On: Wed, 5 May 1999 16:18:31
Mike Higgins <MHiggins@xxxxxxxxxxxxxxxx> wrote
Subject: RE: [OM] X/FP?
>I know I'm getting way off on a tangent here, but why FP "Focal Plane" flash
>bulbs? What has putting the flash on the focal plane got to do with
>anything? Surely they would work just as good off the Focal Plane? (I love
>knowing the origins of the markings on my OM-2... and I had assumed that FP
>stood for Flash Bulb, but in another language! in fact I'm sure I've told
>people this! oops.)
Mike
-------------------------------------------------
I think this is some "ancient" history, Mike. There were earlier
descriptions of why X and FP sync are different... long rise time of
bulb vs near instantaneous electronic flash + having to wait until a
focal plane shutter is completely open.
But, if memory serves me right, the nomenclature "FP" for flash with a
focal plane shutter pre-dates the invention of electronic flash. As I
recall there were two different types of flash bulbs. One for use with
leaf shutter cameras and one for use with focal plane shutters. Between
flash bulbs the "FP" bulbs had a longer burn time and maybe a slower
rise time.
With a focal plane shutter the shutter is practically resting on the
film. To expose the film evenly the shutter needs to be completely open
during the flash. While it's opening or closing part of the frame is
obscured.
With a leaf shutter, the shutter is located away from the film and is
generally in the vicinity of the optical center of the lens (in between
the elements). Since all parts of a lens contribute to all parts of an
image, as soon as the shutter starts to open all parts of the frame are
getting an even exposure. As the shutter opens wider it lets in more
light but the illumination is still even across the frame no matter how
much or little the shutter is open.
So, the point of all this is that there were still different timing
considerations for flash photography even before the invention of
electronic flash. The bulbs had to be differentiated as to their use
for focal plane or leaf shutters and the designation hangs on today.
Now, somebody else help me out here if I've got this wrong. As I said,
it's been a long time.
Chuck Norcutt
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