The 18% myth

["Lennart Rydstrm" <clrphoto@xxxxxxxxxx>]

THE 18% MYTH

To Helmut, Cameron and Jim who showed interest.

What I refer to when I state 18% gray to be a myth is the widely spread common statements:

1.  To provide proper exposure, take a reading from a l8% greycard.

2.  Anything you take a reading from will be rendered as l8% density.

3.  Zone 5 corresponds to l8% gray.

Unfortunately l8% is the wrong shade of gray for photographic purposes.
It is NOT properly related to ISO.

Some definitions.
Std BLACK reads 1.6 in a densitometer and
std WHITE reads 0.1.
Range black to white: (1.6 ./. 0.1) =1.5 = 5 full stops. (5x0.3=1.5).

The exposure meter.
If we, theoretically, divide the meter into two parts:
1.  the photodiod (cell) and
2.  the calculating dial
we find: 

The explanation of the 18% myth.
The photodiode is a device which cannot know what it is aimed at.
It cannot differentiate for eg. light from shadows. 
It just reads the amount of light falling upon it - in a 5 step luminance scale.
The "output" value is displayed with som kind of pointer.

Should a meter require service, to function with absolute accuracy,
the cell output is adjusted against a 18% gray reference.

In the good old days
you had to manually transfer a value from the pointer to the calculation wheels.
Today it is automatically done.

When transferred, the "5-step pointer value" is recalculated to a value equal to
the exact mid tone in a 7 step grayscale. READ THAT AGAIN!

In Practice:

A.  Taking a reading from a 18% gray card renders a "print tone" different
from 18%.

B.  Taking the reading from a referens equal to the mid tone in a 7 step grayscale
      renders an unchanged outcome.

Taking the reading in an open shadow (in the subject area) will be congruent
to the old rule:
Expose according to the shadows.

This is 100% applied ISO. 
You control your camera settings to a fully predictable result. For every
single frame!

Why I am "lyric" about the CLR system?
It works with great accuracy in all light conditions. Whether one understand
it or not.
I do not need to pay a lot attantion to "exposure" any more. It just works.

The package contains two cards.
The first card is the one I described obove. 
The second card provides accurate exposure for "C-41 and E6" films, 
where highlight WHITE is the point of reference.

You will find CLR homepage on http://algonet.se/~clrphoto/

A double click on the CLR head logo opens up a compendium with full description.
Unfortunately its given in swedish. Nonetheless the pictures and diagrams
will be
very instructive.

Cappuccinotime . . .
- -Lennart R

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