Photography question, 18% grey?

Question

When using the zone system, why is middle grey [zone 5] also called 18% grey? Why not 50% grey?

Answer 1

Or to put it very simply, 50% grey looks very dark to us. It looks more like charcoal. 18% grey looks to the human eye as halfway between black and white.

Answer 2

I think it has something to do with the way camera meters typically meter a scene. They try to balance everything in the scene to an 18% grey mix.. This means for example, if you were going to take a picture of a bright WHITE wall, when the camera gives a recommendation, it gives a recommendation that will result in the white wall being 18% GREY in the final photo. Therefore for a proper exposure of the pure white wall, you would need to manually overexposed the image or use some form of exposure compensation to bring it back to WHITE. Therefore, you don't want a PURE white wall as a background, that's for sure. You will be spending all day continually making exposure compensations. With an 18% grey wall as the background, your exposure readings will be right on the money probably more often than not, reducing the need for extra work.

Answer 3

Because 18% Grey represents the "average" of dark and light tones in the majority of photographs and is the value that designers of light meters use to calibrate the settings. The light meter is always trying to provide the setting to achieve 18% grey tones. When you think about it, daylight is the time when most pictures (without flash) are taken and it is much brighter than half black and half white, so that is the condition you want to expose for.

Answer 4

From "How To Select & Use Minolta SLR Cameras" by Carl Shipman, 1980, p74

An Average Scene
By measurement of many typical scenes, it has been found that most outdoor scenes and many shots in a man-nade environment have the same average amount of light coming from the entire scene. Some parts are brighter and some are darker but they average out, or integrate, to the amount of light that would come from a certain shade of gray. This gray shade is called 18% gray because it reflects 18% of the light that falls on it. ...

Not only do average scenes reflect 18% of the light, the middle tone or density of an average scene is also 18% gray.