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Simplified Zone System

ALL the exposimeters of ALL (and I do mean ALL) the cameras on the market are factory-calibrated on a so-called MEDIUM TONE subject, which corresponds to an average of the real-world subjects a photographer is likely to aim his camera at. (This also applies to ALL the expensive stand-alone exposimeters - just in case you were wondering) If your specific subject is MEDIUM TONE too (such as e.g. a typical meadow), all you have to do is blindly trust your camera exposimeter, and shoot. Complex scenes (e.g a group of people dressed in all sorts of light and dark colours) will not require any particular exposure adjustments either, as long as the different "patches" of colour are distributed so as to give an average reading that is not too far from the famous "MEDIUM TONE subject".

BUT if you want to photograph a scene whose TONE is globally lighter or darker than normal, some kind of exposure compensation will be necessary. For instance, if your subject is a bride in a traditional white dress, posing in front of a light yellow wall, the camera will suggest an exposure that will make the scene match its factory-set MEDIUM TONE standard. In other words, you will get a grey dress and a deep yellow wall - not exactly a flattering picture you could sell ! In this case, it will be necessary to INCREASE the exposure, by opening up the diaphragm and/or using a slower shutter speed. On the contrary, dark subjects require LESS exposure (narrower apertures and/or faster shutter speeds) than the camera suggests.

The TABLE below indicates how much you should alter the exposure in order to match the TONE of the subject.

...but it does more than that, really. And here is where the fun begins.

ZONE Exposimeter reading Resulting tone Examples
II -3 STOP BLACK (almost no detail) -
III -2 STOP Very dark Dark shadows in bright sunlight, black subjects with detail (darkest areas of the frame with detail)
IV -1 STOP Dark Dark vegetation (e.g. fir trees), soft shadows, black skin
V 0 STOP MEDIUM Green grass, deep blue sky, tanned Caucasian skin
VI +1 STOP Light Light rock, yellow subjects, shady snow, pale blue sky, sky at sunset, light Caucasian skin
VII +2 STOP Very Light Bright white subjects, sunlit snow (brightest areas of the frame to retain detail)
VIII +3 STOP WHITE (almost no detail) -

Actually, this TABLE can be used as a reference in order to apply a "Simplified ZONE System" when measuring exposure.

The famous American landscape photographer Ansel Adams first devised the ZONE System to be able to extract the maximum amount of detail out of his black-and-white prints, but the basic principle applies to all kinds of photography.
In fact, Ansel Adams referred to MEDIUM TONE as ZONE V (5), because by using large-format black-and-white sheet film and advanced developing and printing techniques he was able to squeeze a maximum of TEN full STOPs of dynamic range out of his photographs.
However, most colour slide films and digital sensors are capable of faithfully recording a more restricted range of luminosity, spanning roughly FIVE to SIX STOPS. Anything outside that range will be recorded as plain black or white.

So: how does one go about putting all this into practice? To cut it short, it works more or less like this:

1. Setting your exposimeter to "SPOT", you measure a small area of interest in the frame, bearing in mind that the exposimeter is calibrated to ZONE V (= MEDIUM TONE). If you want the measured area to appear of MEDIUM TONE in your photo, you set the Aperture and Speed controls to match the indications of the exposimeter; otherwise, if you want it to be lighter or darker, you respectively increase or decrease the exposure, by adjusting the Aperture and/or Speed (within a maximum of +/- 2 STOPs). By doing this you are effectively POSITIONING that area in one of the available exposure ZONES. E.g. If you measure a slab of granite rock and set the controls so that the exposimeter indicates +1 STOP, you know that it will be recorded as a medium-light TONE in your photograph (ZONE VI).

2. Leaving the controls as they are, you then aim the SPOT meter at other areas of the frame, to check in which ZONES they will "fall". E.g. if those trees at the bottom read -1 STOP, you know that they will come out a medium-dark TONE (ZONE IV).

3. If all the measurements are within a range of -2 to +2 STOPs (i.e. ZONE III to ZONE VII), all is nice and well. Otherwise, if the scene has a luminosity range that exceeds the dynamic range that can be recorded, i.e. if some of the SPOT measurements you have just made read -3 STOP (ZONE II) or under, or +3 STOP (ZONE VIII) or over, you then know that these will respectively be reproduced as pure black, or pure white. It is then YOUR CHOICE (*) if and how to alter the exposure settings, either by INCREASING the exposure so as to "rescue" the darkest areas of the frame (at the same time making everything else lighter, of course), or on the other hand by DECREASING the exposure, thus preventing those brightest areas from being "burnt out" (in this case, everything else will come out a little darker).

(*) As a general rule, using slide film or digital cameras, it is better to avoid overexposure (i.e. areas of the frame that are "burnt out" and recorded as pure white), while it is often subjectively acceptable for deep shadows to be reproduced as even darker (i.e. black).

Finally, an EXAMPLE photo is provided as an illustration of the procedure.

[EDIT: Another nice explanation of the Simplified Zone System and how it applies to digital photography is given here.]