Here's a short piece from the 1961 BJ Almanac that explains the types/classes
Get more Popcorn & read . It's from a British article so references typical shutters found in the UK at that time:
FLASHBULBS
A flashbulb consists of a glass bulb containing an accurately controlled quantity of an inflammable filling in an atmosphere of oxygen and provided with a thin tungsten igniter filament which is connected to a cap or contacts sealed to the glass. When placed in an electrical circuit the igniter filament becomes white hot and breaks, igniting the filling to produce a brief flash of light. The light output is given in lumen seconds and ranges from about 7500 with small flashbulbs to as high as 95,000. As the duration is of the order of 1/50th sec. this means that a very intense illumination is produced for that period, and peak outputs of around 1 and 2 million lumens are quite common. In the case of paste type flashbulbs the filling consists only of a quantity of zirconium priming paste attached to the filament and lead-in wires. This gives a rapid flash, but the total light output is limited by the small amount of paste that can safely be used in a glass bulb.
In other types the filling consists of an alloy of aluminium and magnesium or zirconium foil, shredded foil, or wire which is ignited at many points simultaneously by the bursting of verv small quantities of priming paste placed at each end of the igniter filament. The light output is controlled by varying the amount of foil or wire contained in the bulbs, but the nature of the filling has an important effect on construction, performance, and particularly the length of time which elapses between making contact and the attainment of maximum light output. The British Standards Institute classify flashbulbs in the following groups in relation to their time to peak.
Class F. Fast time to peak, which reach peak light output 5-9 milliseconds after contact. These are of the paste type.
Class M. Medium time to peak, which reach peak output 18-24 milliseconds after contact. These are wire or shredded foil filled.
Class S. Slow time to peak, which reach peak at least 30 milliseconds after contact. These are large flashbulbs with wire or shredded foil filling giving a high light output.
Class FP. For focal-plane shutters, having a long flash which is maintained above half-peak intensity for at least 25 milliseconds and which reach half-peak intensity between 16-18 milliseconds after contact. These are specially constructed with wire filling.
SYNCHRONISATION
The flashbulb, being fired electrically, offers a useful means of synchronizing the flash with the opening of the camera shutter. This is done by fitting flash contacts in the shutter mechanism so that when it operates contact is made between them. The contacts now act as a switch, and when connected to the flashbulb firing circuit, close it during the operation of the shutter mechanism. Synchronisation can be by one or more of the following methods which are classified in relation to the time period between contact and shutter opening.
" X " type synchronization. No delay. Contact is made when the blades are fully open.
" F " type synchronization. 2-5 milliseconds delay. Contact is made as the blades start to open.
" M" " type synchronization, 16-17 milliseconds delay. Contact is made when the shutter is fired, but a gear train introduces the delay so that the shutter opens after contact.
Synchronisation of popular shutters
Modern snapshot camera shutters are of the " F type although a few mild are " X " synchronized Speeded shutters of the following manufacturers arc synchronised as below, alternative settings being marked on the shutter;
Shutter . . . . . . . . . . . . . . . . . Synch.
Epsilon (later model) . . . . . . . . . X only
X Compur Rapid . . . . . . . . . . . . X only
Pronto. . . . . . . . . . . . . . . . . . X only
Pronto S. . . . . . . . . . . . . . . . . X only
Vario . . . . . . . . . . . . . . . . . . . X only
Velio . . . . . . . . . . . . . . . . . . . X only
Agilux. . . . . . . . . . . . . . . . . . . F only
Epsilon (early model). . . . . . . . . F only
Epsilon . . . . . . . . . . . . . . . . . . FX
Synchro Compur . . . . . . . . . . . . XM
Prontor SV. . . . . . . . . . . . . . . . XM
Prontor SVS . . . . . . . . . . . . . . . XM
Supermatic XM . . . . . . . . . . . . . XM
Recommendations for synchronisation
" X " Setting
Use electronic flash at all speeds.
Use Class F flashbulbs at speeds up to 1/50th sec.
Use Class M flashbulbs at speeds up to 1/25th sec.
Use Class S flashbulbs at speeds up to l/10th sec.
" F " Setting
Electronic flash cannot be used.
Use Class F flashbulbs at speeds up to 1/50th sec.
Use Class M flashbulbs at speeds up to 1/25th sec.
Use Class S flashbulbs at speeds up to l/15th sec.
" M " Setting
Electronic flash cannot be used.
Class F flashbulbs cannot be used.
Use Class M flashbulbs at all speeds over 1/25th sec.
Use Class S flashbulbs at speeds up to l/25th sec.
It will be notch that limitations are imposed on the shutter speeds that can be used with certain shutter and flashbulb combinations, due lo the various times to peak and delays involved. These are to ensure good synchronisation, but do not necessarily limit the effective flash exposure. This is conventionally taken as the duration above half-peak, and in she case of Class F flashbulbs is about 1/50th second duration. With Class M flashbulbs it ranges from about 1/50th second to less than 100th second with the small types, so that exposures at open flash and speeds up to 1/25th sec. (" F " or " X ") or 1/50th sec. (" M ") produce the same result. There is little point in using shutter speeds between 1/25th and 1/100th sec. with the small flashbulbs. Higher speed synchronisation is not recommended with Class S flashbulbs as it is assumed that the total light output will be used.
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The Acme synchro shutters allow X, F, M & S synchronisation:
Time (ms) . . . .Dot Colour . . . . .Description
0 . . . . . . . . . . . red. . . . . . . . . . . X (electronic flash)
5 . . . . . . . . . . . white . . . . . . . . . F (gas filled flash bulbs)
20 . . . . . . . . . . blue . . . . . . . . . . M (wire-filled flash bulbs)
23 . . . . . . . . . . yellow. . . . . . . . . S (large slow flash bulbs)
Ian
