On Wed, 14 Sep 2005, Jens Bladt wrote:
I wonder how the flash actually DOES work, if the light output is
indifferent to the ASA setting?
The flash quenches when its sensor is happy. If your lens aperture and
distance were correct (indicated by the slider and thus affected by
the ISO setting), you are OK, if not, you are not OK :-)
Or so I think. From Boj:
<http://kmp.bdimitrov.de/technology/hot-shoe/index.html>
"Automatic Flash
---------------
An automatic flash is one that can limit the flash duration and
therefore the amount of light that it outputs. The control function
is performed by electronics contained inside the flash and guided by
a light-sensitive sensor located on the front of the flash. When the
sensor detects that enough light has returned back to the flash, the
control electronics terminate the flash discharge. In the simplest
flashes, the flash capacitor is shorted (so the extra charge is
lost), but in more advanced ones (those labeled thyristor), the
charge is preserved for the next flash discharge.
An automatic flash achieves proper exposure not only when GN = d * F,
but also when GN > d * F.
Automatic flash operation achieves correct exposures not only in
"straight-on" applications, but also when the flash head is tilted,
swiveled or covered with various gels and reflectors. To perform its
calculations, the flash assumes that some F (usually f/4 or f/5.6) is
selected on the lens. Failure to match F results in under- or
over-exposure.
Program Flash
-------------
Pentax did not make any purely automatic flashes. Instead, they added
another feature, called "program flash," and gave the flashes in this
category the SA designation.
Program flashes solve the problem of the photographer forgetting to
set the correct F before every flash exposure. When used in program
exposure mode, the flash and the body work together to set F. This
capability requires an extra contact, mode, so the flash can "tell"
the body what brightness it will produce. Knowing the film speed, the
body calculates and sets F.
Some SA flashes have the extra functionality of signaling when proper
exposure was achieved. This is done either by lighting up a lamp on
the back of the flash or communicating with the body (via the ready
contact) and letting it make the flash symbol in the view-finder
blink."
For the reasons mentioned earlier, I harly ever use a flash in bright
daylight. I know PJ's do it all the time. I never understood why.
Contrast control. You get even illumination and remove the shadows
under the eyes.
I believe light looks better comming from above.
I agree, it is softer and more diffused, but requires some direct
flash as well to eliminate the shadows. Thus the common "white card"
(or diffuser) trick.
Kostas
