jaz@lucid.com (Jim Zaun) (01/21/89)
>> [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)