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RE: [OM] (silly?) incident metering question

Subject: RE: [OM] (silly?) incident metering question
From: Brian Windrim <brian@xxxxxxxxx>
Date: Mon, 30 Aug 1999 14:07:32 +0100 (BST)
Acer Victoria asked:

> Since reflected meters assume scene is 18 grey, they have to be
> over-ridden in special cases (a la the 4s shadow and hilite button), yes?
> What about incident? Are they immune to this kind of error?

A reflected-light (e.g. spot) reading taken off an 18 0ray card
should be exactly equivalent to a reading from an incident-light meter
at the location of the card.

Incident-light metering therefore suffers from the same limitations as
gray-card reading, in that it fails to take the subject's tonal range
into account.


> Say, I'm in a valley, and there are black shadows under some
> overhangs and sunlit rocks in the general scene. Reflected metering
> will try to render it all 18%, so the lucky 4 user takes a spot off
> the shadow and hits shadow on the body to u/expose by 2.x stops
> getting proper black shadows.

Yes, but at the risk of overexposure of the sunlit areas if the
shadows are more than the 2.x stops darker. This is particularly risky
with slide film (where white-out must be avoided at all costs).


> (or, the smart non-2S/3/4 users simply does it manually <G>).

I've been using OM4 for 12 years and I can't remember ever using the
hilite/shadow buttons (except maybe highlighting on snow). If the
light is *at all* difficult I'll flip to manual where I can see all of
the important tonal values at the same time.

In the case of your hypothetical scene, this would involve spotting of
both the sunlit rocks and the shadow areas and then seeing if both
values can be fitted into (say) the +1 to -2.5 range.

Compromise may be required, but the manual mode display lets you
choose an optimum.

Don't get me wrong, the hilite and shadow buttons are an important
move away from the tyranny of 18 0ray, and may well be useful if
you're in a real hurry (I seldom am). It should be noted, however,
that their bias values (+2 and -2.5 respectively, I believe) are
chosen to match the working tonal range of typical colour slide films
and may not be optimal for negative films (wider range) or for
high-contrast films like Velvia (narrow range).


> What would an incident say?

It doesn't know a thing about your shadows, or anything else except
the light falling on it.

Incident metering tends to be used where colour accuracy is
all-important and where the subject's tonal range is controlled by
lighting, for example in fashion photography.


-Brian

PS. I would recommend Ansel Adam's "The Negative" for anyone
interested in a better understanding of these issues. It opened my
eyes.


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