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TECHNICAL NOTES



3b. In the Field - Exposure



Details, Junction Falls, 2000


Exposure: I expose and develop my film with a passing nod to the tenets of Ansel Adams' zone system. Contrary to popular photographic myth, this is not a magic formula for producing perfectly exposed and developed negatives. But it does provide a sound framework for understanding and controlling how the real world can be represented on analog photographic materials given their inherent characteristics and limitations.

Whole books could be - and have been [Note 1] - devoted to describing the zone system and its application to photography. So I can't hope to do more here than touch on some basic concepts. Bear in mind also that my remarks are made in the context of the theme of this site: black and white photography. They will have some - but limited - application to colour photography.

Now to begin...

Imagine a continuum of tones of gray that ranges between pure black at one end and pure white at the other. Now split that continuum into ten equally spaced "zones". Each zone between pure black and pure white is assigned a number between 0 and 10 (the convention is to use Roman numerals plus 0), so Zone 0 is pure black, Zone X is pure white, and Zone V is a mid-gray half way between pure white and pure black. Like this:

Here's the magic bit: Moving from one zone to another (e.g. from Zone V to Zone VI) is equivalent to a one f-stop change in exposure.

The camera's exposure meter will provide an exposure that renders whatever it is pointed at as a mid-gray or Zone V. We want to be able to measure the brightness of specific, tiny parts of the image and we therefore use the camera's meter in "spot-meter" mode. This will provide an exposure that renders those small details as a zone V - irrespective of whether it is pointed at a deep shadow or a bright sun-lit highlight. The user therefore needs to adjust the exposure indicated by the spot-meter to ensure the tone of the subject matter is appropriately rendered by the film. Photographic convention says that the darkest part of the scene that is to contain detail should be on Zone III, while the lightest part containing detail should be on Zone VII. So, in the deep shadow example, by giving two f-stops less exposure than the spot-meter's indicated exposure, the tone of the subject matter has been moved from Zone V down to Zone III. (In Zone system language, the subject has been "placed" on Zone III). The same can be done for the high-values: In the case of the bright sun-lit highlight, giving two stops more exposure than the spot-meter indicates will move the highlight from Zone V to Zone VII.

For technical reasons to do with the way film responds to development (more on this shortly), when using black and white film we usually determine the correct exposure by reference to the lower zones (i.e. the shadows). Hence the first half of the old photographic rule: "expose for the shadows, develop for the highlights". This means give enough exposure to the retain some detail in the shadows (too little exposure and they will just go black in the final print).

Having determined the correct exposure, there is one other piece of information that will be of assistance when developing the film: the range of the spread of tones in the scene - the subject brightness range ("SBR"). Having "placed" the shadow tones on Zone III, spot-meter the brightest tone in the scene that we want to have detail in the final print to see where it "falls" (i.e. which Zone it occupies). If the brightest highlight where detail is required lands on Zone VII, then the scene has a SBR of 5 Zones. Lets call this "normal". On the other hand, if the brightest highlight falls on Zone VIII or Zone IX then we have a SBR of 6 or 7 Zones - indicating a contrasty scene. Conversely, if the brightest tone falls on Zone V, we have a SBR of 3 Zones - indicating a "flat" scene. Make a note of the SBR. We will refer to it later when developing the film.

So, in summary, to obtain the correct exposure and the information we require for optimal development:

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Spot-meter the darkest area in the scene where you still want to see some detail in the print. Close down two stops. You have just placed the shadow area in Zone III. In many (most?) circumstances, that will be the proper exposure for the scene.

Now, aim the spot-meter at the brightest area in the scene where you want to see detail in the print and write down the number of zones (f-stops) between the darkest and brightest readings.

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After tripping the shutter and exposing the film you are ready to develop it. There's more on developing in my dark page. If it is of interest to you, details of exposure and development for each of my images (where available) are included below the image.

Concluding comment: I have not touched on a further issue that you should be aware of: film speed. Just because a manufacturer rates their film at (say) ISO 400 does not mean that this speed will necessarily give you the best results. In fact, many photographers find that using a slower speed than the manufacturer's rating provides better results. Again, Zone System devotees have all sorts of testing methods for determining the "exposure index" ("EI") that is appropriate for their (and your) process. I'm not going to go into detail here. I'll simply refer you to the following link for more information: "Use Your Eyes!" by Paul Wainwright. But if you'd prefer to get out there shooting rather than running tests then a good rule of thumb is to use half the manufacturer's rated speed. E.g a film rated at ISO 125 is exposed using a personal exposure index of 64. Basically this means setting the ISO/ASA speed dial on the camera/meter to 64 instead of 125 and thereby fooling the camera into thinking is using ISO/ASA 64 speed film.

Note 1: For more information refer to "The Negative" by Ansel Adams.




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