Gary,
Thank you for this beautifully clear discussion of "inverse square law."
While I understood this flash problem/effect in principle, I didn't have a
name for it, let alone a "law" to help me figure my way around it.
Your discussion, however, raises a question about testing for exact guide
numbers. How does one go about doing such a test?
If it is complicated to describe and there is an obvious resource (like the
list archives or someone's "flash page"), please just point me in the right
direction.
Thanks,
Glen Lowry
-----Original Message-----
From: Gary Edwards <edwardsg@xxxxxxxxx>
To: olympus@xxxxxxxxxxxxxxx <olympus@xxxxxxxxxxxxxxx>
Date: May 6, 1999 10:05 PM
Subject: Re: [OM] T32 Auto setting inaccurate
>There has been much discussion of the "inaccuracy" of TTL Auto flash
metering.
>Some of the problems may not be truly inaccurate metering. Instead, they
may be
>at least partly a lack of understanding of the physcial limitations of
on-camera
>flash.
>
>One needs to consider some of the physics and other limitations of this
kind of
>flash metering. Remember the inverse square law? It applies with a
vengeance
>here. If you shoot a subject from, say, 4 meters against a background that
is
>another 4 meters behind the subject the light to the background will fall
off by
>two stops. In other words, whatever the flash output, the light on the
subject
>and background will differ by 2 stops! (Ignoring, of course any
contribution by
>the ambient lighting? more on that later.) Since for all OMs featuring TTL
flash
>metering, the exposure measurement is centerweighted, the actual exposure
>relationship of the subject and the background will largely depend on the
relative
>area of each image within the centerweighted portion of the frame. If the
subject
>is smaller than the area of the background, the metering circuits will tend
to
>pump out enough light to "properly" expose the background while
overexposing the
>nearer subject by as much as 2 stops. This is what usually happens. You
can help
>a lot with a 1 to 1-1/2 stop minus exposure compensation dialed in if you
are
>willing to have a dark background
>
>What else can you do about this? First off, recognize when it will happen.
Then
>there are several approaches to mitigate the problem. Try moving the
subject
>closer to the background. Look for a more reflective background. Get
closer to
>the subject (fill more frame with the subject). Move farther away from the
>subject. Wait a minute, I just told you to get closer! How does moving
farther
>away work?
>
>If the subject is still 4 meters in front of the background but you move to
8
>meters away from the subject (and use a longer lens to maintain the framing
of the
>subject), you are now 12 meters from the background. The background is
12/8 of
>the distance to the subject away so the light falls off at the background
by 1
>over the square root of 12/8 (this is not tough math!). Now the background
gets
>82% as much light as the subject, easily with in the exposure latitude of
print
>film and, for most uses, of transparency film. Another approach moves JUST
the
>flash farther away - by bouncing it off a ceiling or wall. The total
distance
>traveled by the light from the flash to the subject and background is what
matters
>- not the distance the light travels from the subject to the camera. (Isn't
>physics fun?)
>
>You can also open up the aperture so that more ambient light is used (still
>keeping the shutter speed with in the synchronization range). This helps
because
>it reduces the proportion of the exposure due to flash.
>
>When exposure is really critical (Gary Reese's endangered species example
is
>great) manual flash and guide number calculations (or an incident flash
meter) are
>the way to go. Either guide number or flash meters MUST be calibrated by
test.
>Don't trust spec sheets. For snapshooting and photojournalism with print
film,
>careful composition and TTL Auto flash can work very well, and is a lot
quicker.
>For other uses between these extremes, you be the judge.
>
>Gary Edwards
>
>
>
>
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