big bowls. Likewise, the M5 is the same light output as a bayonet #5, and in the same reflector has the same guide number (but M5 usually got only a 3" reflector, while bayonet bulbs often had 6", so the guide numbers look different). However, once you get numbers bigger than #11 (which was a screw base), the relation starts to break down; as well, there were #6, which were long burning focal plane versions of #5. A P25 does not give more light than a #11 (there was also a P40); I'm not sure where the number for that one comes from. For the ones with a direct relationship, I almost think the number was the number of grams of magnesium (or equivalent, since a few bulbs used zirconium, magnesium/aluminum, or other metals instead of plain magnesium) inside the envelope. If the oxygen in the bulb could be maintained at enough pressure to provide complete combustion, that would be a very linear relation to total light output, and if the wire diameter was constant, the burn time would be constant as well. In general, the relation between number and power seems to have started around 1950; older type bulbs (starting with the introduction of electric flash photography in the 1930s, as opposed to "flashlight" photography using photoflash powder, magnseium ribbon, and flash sheets) were given number or other designations more or less at random.
Number relates to socket size only in that it took a bigger bulb envelope to hold more magnesium and oxygen, and there was a limit to how big an envelope it was practical to mount on a given base. So, you see AG-1 and AG-3, but never an AG-5 (even if they could have made an AG base on a 5 size bulb, it would never have fit in the tiny flashes that used the AG series bulbs). Likewise, you see M2, M3, and M5 (which is quite visibly bigger than M2/M3), but never an M1 (too small, and also wasn't practical with common film speeds at the time the midget bulbs were introduced), and never anything bigger than a 5 on a midget base. Even bayonet base couldn't accomodate a #11 bulb (they were the size of a 40 W household lighting bulb, and used the same socket).
BTW, if you ever use older bulbs that aren't plastic coated, use extreme caution -- it was *very* common for bulbs, especially small sizes, to explode on firing, prior to the advent of plastic coating. Kodak used to sell a plastic hood to put over the reflector of the Kodaflash units; it was clear on one side, to protect against bulb fragments, and if you turned it around it was matte to act as a diffuser for close shots (essentially, allowed opening up one stop from guide number calculation by spreading the light more, which in practice let you cut your distance by 30% at the same opening). By the mid-1960s, practically all bulbs sold in the USA were coated, and a shattered bulb was something you'd see only if (as I once watched a teen photogapher do in the early 1970s) somone peeled the coating off a bulb -- as when converting a blue bulb to clear to get the extra little bit of guide number on B&W film.