Why is Zone V 50% gray?

[robc]

The obvious has just occurred to me. (I think).

to answer your straight question:

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

Of course it will render a Kodak 18% grey card as K50%. It would render a 70% grey card as K50%. And it would render a 20% grey card as K50%. Regardless of orientation of the card.

There is a light meter in the camera and, as we all know, camera light meters give enough exposure to render what ever they are looking at as a mid grey. If that always turns out to be K50% then that is correct. The camera does not know what reflectance or colour your grey card is.

And by the way, an 18% reflectance card is only approx 2.5 stops less than 100% reflectance and therefore on a 10 stop scale it is between zone VII and VIII. Only on a 5 stop scale is it the midpoint.

[Helen Bach]

I'll put this as a separate post to keep it simple (well, as simple as I can make it) and expand a little on what 50% means as a measurement of lightness.

The 18% reflectance of a grey card is not 50% grey, but it is 50% lightness. Light is a perceptual property, and measurements of it often attempt to reflect human perception. Sometimes it is done with a spectral response curve (lumens etc) and sometimes it is done with a non-linear response curve, as here with 'lightness' or whatever L you want to call the L of Lab.

18% grey is 18% reflectance. It is a simple physical property, with no perceptual factors.

That's why 18% is also 50%.

Best,
Helen

[robc]

I missed your previous post.

Converting your sRGB(or AdobeRGB) file to Lab mode gives you the grey as L=50. 119RGB.

[Helen Bach]

Converting your sRGB(or AdobeRGB) file to Lab mode gives you the grey as L=50. 119RGB.

I must be explaining myself very badly if you need to mention that.

"18% grey, in sRGB, should have an RGB value of about 117 to 119. It would show as L=49% or 50%"

Oh poo.

[Don Bryant]

Helen

Thanks for your patience. Yes, I meant K=50%. I hope I didn't refer to reflectance density when talking about the monitor anywhere. I measure the relative monitor brightness with a densitometer, set to reading transmission and zeroed at K=0%. When you do that, the 50% patch reads 0.66 'density' for a gamma 2.2 monitor, which is exactly 2.2 stops darker than the 0% patch.

I'm curious to know what kind of instrument you use to measure your monitor values. Notice that I didn't say reflectance since reflectance values of a monitor make no sense.

[robc]

well I created a test file in PS with a gradient from black to white. In sRGB mode the mid point of the gradient(400 pixels along an 800 pixel image) is 128RGB. At that point the RGB brightness is 50% and L=54. Converting the image to Lab mode changes L to 50 and RGB to 119 B(brighness)=47%

I was just playing to see if understand this. I think what I did was what you said.

[neil poulsen]

It comes down to how you metered. You indicated that you delegated the job to Mr. Nikon. Were you using spot metering? Did the gray card fill the frame?

[Ralph W. Lambrecht]

I'm curious to know what kind of instrument you use to measure your monitor values. Notice that I didn't say reflectance since reflectance values of a monitor make no sense.

I use a regular densitometer set to transmission mode. Mine is from Heiland, but I did it with an Agfa densitometer before and see no difficulty is using any other brand.

[Ralph W. Lambrecht]

It comes down to how you metered. You indicated that you delegated the job to Mr. Nikon. Were you using spot metering? Did the gray card fill the frame?

Neil, I think we've been there. The metering is not the issue. Use a white wall and you get the same result. The point is the camera puts an average reading at 50% gray and prints it lighter than the density of a gray card. Therefore, a gray card is not accurately represented by a digital system, and the question is WHY?

[robc]

I need some clarification here.

First, is a grey card a true middle grey? If so then where's the proof. Ralph has already said:

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.

If you don't think its a "mean or medium on a grayscale" then why do you expect it to be reproduced the same as its colour when you know that a light meter will change it to a middle grey in the sRGB colour model?