[LargeFormat] Re: flash exposure

[jpetrush]

Weeellllll, this is a tad OT for LF, but a legitimate photographic question,
so, to answer your second question first, every flash maker I have ever
encountered over rates the output of their flash units by at least a half
stop, sometimes more.  Either bias the film's ISO setting or dial in your
"over exposure" factor.

To find the output for a given unknown flash, try the following:

Set your camera on a tripod at a fixed distance from the subject
Load it with ISO 100 slide film.  This eliminates any processing
"corrections"
Set the lens at f/5.6 for a small unit, f/11 for a large powerful one.
Begin a set of exposures with the *flash* 4 feet away from the subject, then
start backing up to 5.6 feet, 8, 11, and 16.
Keep lens aperture and shutter speed (x sync) fixed throughout
Take notes
Evaluate the best of the batch
Determine the guide number by multiplying the f number times the distance of
the best exposure.  This is your guide number in feet for ISO 100 film.
To determine what f number to use at a given distance, divide 100 by the
distance
For films faster or slower than 100, add or subtract the number of stops
from 100 to that f number.

Or, get yourself a good flash meter <g>

jp

----- Original Message -----
From: "philip Lambert" <philip.lambert@virgin.net>
To: <largeformat@f32.net>
Sent: Saturday, December 30, 2000 10:03 AM
Subject: [LargeFormat] Re: flash exposure


> How does one achieve correct flash exposure?  I bought an old Canon flash
at
> a fair without a manual and ran off several exposures of Fuji colour
> negative film of an Xmas arrangement of bottles of alcohol standing on
brown
> wood furniture against a white wall, at various apertures, with the camera
> on a tripod at a fixed distance.  The only differences I noticed in the
> prints were that the least bright objects, the furniture, improved in
colour
> as exposure increased.  The negatives became denser, but the
light-coloured
> objects looked much the same in all the prints.  I am inclined therefore
to
> give generous flash exposure for negative film but what would readers
advise
> for transparency film? Is a good flash meter the answer?
> As a separate matter, my Nikon F90 and SB22 seem to automatically
> underexpose slightly (using TTL flash metering) so I usually give plus
half
> a stop. Do I have a defective outfit or is a half-stop adjustment regarded
> as insignificant? Or are the manufacturers' claimed film speeds only an
> approximation?
> Philip Lambert