Re: [OM] 18 0rey Card

[Lars Bergquist]

>Expound away!, by all means.
>
>Giles
>> Note that incident or grey card metering is never misled by overly dark
>> or light parts of the subject, by light sources such as the sky in the
>> picture field, etc. So with reversal film, the critical highlights are always
>> correctly exposed. (If there is enough interest, I could expound at lenght
>> on this).

Well, I presume I will have to take the time.

Incident light does vary, but we compensate for that of course in order to
have correct exposure. Otherwise, life would be simple and it would be true
as one wag stated, that "f/5.6 and 1/125 is always the right exposure"! So, let
incident light be = 100 %.

Subjects nevertheless vary in reflectance. After decades of experimentation
however, exposure meters now assume (at least in principle) that the average
subject has a mean reflectance of 18 %. Hence the 18  0rey card, which
then should give the same reflected reading as an incident reading made 
in the opposite direction. I assume that you know that in incident metering,
you measure from the subject toward the camera. I know that there are other
schools of thought, but that is my school.

However, a snowscape has a different reflectance from a coal cellar
(assuming that you can still find a coal cellar). What is the range of
possibilities?

A matte purely white paper is representative of a  diffuse highlight, which
should be rendered with some detail, at least potentially. This reflects a little
more than 80 %. Anything which reflects more than that is a specular
highlight (a Spitzlicht in German, spetsdager in Swedish) which is
essentially a mirror image of a light source, such as the sun, and should be
rendered as absolute flat white (base + fog). Now, 80 % reflectance would
represent an exposure about 2 1/3 stop above par (which par is "Zone V" in
the Anselist religion). With reversal film, this places the diffuse highlights
just below the shoulder of the film's characteristic curve, where detail begins
to fade away or "block up". With neg film, there is still lots of leeway, but
here, the limiting factor is the curve (i.e. contrast range) of the printing
paper.

So, if we can ascertain what 100 % represent, we automatically know where
the 80  0iffuse highlights are, and we have correct exposure! And this is
of course exactly what we do when we meter incident light. With this
exposure, all other reflectances from light to dark are represented reasonably
correctly -- subject to the errors introduced by the fact of life that the
representative curves of films or papers are curves, not straight lines.
There is nothing to worry about.

The situation is essentially the same with shadows. Black velvet reflects
2 -- 3 %. This is a little more than 2 1/3 stops below 18 %. Both reversal film
and also neg film given a minimum exposure start their curve toes here,
meaning that detail begins to give. It is of course important that in both
slides and prints, black is rendered as black ("good Dmin and Dmax",
respectively). The shadow end has of course no absolute end value, as
in the highlights where it is 100 %. Say that with a spot meter you are
surveying a subject which is a front-lighted house. The white trim should
be 80 -- 85 %, OK. And that open basement window should be black, as
this is the mythical coal cellar. But hey -- there's a blonde in a black
velvet dress in that cellar! In other words, reflectance can asymptotically
approach 0 (and the deepest shadow tone your spot meter measures
depends on the acceptance angle of said spot meter. A .1 degree meter
would find deeper shadows than a 1 degree meter, which is of course just
a one-degree averaging meter! In other words, a spot meter makes more
problems than it solves, while an incident meter solves nearly all problems).

With neg film of course, the only important thing is that both diffuse
highlights and important shadow areas have printable, that is sufficiently
contrasty detail. Then you print to the paper's Dmax (deep black) and
near-Dmin (nearly white, reserving real Dmin for specular highlights).
A good B/W neg film, and most colour neg films, can take at least 3 stops
above minimum without losing highlight detail. Up to 1 -- 1 1/2 stops over,
shadow detasil also improves. Remains only to find a lab which will print
the thing right!

Vänliga hälsningar/Best regards
Lars Bergquist
Välkommen till/Welcome to ...
<http://www.bahnhof.se/~timberwolf/>

<thismessagewasdeliveredviatheolympusmailinglist> <forquestions,mailto:owner-olympus@xxxxxxxxxxxxxxx> <webpage:http://zuiko.sls.bc.ca/swright/olympuslist.html>