Re: Using Flash w/Twin camera

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>     I know that using a pair of flashes will produce a twin-sun effect,
>     and probably over-expose both.

  This is a very good question.

  If both flashes are near the minimum distance for the auto
or TTL (because they can't cut off the light instantly) then
two flashes will cause an overexposure.

  At intermediate distances I don't know. It depends on how
fast the flash light is cut off after the auto circuit ends
it. If it is instantaneous then 2 auto or TTL flashes firing
at the same time will see the light from both and cut off
instantly when there is enough. So no overexposure.

  If the flash puts out say 1/16 power after the "stop" signal
from the sensor you will get a small overexposure.  Suppose
the subject is at a distance that takes 1/2 plus 1/16 power
from a single flash.  The auto sensor will say "stop" after
it sees 1/2 power. Combined with the extra 1/16th that
happens afterwords gives the proper exposure.

  With two flashes and the same subject: After 1/4 power from
each the two sensors will see 1/2 power and say "stop". Both
flashes will put out an extra 1/16th after, for a total of
1/2 plus 2/16. So we get an extra 1/16th power of overexposure.

  The above assumes the sensor cuts off early to compensate
for the light that comes after the "stop". If they don't, all
shots (except max range) will be slightly overexposed and
twin flash shots a bit more.

  You might be able to estimate this from the minimum range
spec for your flash. For my old Pentax 280T flash I recall
the min distance was about 1/5 of the max. This is 1/25th
of the light at max range (ie full power). It is probably
a combination of the minimum response time of the sensor
circuit plus the light after the "stop" signal.

  Does anyone know for sure about this?

Greg (speculation is my middle name) Erker


------
Greg Erker, Research Engineer at, but not speaking for,
  Telecommunications Research Laboratories (TRLabs)
  #108, 15 Innovation Blvd.,  Saskatoon, SK, CANADA
  Ph: 306-668-8209  Fax: 306-668-1944



------------------------------

End of PHOTO-3D Digest 1073
***************************