[Leica] grey cards

[Erwin Puts <imxputs@knoware.nl>]

Much confusion about greycards still do exist. This is a post I sent 
to the Lug more than a year ago.


The facts: the greycards from Kodak, Fotowand etc do reflect 17,8% of the
incident light. Any densitometer reading will confirm this. I have used many
different cards and densitometers and they all agree. Now this 17.8% is
certainly not middle grey, that would be 50% reflection. So the designation of
the card as a midgrey card is wrong. In fact the grey value is a darker grey.
BUT: CIELAb measurement of the same grey card show that the perceived
luminance value seen by the famous 'standard observer' is 50% luminance. In
this respect the designation of the grey card as mid grey is correct. (seen
from the pschometrical value of the eye).
BUT: if you analyse the typical scene photographed by many persons (landscape,
persons on a beach, scantily dressed ladies) you will notice (by statistical
analysis and Gaussian curves)that the typical scene has a reflectance value of
13%. SO Bob Shell and Kodak are also right. The Kodak instructions will tell
you that the use of the grey card should ideally be restricted to studio work,
(the ladies again)as there the lighting conditions are more appropriate for
this card's reflectance. This knowledge is not new and could be gleaned from
the Kodak leaflets 20 years ago.

The reflectance value of a card and/or a real scene is one side of the
equation. The other is is the calibration of the exposure meter. I have tested
all meters (Gossen, Seconic, Minolta) on a calibrated Gossen test bench (of
course!) and noted a variance of about one stop. That is partly explained by
the K-Value Walt mentioned. BUT: official instructions for calibrating meters
do not exist. The only recommendation is to calibrate in such a way that the
greycard density on the negative (D=0.75)will be placed somewhere in the
middle of the straight line portion of the characteristic curve.
All this depends on the development and exposure process. In general handheld
meters conform to this advice (and I never saw a bigger difference than 1
stop). Most meters are however adjusted and calibrated for about 3800 Kelvin
as this is a single exact value. White light as we all know can vary from 5500
K to 12000 K and is therefore more difficult to calibrate.
Therefore white light measurements may vary.

Incamera metering is more flexible as there are no industry agreements as in
the handheld sector. Any camera manufacturer will adjust the metering to suit
the perceived taste of its prospective users. (biased to transparancy etc).

When using handheld metering we are on relatively safe ground. Calibration is
known and a slight adjustment of EI and/or development times will do.
In camera metering needs a more prolonged user adjustment programme. But that
is so nice about our technology.


Erwin