Why is Zone V 50% gray?

[Ralph W. Lambrecht]

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

Gray Gamma 2.2 or 1.8, whatever the monitor is set to and what is tagged to the file.

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.

Yes, I understand that.

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?

Nope, lost you there. 50% always seems to be 50% in my tests, as long as the attached color space matches the file's intention.

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.

That's a biggy!!! And it might depend on the Dmin too? Oh, I'm getting worried now. It does come down to the absolute vs relative discussion again, doen't it?
But for now, let's stick to the monitor's representation. Why not 0.75 for Zone V? It can do it, and Dmin and Dmax support it comfortably. It this whole thing is really relative, then Zone V should actually be darker than 0.75 on a monitor, because the monitor's Dmax is far beyond the print's Dmax.

[Helen Bach]

OK, my head is beginning to spin with all these numbers.

When you say 50% grey, do you mean K=50%? I guess that because you say that 50% is two stops darker than 0%. Can we determine which value we are using before continuing? How about using the L of Lab?

How are you determining the 'reflectance density' or Dmax of your monitor when you say that it is 0.66 for example?

Thanks,
Helen

[Brian Ellis]

Like everyone else here with the exception of Helen, I don't understand the question very well and I certainly don't have a clue as to the answer.

[robc]

OK, my head is beginning to spin with all these numbers.

When you say 50% grey, do you mean K=50%? I guess that because you say that 50% is two stops darker than 0%. Can we determine which value we are using before continuing? How about using the L of Lab?

How are you determining the 'reflectance density' or Dmax of your monitor when you say that it is 0.66 for example?

Thanks,
Helen

I think he means hex 808080 or R128 G128 B128 which are 50% brightness from black.

I could be wrong though.

[Ralph W. Lambrecht]

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.

[Ralph W. Lambrecht]

Correct.

[robc]

when you say a digital camera records a grey card as 50% I think we need some more information:

1. did you set the grey card at the correct angles to make the exposure? i.e.

Kodak’s instructions which come with a grey card say:

“Position the grey card in front of and as close to the subject as possible. Aim the surface of the gray card toward a point one third of the compound angle between your camera and the main light. For example, if the main light is located 30 degrees to the side and 45 degrees up from the camera to subject axis, aim the card 10 degrees to the side and 15 degrees up.”

This means you need to buy a sextant to use a grey card as it is meant to be used

2. How were you metering? If you were spot metering from the camera then ignore 1 above because it becomes irrelevant except you need to know the K factor for the camera meter. i.e. how it deviates from a true reading with no adjustment.
If you weren't spot metering from camera then 1 above may not be irrelevant. Thats why we calibrate film speed. You may need to calibrate camera ISO.

3. Are you talking about all digital cameras and have you tested them all?

[Helen Bach]

Everything seems to be in order. With a gamma of 2.2, 50% input brightness is 2.2 stops down in output from 100% brightness, representing an equivalent 22% reflectance. If you wanted 50% input brightness to be 2.47 stops down from 100% (ie representing 18% reflectance) then you would need a gamma of 2.47.

But, on the other hand, you want the display to show shadow detail as well. For a constant gamma (albeit at different values) and a fixed display dynamic range, the lower the gamma the greater the input range you can display. So the greater the dynamic range of the display device, the greater the gamma can be. All to the greater gamma, so to speak.

Of course, gamma need not be constant. In reality you can use a Tonal Response Curve in place of a constant gamma.

How's that?

Best,
Helen

The business about gamma also determines where 18% grey is. If you use a gamma of 2.2 (or if the colour space effectively uses it when converting the file, not just tagging it) then 18% grey should be about 117 in RGB, not 128. So if you take a snap of a grey card with your digicam and it gives you a file with the card at a value of 117, you know that the gamma is effectively 2.2

[Ralph W. Lambrecht]

Kodak’s instructions which come with a grey card say:

“Position the grey card in front of and as close to the subject as possible. Aim the surface of the gray card toward a point one third of the compound angle between your camera and the main light. For example, if the main light is located 30 degrees to the side and 45 degrees up from the camera to subject axis, aim the card 10 degrees to the side and 15 degrees up.”

This means you need to buy a sextant to use a grey card as it is meant to be used

Funny, and not all false, but yes, I did do it that way.

How were you metering?

I delegated the job to Mr. Nikon.

Are you talking about all digital cameras and have you tested them all?

I'm not THAT well off, but I (we) tested 5 different DSLRs from 3 different top-brand manufacturers. Same result from all.

[Helen Bach]

I was in a bit of a rush with the last post, so I didn't write much of an answer to the first part of the question about cameras.

18% grey, in sRGB, should have an RGB value of about 117 to 119. It would show as L=49% or 50% (I will forget about K and I suggest that everyone else does, because CMYK is a whole different can of worms). sRGB uses a gamma of 2.2 in effect (it actually uses 2.4, but with an offset that makes it look like 2.2). An RGB value of 128 (50% of the full input signal) represents an L of 54% in sRGB. It's a mismatch between the perceptual curve that places 18.4% reflectance at L*=50% and the colour space curve that places 21.8% grey equivalent at 50% of the full input signal. Adobe RGB(1998) is similar in that it uses a gamma of 2.2.

Now, exactly where the camera's meter places the grey card (or any card, because if it fills the frame it shouldn't matter what reflectance the card is) depends on the metering constant that the manufacturer programs in to the meter.

That's why it is important to keep track of what number represents what. Easy, isn't it not?

Best,
Helen