🚀 go-pugleaf

>> [Mike Borza] -- ... doesn't this mean that shutter speed is essentially ir-
>> relevant for ... an electronic flash?

> [Andrew Koenig] -- Correct...

Yes,  the flash duration is so short that the shutter speed is irrelevant  for
the  purposes of controlling exposure.  However, the shutter speed is relevant
on FP (focal plane) shutters for altogether  different  reasons.   If  the  FP
speed  is  faster  than its X sync speed,  the opening between the 1st and 2nd
curtains will be less than the total film width.  Hence, with very short flash
durations only a portion of the film will be exposed.  One notatable expection
is the dedicated, OM-4T flash system which takes into account the  FP  shutter
slit width and velocity thus allowing flash photography at any shutter speed.

> [Andrew Koenig] -- Some flash units do not allow the intensity to be varied.
> Others have sensors that measure the amount of light reflected from the sub-
> ject and adjust the intensity appropriately.
           ^^^^^^^^^^^^^^^^^^^^
That last statement is slightly inaccurate.  Prior to exposure the  flash  in-
tensity  can  be  adjusted  either manually or automatically (via a preflash).
During exposure the flash duration (T) is adjusted by an electronic switch  (a
thyristor) based on the light intensity (I) received at the sensor in the fol-
lowing way:
                       E = integral [ I(t) dt ]

When the exposure (E) reaches a predetermined value the thyristor  cuts  power
to  the  flash.  In most flashes I(t) is not constant so the formula cannot be
reduced to E = I x T.  Cheers,
-- 
Jim Zaun, c/o Lucid, Inc., 707 Laurel St., Menlo Park, CA 94025 (415/329-8400)
Internet:   edsel!jaz@labrea.stanford.edu   jaz@lucid.com
Uucp:       ...decwrl!fernwood!edsel!jaz   (new unreliable path)