My Photoshop Classroom

This article explains the Zone System, created by Ansel Adams, that can help you take better photos.

Source: photo.tutsplus.com

The Zone System’s Key Concepts

The zone system divides a scene into 10 zones on the tonal scale (though there are variations of 9 and 11 zones). Every tonal range is assigned a zone. Every zone differs from the one before it by 1 stop, and from the one following it by 1 stop. So every zone change equals 1 stop difference. Zones are identified by roman numbers, with the middle tone (with 18% reflectance) being a zone V which is zone 5.

For us digital photographers, we are only concerned with zones III through VII (zones 3 through 7). The darkest part of a scene would fall into zone III, while the brightest part of a scene would fall into zone VII. Anything darker than zone III would render as pure black with no detail (under-exposed), while anything brighter than zone VII would render as pure white with no detail (over-exposed).

If you point your camera at an area with average reflectance and obtain the correct meter readings (a zero on the light meter), that area would be rendered as average. If you open up your lens or slow down your shutter speed by one stop, that area will become over-exposed by one stop. If you close down your lens or increase your shutter speed by one stop, that area will become under-exposed by one stop.

Now, we’ve agreed that an average tone is naturally placed into zone V. If you over-expose it by one stop, you’ll be placing it in zone VI (zone 6), causing it to render brighter than it actually is. If you under-expose it by one stop, you’ll be placing it in zone IV (zone 4) causing it to render darker than it actually is.