Joe Gwinn wrote:
>>From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
>>Subject: [OM] Re: Flash photography
>>
>>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
Actually, a T-32 has enough spread to cover a 24mm lens within its
stated guide number. The loss of light due to extra distance and less
than perfect reflectance is well understood but what I didn't understand
is why my T-32's are both measuring a stop below what they should be
when not bouncing off anything. I was deliberately waiting 20-30
seconds after the ready light to make sure they were full charged. I'll
have to do some more testing.
Chuck Norcutt
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