Digital BW, The Print

Just for the fun of it I read my trusty Kodak gray card with
my color mouse and, as one might guess, it's accurately middle gray but 
not
a neutral gray mine showed itself to have a red cast.

the numbers were  123, 114, 113

admittedly, my card is several years old, but I now know it is
still trustworthy for using with my spot meter.

J.Z.

Interesting Jon.
My thoughts.
A neutral middle gray would be 128/128/128
Isn't a Kodak Gray Card 18% gray? Then, it should read 184/184/184
Camera meters are calibrated to 18% gray, no?

John Toles
http://www.dragonflyprinting.com/
http://www.dragonflygallery.ca/

On Jun 21, 2004, at 4:18 PM, Jon Zax wrote:

> Just for the fun of it I read my trusty Kodak gray card with
> my color mouse and, as one might guess, it's accurately middle gray but
> not a neutral gray mine showed itself to have a red cast.
>
> the numbers were  123, 114, 113
>
> admittedly, my card is several years old, but I now know it is
> still trustworthy for using with my spot meter.
>
> J.Z.

(Try not to beat me up too bad on this brief, and less than thorough explanation, pls.)
 
I believe:  Kodak, or someone, many years ago figured that an 'average' scene being photographed would reflect 18% of the light falling on it.  In order to capture as much information as possible in both the high and low values, one would want to to expose for the mid-point between the metered high and low values.  However, since all of the early reflected light meters were 'averaging', and the average scene reflected only 18% of the light, it was necessary to have some compensation in the metering system that would properly place the exposure at the 50% point, instead of the 18% which would cause overexposure.  As far as I can tell, the vast majority of all light meters that are of the 'averaging' type (either incident or reflected) follow this scheme to this day.  That's why, if you take a 'spot' (or even a close up reading with a reflected meter) reading of the 18% card and use it as the mid-point for your exposure, you will get an exposure that is 'off' by about 1 1/2 stops, when
 compared to an incident exposure meter reading. 
 
If you place an 18% card in a properly exposed scene, it should show an 18% (or so) reflectance.  But, as we all know, an 18% gray card can be exposed to produce any value we want on the final image.  The Kodak gray card is just a quite good, reliable reference for most of us engaged in 'general' photography.  Even with a few good meters, I always feel more comfortable having an 18% card around to help verify what I'm measuring.  And, having an 18% gray card in an image can help when evaluating a final print, b&w or color.
 
Of course this is all theoretical because of the imperfections/variations of the capture media (positive/negative film, digital, etc.), but it is very useful.  It's kinda' where the Zone system evolved from.
 
 
 
Tom Baker

Dragonfly Imaging & Printing <dragonfly.printing@sympatico.ca> wrote:
Interesting Jon.
My thoughts.
A neutral middle gray would be 128/128/128
Isn't a Kodak Gray Card 18% gray? Then, it should read 184/184/184
Camera meters are calibrated to 18% gray, no?

John Toles
http://www.dragonflyprinting.com/
http://www.dragonflygallery.ca/

On Jun 21, 2004, at 4:18 PM, Jon Zax wrote:

> Just for the fun of it I read my trusty Kodak gray card with
> my color mouse and, as one might guess, it's accurately middle gray but
> not a neutral gray mine showed itself to have a red cast.
>
> the numbers were 123, 114, 113
>
> admittedly, my card is several years old, but I now know it is
> still trustworthy for using with my spot meter.
>
> J.Z.




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[Non-text portions of this message have been removed]

> From: Dragonfly Imaging & Printing
> 
> Interesting Jon.
> My thoughts.
> A neutral middle gray would be 128/128/128
> Isn't a Kodak Gray Card 18% gray? Then, it should read 184/184/184
> Camera meters are calibrated to 18% gray, no?

In a gamma 2.2 color space, it would read 117,117,117.

-- 

Ciao,               Paul D. DeRocco
Paul                mailto:pderocco@...

> > From: Dragonfly Imaging & Printing

> > Camera meters are calibrated to 18% gray, no?
> 
Are they really ? I've read reports that they were actually 
calibrated at around 12% grey with people expecting 18% grey.

http://www.bythom.com/graycards.htm

Cheers,
Andre