Why is Zone V 50% gray?

[Ralph W. Lambrecht]

Why does a digital camera record a Kodak Gray Card (Zone V) as 50% gray?

A Kodak Gray Card represents an 'average' scene of 18% reflectance, but that has nothing to do with a mean or medium on a grayscale. A gray card has an absolute log reflectance density of about 0.75. Medium gray on a 2.2-gamma monitor and a calibrated print thereof is about 0.66. So the image of the card is lighter than the card itself. Why? Who came up with this assumption or standard? Is it a mistake? Did someone assume 'average gray' meant 50%?



Disclaimer:

I'm aware that this is a mainly or exclusively analog forum, but I doubt to get a satisfying answer for my question in a digital forum, since they usually understand little about the Zone System. That's why I post the question here. Besides, I need this to work out some details about making digital negatives, which will be used to make pure-silver contact prints. So, it's an analog question in a way.

[Eric Rose]

There is a company that produces "gray cards" for digital cameras. Evidently digi cams do not read the usual cards we use properly. Check out this link:
http://www.betterlight.com/pdf/white...gray_cards.pdf

[Ralph W. Lambrecht]

There is a company that produces "gray cards" for digital cameras. Evidently digi cams do not read the usual cards we use properly. Check out this link:
http://www.betterlight.com/pdf/white...gray_cards.pdf

Many thanks, but that's not the point. The question is, why does it place a regular gray card on 50%, where it ends up at a lower density (Lighter) than the card?

[robc]

A grey card is 2.5 stops less than 100% reflectance. So go do a little test and see how many stops less than 100% reflectance your gray in the print is. i.e. zero your densitometer on paper base white and then see how many stops less the print of the grey card is. Let us know how you get on.

[edit]

re-read the question and I now see that was your test.

sRGB specifies a screen luminance of 80cd/m2

[/edit]

[Ralph Barker]

It's set for the factory-mis-calibrated monitors of "average" users?

[Ralph W. Lambrecht]

Relative screen luminances are set by the gamma value of the monitor. The gamma value defines the relative luminance of the midpoint. At gamma 2, the 50% patch is 2 stops darker than the 0% patch.

[Ralph W. Lambrecht]

It's set for the factory-mis-calibrated monitors of "average" users?

Is this your assumption, or do you know for sure? Really interested!

[Helen Bach]

Ralph,

When you say that it is represented as '50% grey' what does that mean, and in what colour space?

If you are looking at it in say sRGB or Adobe 1998 then the numerical representation will reflect the non-linearity of the conversion to the colour space. There is then a reverse non-linear conversion from the numbers to the print or display.

So 18% grey can be represented as L=50 in Lab (and somewhere between 110 and 128 in RGB, depending on the colour space) while still being identified correctly as 18% grey, and print or display as 18% grey, or thereabouts depending on this and that, but mainly the other.

Does that make sense?

Best,
Helen

Later:

I've re-interpreted your question. I think that the above was not quite what you wanted, but I read two different questions, I think.

In a print the density of an L=50 patch (ie 50% in the image file) depends on the D-max of the print: it is relative and not absolute.

[Ralph Barker]

Is this your assumption, or do you know for sure? Really interested!

It was an attempt at humor. I'm also curious to hear the real answer.

[Ralph W. Lambrecht]

It was an attempt at humor. I'm also curious to hear the real answer.

I'm afraid, it might be all wrong! We'll see.