I just read Bob Shell's typical disinformation in the
current "Shutterbug" that has been quoted here on the list.
He has a side bar article about how meters are calibrated to
13% not 18%. The problem is that with his usual
misunderstanding he is comparing apples and oranges.
Meters are calibrated with a 4 stop neutral density filter
(12.5%) from a standard calibrated bulb. That means all
meters that are calibrated exactly will give the same
reading.
What that has to do with the average reflectance of real
world scenes is exactly nothing. They have nothing to do
with each other except the measurements are quoted in
percentage. It is like saying something is wrong with your
mike because 8 ounces of it does not weight 8 ounces. They
are two entirely different measures.
For those of you who like take Shell as the gospel he has
another misunderstanding in his sidebar about incident
meters. The flat disk is not used to measure light on flat
objects (ala copying) as he says. It is used to give a
directional reading while the dome is used to give an
integrated reading. Users of hot lights know this for the
rest of you I will explain.
Say you have two lights, a main and a fill, using the dome
you get an overall exposure reading for the set. To check
the light balance you can turn on just one of the lights in
turn, or you can use the disk on each light. The difference
is you use the dome by pointing it at the camera from the
subject. You use the disk by pointing it in turn at each
light the difference in the readings from the two lights is
your lighting ratio in stops.
Hope this clarifies things, I know I was confused by what I
read on the list.
--Tom
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