Middle gray vs. neutral gray... Please check my thinking.

[Abitconfused]

As I understand it, "middle gray" is Zone 5 which has a luminance of 50% but reflects 18% of ambient light. Whereas, "neutral" refers to reflected light where all wavelengths of light are equally represented (useful for white balance readings). I think "seeing" in Zones is still useful in the digital age, especially for estimating exposure compensation. Opinions?

[Colin Southern]

Hi Ed,

In real-world terms, a "middle gray" card is used to set exposure (as it's what camera metering is calibrated to), whilst a neutral gray (or "spectrally neutral" as we like to say) is in fact used for white balancing.

18% reflectance is what's commonly mentioned, but I understand that the actual figure is 13% -- but there's only a small difference anyway -- and even correctly metering from a grey card still doesn't guarantee a visually correct exposure.

[Graystar]

That's roughly correct.

I'm not a student of the Zone System, but it was my understanding that the zones themselves did not represent specific increments, and were to be perceived as relative to the media in use. Zone 5 has no special relationship to metering. As has been said, cameras and hand-held meters are calibrated to roughly 13% reflectance (12.7% or 12.8%...depending on who you ask.) This means that you have just under three stops before exceeding 100%. So I don't really know how you break up 3 stops of range into 5 divisions in a useful way. That's why you see web pages on "modified" or "simplified" zone systems with only three or four zones above the metered gray of the camera.

The WhiBal and the RMI Digital Gray Card are both neutral cards but are brighter than anything that can be called "middle gray." The ability to reflect all wavelengths equally doesn't depend on the amount of reflectance. As for "Middle gray", or at least the level of reflectance expected by a light meter, clear blue sky and green grass are frequently used to set exposure, as the reflectance level equals the level that the meter is expecting. So you don't even need gray to set exposure.

I would imagine that your understand would be better served if you just just dropped "middle gray" and "neutral gray" and just hung onto "13% reflectance" for metering and "spectrally neutral" for white balance. That's what really matters.

As for estimating exposure compensation...I don't think that "seeing" in zones is possible. You have to learn, through experience and practice, the compensations required by various tones. The reason that EC is difficult to estimate is that we see in color, and our perceived brightness depends heavily on the color. You can have a red and a blue that meter the same...but the red will appear to be darker. That simply needs to be learned. Then you'll be able to make an EC estimation when you see a similar red.

We don't live in a gray world.