Re: spot, averaging, or incident metering?

Shannon wrote:

> Lately I've been experimenting with a little averaging/incident meter
> that is very small and light. I used it as an incident meter for a
> few weeks and just developed some of the negatives. A lot of them
> are either over or underexposed. I was metering in the shade, with
> the meter set at twice the speed I normally shoot the film at
>
> But since the results were rather spotty, I have been using the meter
> in averaging mode recently. This seems to yield slightly better
> results, but still not as good as the spot meter.
>
> Am I doomed to carry around my heavy spot meter everywhere, even when
> using small cameras? It seems that when I meter a scene with all
> three meters, I rarely get the same reading from even two out of
> three. I believe the spot meter, as I said, because I get very
> consistent results when I use it. But the incident and averaging
> meters only agree with it under rather special circumstances, when
> the light is perfectly even everywhere and the scene is not too
> contrasty. Like under overcast skies.

The first thing to do is see if the meters all agree. Here's one easy way
to do this: take a Kodak 18% grey card to a location in wide open shade
(i.e., not sunlit but exposed to a large amount of sky) on a cloudless day.
Read the grey card with the spot meter and the averaging meter, and the
incident light with the incident meter. (Make sure the spot and averaging
meters are close enough so that the card fills their field of view.) The
readings should all be the same. If not, the meters are not calibrated
with each other and you need to address this before trying any further
comparisons.

A spot meter is generally used to read a uniform area of scene luminance,
and allows the photographer to decide how to expose that scene area. (In
practice, one usually meters several areas and may decide to adjust
processing [N-2, N-1, N+1, N+2, etc.] and choose a print material to
"place" those subject luminance values closer to a desired print reflection
density.)

An averaging meter looks at a much larger area of the scene, which (barring
test conditions such as above) almost certainly contains a wide range of
luminance values. It does not average based on the relative importance of
each luminance value in the scene, however, but on the relative sizes
(areas) of the different luminances in its field of view. For example, if
you point an averaging meter at the horizon, roughly half of its field of
view will be taken up by the relatively bright sky, and the resulting
exposure will generally be too low. Thus, the averaging meter makes the
assumption, "The areas of all of the different important luminances in my
field of view are approximately equal." If this is true, the meter will
give good exposure recommendations. If not, it will provide poor
recommendations. (Note that you can often point the meter differently to
compensate for this -- for example, by pointing the meter down in the
previous example you might include more land that has luminance values
close to the land just below the horizon, which is a primary
compositionally important area.)

The incident meter makes the assumption, "I am in the same light that is
falling on the important objects in the scene, and the reflectance values
of the important objects in the scene are clustered symmetrically and
rather closely around 18% grey." Again, if this is true (for example, for
the mythical "average subject" on an overcast day), the meter will give
useful readings; if not, it won't. And again, you can do some degree of
adjustment by intentionally placing the meter in *different* light than
the important objects in the scene are in, but this is at best very
approximate.

Best regards,

etienne
Received on Fri Sep 30 22:14:26 2005