Focal Plane (FP, or high-speed sync) flash is, as Bob Blakely mentioned, an old technology. The history, the potential utility, and the limitation of FP flash are discussed in the Introduction to Flash Technology section at http://www.photozone.de/ "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." [This required special flashbulbs.] "Focal plane (FP) flash. . . 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. . . [M]ost 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. . . 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." A table from the site shows how the effective guide number (of a Canon 380EX at 50mm (GN: 31)) decreases as the shutter speed increases: Shutter Effective GN 1/250 14.8 1/500 10.8 1/1000 7.7 1/2000 5.4 1/4000 3.8 Brian Walsh - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
