The first thing you need to know is that exposure meters are "stupid", really very "stupid". They do not have any idea of what they photograph and they assume that all the scenes have the same tonal value: middle gray (famous gray of 18%). For example without correction, a snowy scene will be underexposed. If you follow scrupulously the indications of the exposure meter, each scene will have the same middle gray density and you will get lots of bad exposures, especially if you photograph in difficult light.

Ansel Adams developed the Zone system to cope with this situation! It provides photographers with a simple and effective method to define with precision the relationship between what they see and the results that they will get. It allows for a direct correlation between the visual world and the real photographic print. Originally the system was developed for film, but is easily applicable to digital photography.

To know : lthe only way to a complete understanding of the zone system is through trial and error. You must try and try before complete mastery of the subject.

What makes the conceptual basis of the zone system necessary is that modern photographic papers and current digital sensors are very limited in the range of tonal values they can reproduce. The human eye does not have these limitations, it can perceive an extremely wide range of tonalities. From the darkest value that we can see, to the most luminous light we can tolerate, the dynamic range of the human eye goes from 1 to 1.000.000.

This means that photographers must use a method that allows for the final prints to approximate the way the world looks to our eyes. The zone system does this by providing photographers with a simple way to control the contrast in their cameras (film or digital). This method makes it possible to easily translate the range of the visible tonalities in a range of reproducible tonalities using a "visual rule". This process is known as "previsualization".

Here a grey scale:

Divide this grey scale into 10 parts so that each part contains only one value of gray. Then, number each part from 0 for black to 9 for white.

Now, you have simplified grey scale for the Zone System. Ansel Adams defined the various zones as follows:

The metering system of your camera is calibrated to read the middle gray 18% as being the correct value of exposure. Thus, it will make the average of the shadow lights and high lights causing a lot of under-exposure or over-exposure. The Zone System eliminates this problem by assigning these familiar "values" to each zone.

The key of success with this method is to carefully pre-visualize the subject and apply the correct Zone values to the important exposure areas. Then, you must adjust your exposure settings accordingly to accurately produce the results you want.

Select a portion of the scene to meter. Middle to highlight areas are generally preferred digital, but the most important part of the scene is often the best choice (you'll learn from experience) and use this 3-rules of Zone System :

• 1. The Zone scale is a progressive series of tone values. Each value is the equivalent of one full f-stop (or one EV step).
• 2. The spot meter provides exposure readings for Zone 5, giving you a correct exposure for a known Zone.
• 3. Adjust the meter's exposure by the difference between your chosen placement and zone 5 (F/Stop or EV).

The zone system can be used in digital photography just as in film photography. The important difference is that one exposes for the shadows and develops for the highlights when using negative film; and one exposes for the highlights and develops or post-processes for the shadows digital imaging.

The DSLR sensors code the images on 12 bits in RAW mode and support a maximum of 6 F/Stop. A simple calculation shows that 12 bits represents 4096 (2^12) values of grey for each layer (red, green and blue). The digital sensor records these values in a linear way (divided by 2), the values of gray are distributed as follows:

• 1 : 2048 values (4096 / 2) - highlights
• 2 : 1024 values
• 3 : 512 values - mid tones
• 4 : 256 values
• 5 : 128 values - shades
• 6 : 64 values - shadows

If your exposure is based on the mid tone (18% gray) as the build in meter suggests, we see the image will be poor in values of grey in the shadows. It results noise in the shadows and posterisation in the highlights. That’s why you need to expose for the highlights.

Here is what a digital sensor is able to understand of the Zone System grey scale:

Suppose you have snowy mountains at zone 7. For a correct exposure (zone 5), meter them, then close down 2 f-stops (+2 EV).

You want to do a studio portrait of someone with light skin. We shall place the lit side of the subject's face (the most important portion of a portrait) in Zone 6. First, meter the face. What the meter gives you is the setting for a Zone 5 face. You will have to give the face more exposure than indicated by the meter (more light on the sensor) to place it in Zone 6. Opening the lens 1 f-stop (+1 EV) from the metered exposure will have this effect.

Imagine that you are photographing an aubergine. It's not black, but it's dark. Maybe you would like it to appear in Zone 3 in the final print. Again, your spot meter indicates exposure for Zone 5. By closing down 2 f-stops (-2 EV), the aubergine will be placed in Zone 3.

Let us imagine a woman with clear face, black dress and walking on a sunny beach. The metering system will give the following indications:

• - Black Dress: F/4
• - Clear Face: F/16
• - Sand: F/22

On veut donner au personnage un air assez naturel où apparaissent des détails à la fois dans la robe et dans le sable. Si l'on expose pour la robe elle sera grise (zone 5) et tout le reste sera surexposé.

Si l'on expose pour le sable ce sera le contraire. C'est le sable qui sera gris, tout le reste étant sous-exposé.

Si on fait une moyenne en posant entre f/8/11 on perdra des deux cotés

Ce n'est qu'avec une ouverture de f/8 que l'on obtiendra des résultats à peu près satisfaisants. Cependant le sable et le visage seront traduits par un gris clair. Pour obtenir des détails dans le sable et pour donner une impression naturelle du visage (celui-ci devrait se trouver en zone 6) il faudra post-traiter l’image. Ceci fera descendre le sable en zone 7 et le visage en zone 6