>Date: Fri, 20 Feb 2004 07:08:53 -0500
>From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
>Subject: [OM] Re: Flash photography
>
>Even if the ceiling is white it's still not a mirror and will absorb a
>lot. Since I have a flash meter I decided to do a little experiment.
>
>Test conditions: Shooting from either 8 feet or 12 feet and measuring
>straight-on. Or bouncing off an 8-foot ceiling at the midpoint between
>flash and subject. The ceiling is white, unpainted plaster with a brush
>textured swirl pattern.
>
>Since the flash was only about 3 feet below the ceiling the additional
>distance traveled was not far. By guide number calculations, using
>distance only, the additional exposure would be less than one stop. The
>measured exposure in both instances, however, was approximately two
>stops more for bounce vs straight-on.
>
>I didn't try it but I think if I was angling off both ceiling and wall
>as I normally do with indoor TTL shots it would probably require at
>least 3 and perhaps 4 stops more exposure.
>
>One slightly disconcerting outcome of this experiment was that I tested
>two T-32's and both gave me the same results with the flash meter.
>However, both were a full stop below what would have been indicated in
>the straight-on shots using guide number instead of the meter. Don't
>know why yet.
I would guess that it's the angular spread: When shhoting straight on, the
light from the flash spreads out only enough to fill a 35mm frame using a 50mm
lens. When the flash bounces off the ceiling, it instead spreads out in all
directions, and so most goes the wrong direction and so is lost to the process,
even though white paint has ~90% reflectance.
Joe Gwinn
The olympus mailinglist olympus@xxxxxxxxxx
To unsubscribe: mailto:olympus-request@xxxxxxxxxx?subject=unsubscribe
To contact the list admins: mailto:olympusadmins@xxxxxxxxxx?subject="Olympus
List Problem"
|