Focal plane (FP) flash seems to be a quite new technology
but surprisingly the concept is already known for some decades (e.g. Olympus
OM-3Ti, OM-4T) - see below. It allows faster sync. speeds
than just e.g. 1/200s. With very fast shutter speeds the opening between
1st and 2nd curtain is never as large as the whole film so a single flash
burst would lead to a partially unexposed film. As a solution you have
to have a constant flash light for the whole exposure time. Unfortunately
modern flash units have a peak emission characteristic so a single flash
is not usable for this purpose. Today most manufacturers use a series of
high frequent flash bursts (say 50 kHz) with reduced single light emission
to simulate a (theoretically) long single constant flash burst.
Obviously there's also a drawback with this kind of
flash exposure. The following picture illustrates a single flash burst
(in FP mode). As you can see most of the light is blocked by the 1st and
2nd curtain so the effective guide number is reduced dependent on the shutter
speed. The faster the shutter speed the smaller is naturally the opening
between the two curtains and the less flash light reaches the film. The
table to the right below shows the effective GNs for the Canon 380EX. As
far as I know these GNs may vary from flash unit to flash unit and between
the manufacturers. You can see the practical possibilities are a bit restricted
at really fast shutter speeds.
||Example: Canon 380EX at
50mm (GN: 31)
Here're some background information about the history
of high speed sync (FP-Flash) provided by
A high speed flash for focal plane shutters is
not new. It is about 50 years old (perhaps more). A set of principles similar
to what is outlined for electronic strobe above was used with flashbulbs
for several decades and ended with the demise of flashbulbs and flashbulb
manufacturing. Starting in the early and middle 1950's a number of cameras
were made with a flash sync position called "FP" which means the same then
as it does now. It is a high-speed flash sync for use with focal plane
shutters at speeds of 1/100th second and faster, and flash bulbs. As today,
this feature was found generally only on the higher end cameras during
the flash bulb era, mostly professional grade models. An examples of these
cameras is the Zeiss Ikon Contax IIa and IIIa Color Dial. These were
introduced to the market in 1954 and had "M" sync for shutter speeds of
1/30th second and slower, "X" sync for 1/50th second (for electronic strobe
flash!), and "FP" sync for 1/100th second and faster.
Standard flashbulbs (designed for "M" [20ms to
peak output] and "F" [5ms to peak output] synchronization) did not have
a long enough peak light output for the "FP" sync even though the duration
of their light output is very long compared to an electronic strobe's.
A special long-duration or "FP" bulb had to be used with this sync. (e.g.
M-3, M-3B, 6 etc.). The concept of how the long peak duration flashbulb
worked is similar to the FP strobe. If the sync was adjusted correctly,
these bulbs provided a relatively constant peak light output starting before
the opening shutter curtain was released and continuing until after the
closing shutter curtain completed its travel.
I have tried to find the lead time required between
bulb ignition and opening curtain release for these bulbs but without success.
My best guess, based on the original factory repair manual for the Contax
IIa and IIIa CD is somewhere between 15ms and 20ms which is similar to
the "M" sync.