Is 18% gray a problem? Who says 18% is "correct"

[administrator]

There is debate about the accuracy of one of the most common dogmatic statements made about proper exposure. Unless you have been living under a rock, you know that "to arrive at proper exposure, an 18% grey/gray card must be used because the exposure meters in every camera in the world are set to 18%."

This statement has been repeated on websites, in beginner and advanced photography classes, and printed in texts. The accuracy of this statement is never questioned, even while the source of this dogma is obscure.

Some claim the the 18% number is just plain wrong - 12 or 13% is the correct number. To quote Richard Hess:

"Using the 18% gray card as a metering reference will cause approximately 1/2 stop underexposure as the reflected light meter is assuming 12%."

Please see the following page where this somewhat abstruse topic is analyzed mathematically. I cannot claim to be able to follow the reasoning set out there, but I'm sure others can.

http://www.richardhess.com/photo/18no.htm

To boil it down to the essentials, I believe, Mr. Hess is stating that the ANSI standard for photographic exposure meters, when applied precisely, leads to a meter that adjusts exposure to about 12 or 13% reflectance. Or, perhaps better stated, that a 12% or 13% reflectance gray /grey card, will result in a "correct" exposure, NOT an 18% card.

Based on my analysis of the exposure and white balance processes in modern digital SLR's, I tend to think the 18% grey number is fast becoming irrelevant. The Nikon D3, for example, adjusts both exposure and white balance based on multiple parameters, not just reflectance. There is luminance analysis and highlight suppression (so that very bright objects do not unduly affect the exposure calculation) as well as scene recognition technology (matching the scene in the viewfinder against an internal database of pre-analyzed scenes) and, I'm sure, more.

18% is still very much alive if you own a classic style camera. My Minolta SRT-101 has a very rudimentary light meter, which consisted of a single luminance meter with a light intensity needle visible in the viewfinder. The photographer's job was to adjust shutter speed and aperture so that the needle on the light meter passed right through a bull's eye circle that represented the shutter/aperture summation. I'll bet it was looking for 18%, or 13%.

I would appreciate hearing from anyone who has actual knowledge of the way automatic and manual exposure values are determined in modern cameras. Please email and I will make sure you can post here.