What do the numbers mean on my IIIf top and front plate?
Ever wonder what the numbers engraved on the IIIf top plate and selector dial beneath the shutter speed dial mean? They are actually there for setting the delay (in milliseconds) between the flash being triggered and the shutter opening. This pause was needed for bulb type flashes because they took some time to warm up and emit their peak light output.
In May 2000, Michael Darnton <firstname.lastname@example.org> wrote the following detailed explanation:
0 = no delay--for electronic flash, which should go off when the film is fully visible to the lens.
5 was for gas-filled bulbs ("SM"). They ignited faster, and gave a very short burn, to make an effective 1/100 exposure on cameras that had synch at slower speeds (like all Leicas). It was for (poorly) stopping action. Since they were fast burning, they needed to be ignited only 5/1000 in advance of the shutter opening.
Normal magnesium wool filled bulbs (#5 and 25--same bulb, different manufacturers--in the medium bayonet size--about the size of a walnut) took 15-20/1000 to get going, hence needed to be lit 15-20/1000 in advance of the shutter crossing the film. The exact number depended on the bulb and manufacturer.
This was all before M-2s and M-3s (bulbs, not cameras), AG-1s, and the quad-AG-1 item known as the flash cube.
There were also FP bulbs, which looked like the #5, but had slower-burning magnesium foil. These gave a long enough duration that they could be used at any shutter speed on 35mm focal plain shutter cameras. I think, but am not sure, that they needed the dial set to 20. They were too expensive for me to actually buy and use!
[…] In practice one had to experiment a bit to find out what number was EXACTLY the right number for one's camera and bulb of choice.