[Leica] Re: TTL metering and incident light

[Jim Brick <jim@brick.org>]

John Hudson wrote:

>>Having only very minimal experience of TTL metering I am wondering how the
>>R series TTL meters "address" the issue of incident lighting conditions.>>>

Actually, the R camera sees only the reflected light. It has no knowledge
of incident light. If you take a Styrofoam coffee cup and stick it over the
lens, then turn the camera around, pointing toward where you will take the
picture from, The R camera will then read the incident light. The light
"falling" on the subject.

Incident light falling on a subject has no knowledge of the subject. Black,
white, red, green, doesn't matter. It's the same amount of light hitting
the subject. But the R camera sees what is reflected off of the subject. If
the subject is black, not much light is reflected and you get a low
reading. If the subject is white, a lot of light is reflected and you get a
high reading. So reflected light readings can be a mine field. YOU, the
photographer, have to be intelligent about what you are reading and make
necessary adjustments. If you use an incident meter, reading the light
falling on your subject, generally speaking, no adjustments are necessary
because the incident meter does not "see" the black or white subject.

Take a reflected reading of a snow scene and you will get gray snow. Take a
reflected reading of a black suit and you will get a gray suit. Take in
incident reading of (at) either, and you will get white snow and/or a black
suit. A man wearing a black suit standing in the snow. Use an incident
meter and the snow will be white and the suit will be black.

You say, "what about the bright reflected light, off of the snow, hitting
the incident meter." "Won't that throw it off?" No. That bright reflected
light is also hitting your subject and is considered a source of incident
light.

When you use a reflected meter (M, R, Weston, Pentax spot, etc.) you have
know exactly what it is that you are taking a reading of. It's reflective
brightness in relation to the rest of the scene. Generally speaking, for
chromes, an incident meter will give more consistent results as it is not
fooled by highly reflective or black hole subjects.

There is, however, no panacea. Intelligent evaluation of your subject, for
correct exposure, is always a requirement.

Jim