Light meters, either built-in or hand-held, indicate the amount of exposure
to render the metered scene to an 18 0rey. It doesn't matter if its a spot
meter, or center-weighted average meter or what - 18 0rey is the
"yardstick" (meterstick???). It also happens that a grassy field is
approximately 18 0rey, or the palm of your hand. There's many examples.
If you meter an 18 0rey card, then the "scene" exactly matches what the
meter is calibrated to. This works well if your subject is in the same
light. The problems start when your actual subject is much lighter or
darker than 18 0rey. Snow is a good example. You meter the scene and the
meter indicates the exposure needed to render the scene at 18 0rey.
However, the snow is much brighter. To compensate, you need more exposure,
2-3 stops worth. Conversely, metering a coal pile will indicate the
exposure needed to render the coal at 18 0rey; 2 stops (or more) *less*
exposure is needed to make the coal truly black in the final print. If you
metered a grey card in either of these extreme cases, then metered the
scene, the meter would indicate over/under, but the result would be spot on.
The other case that drives many of us nuts is harsh backlight. Again, the
meter is fooled by a dark subject surrounded by bright background. Even if
you spotmeter a subject in a black suit, you have to adjust for the
difference in reflectance to 18 0rey, and adjust the exposure accordingly.
This is a simplistic explanation. For real details, search the web for
information on the Zone System.
John P
Konrad Pizzuto <pizzuto@xxxxxxxxxxxxxxx> wrote:
>
>I've heard about the 18 0rey card and that camera metering systems
>calculate the light received from an object as that coming from
>something of 18 0rey reflectivity. I'm not sure I've got the whole
>concept right... could someone be so kind as to explain the whole
>concept... possibly in relation with the OM-10 metering system as an
>example.
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