P3D Re: Sunny 16 rule, baaaaah

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Grant C. Campos <gccampos@xxxxxxxxxxxxxxx>


>Tom's method of pointing your light meter at your hand might works
pretty well,
>but since we're picking nits:) . . .
>
>For those who are relatively new to photography:  I use to take my
Uncle Hugh's
>old GE light meter and just point it in the general direction of what I
was taking
>a picture of and set my camera to whatever the meter said.  Sometimes
it worked
>and sometimes it didn't.  First thing I found out was that my old GE
light meter
>used a slightly different scale (ASA 100 film was ASA 125 on the
meter), and
>secondly that a light meter is set up to correctly expose a particular
shade of
>gray.  For Example,  If I point my light meter at a dark Navy Blue wall
and set my
>camera accordingly, the picture will make that end up being as bright
as that
>special shade of gray; the picture will be over exposed.  If I point it
at a white
>wall, the setting will cause my picture to be that same brightness of
that
>standard (#18?) gray; it will be underexposed.


Since we're picking nits...

No one's mentioned the fact that there are two general modes of
metering.  Both Tom and Grant are discussing "reflective" metering,
where what's being measured is the intensity of light *reflected* from
the subject (or group of subjects).  Grant is correct in that such a
reading will correctly expose an 18 *percent* gray card ("Zone V", or
middle gray, in the zone system).  Tom's suggestion to meter one's hand
works because, by coincidence, the average reflectivity of (Caucasian?)
human skin closely approximates the reflectance of an18% gray card.
Tom's advice assumes that the light source on the hand being metered is
identical to that on the subject, however.  Metering one's hand in
direct sunlight when the subject is in shadow will underexpose the
subject.  (Knowing the sunny-16 rule here helps in knowing by just how
much.)  And it's completely useless for taking pictures in a darkened
theater, in a sports arena, or anywhere else that the photographer is in
different light than the subject.  So the "meter your hand" rule breaks
down under certain circumstances, too, many of them the same ones that
render sunny-16 useless.  What to do, what to do?

Reflected readings are further complicated if one is taking a reading of
a distant scene.  In that case, the light reflected from all of the
objects within the meter's field of view is averaged together.  If the
field of view of the meter closely approximates the field of view of the
taking lens, AND if the objects *on average* approximate 18% gray, then
the scene will be properly exposed.  But if the meter is seeing way more
(or way less) bright sky, for example, than the camera is, the scene may
well end up under- (or over-) exposed.

There is another form of metering called "incident" metering.  Incident
meters have a hemispherical dome over the sensor.  The dome integrates
light coming from 180 degrees around the direction in which it's
pointed.  To take a proper incident light reading, you bring your
incident meter to your subject, and point the center of the dome in the
direction of where the camera will be.  The idea is that, instead of
measuring the light which is reflected back to the camera (an indirect
reading which is influenced by the subject itself), you are measuring
all of the light striking the subject that the camera is going to see,
directly.  Since an incident light reading is independent of the
subject, no compensation is needed for the subject's reflectance.
However if the subject is in different light, and you cannot physically
get there to take an incident reading, a reflected reading may be your
only choice.

"Spot meters" are a special kind of reflectance meter, with optics to
narrow the field of view.  Think of them as reflectance meters with a
telephoto lens.  In order to circumvent the subject averaging and
field-of-view mismatch problems I mentioned, which are particularly a
problem with telephoto lenses, meter manufacturers came up with the idea
of letting you reflectance-meter a small area of a scene.  The area
metered will expose as 18% gray.  If there are people in the scene, the
obvious choice is to spot-meter skin.  Otherwise, we are back to the
photographer's judgment as to what in the scene should be rendered as
18% gray.  They do, however, let the photographer make that choice
instead of the meter.

Meters aren't "magic", and they don't guarantee you proper exposures
unless you understand how they work and how to best use them.  I'm a fan
of meters because they eliminate guesswork in situations where sunny-16
doesn't apply (theaters, sports arenas, night shots, etc.).  In
situations where it *does* apply, it's useful to know and use as a
"sanity check" for what your meter is telling you.

     -Greg W. (gjw@xxxxxxxxxx)







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