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# Thread: Medium grey 17%, why not 50?

1. I asked several photofriends this question: why do we call a 17% grey card "medium grey", why is it not 50%? Untill now I did'nt get a satisfactory answer. Anyone clear the fog?

2. I believe that medium gray—halfway between white and black—reflects 18% of the light falling on it. That’s why its referred to as an 18%-reflectance gray card.

3. Is that because even pure white isn't 100% reflective?

4. I wondered about this for a long time. I read somewhere, it is 18% because it is a geometric center. I'd really like to see an equation for this.... Maybe one of APUG's heavy hitters will join in and explain it to us. Please?

5. It's 18% gray. Of the zones that are discernable on exposed film and paper (B&W specifically) Zone V is at the center. This zone reflects 18% of the light that falls on it. As it is situated in the middle of exposure/print zones/values, it is chosen as a reference point upon which to base exposures should you choose to do so. That's why a card that reflects the same amount of light as a Zone V exposure is referred to as an 18% gray card.

6. The average scene reflects 18%. Metering an 18% card will, on average, produce the same meter reading as metering the actual scene.

You may want to meter a card rather than the scene if the scene is very dark or very light - avoiding the problem that white cats and black cats both come out 18% grey if you depend on the meter reading.

7. I tell students that if one took all the shades of grey that are in the average scene and mixed them up like paint, the resulting shade of gray would be "middle gray" and have an 18% reflectance...and that is what our meters assume the scene to be.

Don't know if this is 100% true, but it seems to satisfy them...LOL!

8. Well, the average outdoor scene supposedly integrates to 12.5% gray and that is why light meters are calibrated to that number.

18% gray was, IIRC, picked because it is "middle gray" and the progression of grays is a geometric one, with each step being twice or half what the previous one was.

If you carefully read Kodak's instructions on how to use an 18% gray card, you will see that it accounts for the difference between the 12.5% and the 18% numbers.

9. Originally Posted by tkamiya
I wondered about this for a long time. I read somewhere, it is 18% because it is a geometric center. I'd really like to see an equation for this.... Maybe one of APUG's heavy hitters will join in and explain it to us. Please?
Regardless of what the average scene reflects or what lighmeters are calibrated to, it all has to do with how our eyes compare different brightness levels. The human response to reflection (lightness) is not linear. For example, a surface reflecting 18% of the light that falls onto it, is perceived as being only half as bright (50%) as the illumination itself. The response follows the following equation:

L =116*(R)^(1/3)-16

where L and R are the lightness and the reflection in % respectively.

For example, use R=0.18 (18%) and L will return 50%.

10. Originally Posted by RalphLambrecht
L =116*(R)^(1/3)-16
Of course! All makes sense now ;-) Sorry, just kidding. The explanation was great - the maths just did my head in!

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