Unless you are shooting on manual mode (for maximum power), then the flash
sensor is setup to automatically cutoff the flashtube after it has "seen"
a certain amount of light coming back from the subject (e.g., to prevent
overexposure from using too much light). So yes, it is the same cutoff
point each time. In some auto-strobes (vivitar 283..), different color and
density filters are used to provide a series of auto-ranges at different
distances...

The problem is that you keep wanting to point the sensor at the ceiling ;-)
Doing so will cutoff the flashtube based on the amount of light reflected
back from the ceiling, not from the subject. The sensor has to be pointed
at the subject to correctly cutoff the flashtube when the amount of light
has been received by the lens and sensor, both pointing at the subject.

You don't set the guide number based on light reflected from the white
ceiling, but rather on a series of tests with the actual subject. If you
have a flashmeter, you can meter at the subject's position and adjust for
darker or lighter than average subjects and backgrounds (with experience
;-). Since you seem to be measuring the light from your flash with your
"light meter", I am assuming that it is a combo light meter and
flashmeter? If so, the owner's manual should have more info on using the
flashmeter in it.

the key issue with bouncing off the ceiling is it reduces the maximum
range at which you can work, as you lose a stop or two+ in the bounce
setup, depending on distance and ceiling color (which can also impact the
color of the light reflected to the subject, so purple ceiling may mean
purple faces on subjects ;-)

HTH bobm

probably you should look into the Lighting Cookbook or Fil Hunter and Paul
Fuqua, "Light-Science and Magic" (Focal Press, 1990) classic etc.

generally, the main strobe light controls the overall exposure, and fill
lights or hair highlights strobes are kept a stop or more below the main
light, so the exposure is basically set by the main strobe. In these
cases, automatic flash cutoff still works fine for many situations.

when in doubt, you can flash meter the scene and set the required aperture
based on film ISO, again something that should be covered in your meter
manual for specifics.

if the ceiling is far or colored purple, you can always use a flash
diffuser of some sort, ranging from a white card or handkerchief cloth to
an inflated commercial diffuser etc. Lots less light loss as well as purer
color. finally, you can extend your strobes lighting for $5 or so fresnel
from Home Depot, see http://medfmt.8k.com/bronfresnel.html 

hth bobm

Recently, steven.sawyer@banet.net <steven.sawyer@banet.net> posted:

Hmmm. I don't understand what you're trying to accomplish. One wants the
light measured off of the subject, not the ceiling. As has been pointed
out, you may need a beefy flash unit to be able to illuminate the subject
purely by bounce, because the light falls off as the square root of the
distance from the subject plus the absorbtion of the reflector (ceiling)
and loss from scattered rays that don't reach the subject. In small rooms
with bright ceilings and subjects that aren't too far away, this may not
be much of an issue, but the power required goes up dramatically as the
room size, distance from subject, and reflectivity changes.

Neil