Why is Zone V 50% gray?

[Ralph W. Lambrecht]

Not bad, not bad at all.

[Ralph W. Lambrecht]

Guys and Gals

I like to thank everyone who has participated in trying to solve this. Your posts have all been valuable to me. I have collected enough info to go back to the drawing board, have designed a few tests to shed some light on this issue and will share the results as soon as the data is evaluated.

Many thanks again.

[Helen Bach]

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?

It's important to be absolutely clear about what you mean by 50% grey.

An 18% grey is considered to be perceptually 50% (linear vs logarithmic), as I've already explained, and that 50% is what you appear to be referring to because the common colour spaces used in still photography use perceptual values and so are non-linear, not linear. It is perfectly correct.

What the camera meters to depends on the meter calibration constant. For example, for some reflected-light meters and 35 mm film cameras the constant may be equivalent to metering off a 12.5% grey (many complications to that simple statement).

I've only explained (or at least I hope that I've explained) why a monitor displays it 'lighter' than the original: we haven't really got on to prints yet.

Is there a problem with my explanation of why 18% is also correctly represented as 50%, or why a monitor set to a gamma of 2.2 displays '18% grey' lighter than you think it should be?

Best,
Helen

[robc]

If there is a problem, then it is because I always thought that a Kodak 18% grey card is not a mid grey (50% lightness grey). This stems from understanding that it is derived from tests to determine the average luminance of a typical scene and not the the mid point of a perceptual grayscale. Your statement "An 18% grey is considered to be perceptually 50% (linear vs logarithmic)" is at odds with that.

Do you have references stating that an 18% grey card is a 50% lightness grey?

[Ralph W. Lambrecht]

Helen

Your explanation is absolutely fine. It helped me to understand some of the relationships. Many thanks for it. I'm going to try some transfer function now. I will also try an ICC profile, which Richard is offering to make on his RHDesigns website.

This issue is more complex than anything I have encountered in photography ever.

Thanks for the help.

[Helen Bach]

I don't have a lot of confidence in my ability to write in an easily-understood manner.

"Do you have references stating that an 18% grey card is a 50% lightness grey?"

Well, if you convert a reflectance of 0.184 (18.4%) to CIELAB L* it comes out as 50.

If you go to Bruce Lindbloom's site, http://brucelindbloom.com/ then > Calc > Companding Calculator and enter 0.1842 into the 'Y' input, you should get L* = 50.000 out - in fact I think that they are the default values when the calculator window opens. Fish around on Bruce's site and you will find plenty of other info.

Munsell N5 - which is middle grey on the Munsell scale - has 19.8% reflectance, so that isn't quite the same, but it is close. It's based on a slightly different method than CIELAB L*a*b*, having an L* of 51.6 insteads of 50.

If you want proper printed-on-paper references I'll fish some out.

As far as 'average scene reflectance' goes, just because 18%, or thereabouts, is perceptual middle grey doesn't also stop it from being the 'average scene reflectance'. Or it might not be. But that is such a can of worms.

Best,
Helen

[robc]

Thanks,

I will read up on it.

[Ralph W. Lambrecht]

I don't have a lot of confidence in my ability to write in an easily-understood manner.

I think you're doing just fine.

If you convert a reflectance of 0.184 (18.4%) to CIELAB L* it comes out as 50.

If you go to Bruce Lindbloom's site, http://brucelindbloom.com/ then > Calc > Companding Calculator and enter 0.1842 into the 'Y' input, you should get L* = 50.000 out - in fact I think that they are the default values when the calculator window opens. Fish around on Bruce's site and you will find plenty of other info.

Munsell N5 - which is middle grey on the Munsell scale - has 19.8% reflectance, so that isn't quite the same, but it is close. It's based on a slightly different method than CIELAB L*a*b*, having an L* of 51.6 insteads of 50.

Very valuable info. Thanks for that.

If you want proper printed-on-paper references I'll fish some out.

Yes, please, please, please.

As far as 'average scene reflectance' goes, just because 18%, or thereabouts, is perceptual middle grey doesn't also stop it from being the 'average scene reflectance'.

I think you hit the nail on the head here. There might be no argument after all. Two approaches coming up with the same result. Perfect.

[Paul Coppin]

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.

Crap. I thought I understood computers and even tonal relationships quite well, but apparently, I'm a dolt.
Did I just understand that if I wanted to represent, on the monitor, the actual tonal value of an 18% gray card (or its equivalent reflectance) I should have the monitor set to a gamma of 2.47...? (this may explain some issues I have with print density, relative to monitor, or, yes, truly, I am a dolt... )

[Ralph W. Lambrecht]

Yes, or accept that the monitor shows a gray card lighter than in real life. BTW, print your monitor image, that might get the gray card back where you want it. So much for prints that match the monitor. They call it a fully 'calibrated system'. But that's hype. One can't match the two precisely, but one can get close, if sacrifices in the shadows are made. Theoretical gamma goes to infinity, actual monitor gamma still outperforms any output on paper. Analog has the same dilemma when trying to matcha print to a slide.