Re: Bracketing DrT style!

[P3D <PTWW@xxxxxxx>]

Dr. T creates a hypothetical scenario and invites us to attack
his strategy:

>Suppose I am in a very difficult lighting situation and I only have 3
>pictures left in my camera to shoot.  I have to have a decent exposure.  No
>more film, no more chances, once in a life-time situation.  Here is what I
>will do: 
>
>1. Take one picture at "best guess" (BG)
>2. Take another picture at BG + 1 f-stop
>3. Take a third picture at BG - 1 f-stop
>
>This range of 2 f-stops will guarantee a good exposure within 1/2 f-stop! 
>If the "best" exposure is within this two f-stop range, I have hit it
>within 1/2 f-stop and that's plenty good for me.  

Dr. T actually gives his strategy less credit than it deserves, in
that if the "best" exposure happens to be 1.5 stops above or below
BG, he will still have managed to come within 1/2 stop of nailing it.
Thus he is achieving 3 full stops coverage, not 2.  However...

1/3 of a stop is generally considered the normal limit of human ability
to detect variations in lighting.  It is, therefore, the recommended
increment to use when shooting an exposure test.  When I have done
rolls of exposure tests with my equipment, I generally find no more
than three consecutive shots (a range of 1/3 stop over or under "best")
falling within acceptable tolerances.  That does not mean that anything
outside that range is automatically a throw-away.  But it *does* mean
that if "best" exposure in Dr. T's example were actually [BG - 1/2 stop],
I will most likely be at least mildly disappointed with all three of
the shots.  For my purposes, then, bracketing in 2/3 stop increment
would be a better choice, effectively giving a margin of error of
+/- 1-1/3 stops from BG.

Perhaps I am reading incorrectly, but Dr. T's original salvo on this
topic seems to suggest something very different from what he describes
in the hypothetical scenario.  He wrote:

>IMO, there is a range of acceptable ("good") exposures.  This range
>can be as wide as 2 f-stops or more.

This seems to suggest that if BG = BE, Dr. T would generally include
all three exposures within his definition of "good exposure."  I
confess I probably originally read his comment to mean "plus or
minus" 2 f-stops (darn that red port wine!), but even +/- 1 full
stop from BE is, for me, too far off (with slide film) to be considered
"good exposure."

Even the most casual reader of the list is probably aware that Dr. T's
famous "sharp eyes" are extremely sensitive to alignment errors in
stereo pairs, chromatic aberration in viewer lenses, or mismatched
focal length in stereo camera lenses.  In all liklihood, what an
"average" person may consider imperceptible or minor flaws in these
regards will be subject to harsh criticism from Dr. T for their poor
quality.  Similarly, there are certainly those among us who have above
average ability to detect variations in exposure; for them, it very
well may be necessary to bracket in even smaller increments to be
sure of achieving an exposure that meets their standards.  Dr. T thinks
they are wasting film; they may think Dr. T is wasting time and effort
mounting his own slides.  Perhaps Dr. T's sharp eyes have below average
sensitivity to exposure and lighting variations.  After all, I think
Gabriel has previously made note of the fact that Dr. T may have somehow
mistakenly concluded that 750W projector bulbs were brighter than 500W
bulbs! ;) ;) ;)  And Dr. T has previously described an image he submitted
to competition that did not score as high as he thought it should have,
because the judges were critical of the lighting.

This is in no way meant to be judgemental about anyone's abilities or
preferences, merely to point out that many differences in perceptive
ability do exist.  I think what it means is that while Dr. T can
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satisfy himself while being somewhat haphazard with his exposures,
he must be quite meticulous with his mounting and choice of viewer.
Note these adjectives are meaningful only with respect to what might
be called the "average" person.  From Dr. T's perspective, he is
probably being *equally careful* with exposure, mounting, etc.  Also,
from his perspective, many of us appear to be excessively meticulous
about exposure, while being haphazard with our mounting and choice of
viewer; but we believe we are being equally careful with all three.
This is all fine as long as we don't share slides or enter them in
competition!  If we want our work to be highly regarded (technically)
by everyone who views our images, we must all strive to be meticulous
enough to meet the most stringent standards of the viewer who is most
sensitive with respect to each technical aspect of the image.  Of
course, what makes it that so difficult is when we can't *see* the
rotational misalignment or chromatic aberration, and Dr. T can't
*see* the shortcomings with the exposure.  That is where we face our
greatest challenge, and where we must respect and learn from each
other's observations.

Paul Talbot, *not* going for the award for highest word count per post,
             despite appearances to the contrary!


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