[Leica] Grey cards

[imxputs <imxputs@knoware.nl>]

Much confusion here it seems.
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 refectance 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 conditins 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 calibarting meters 
do not exist. The only recommendation is to calbrate 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 
meers 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 sing 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