The Zone System was designed to control the relative values in the negative through exposure and development. It works well for sheet film users who can control the processing of each individual sheet, but roll film users can't develop each frame separately and one roll is often exposed under varying conditions.. Of course you can choose the zone for your most important value and let the rest fall where they may but when the contrast of the scene exceeds the range of zone III to VIII you may be losing detail you would prefer to keep.
It is possible to lower the contrast through pre-exposure rather than by manipulating the development. To use pre-exposure you will need a camera that allows for deliberate double exposure. At the end of this lesson is a list of cameras that will permit double exposure.
The actual procedure is to expose the film once to a gray card and then re-expose it to the scene you wish to record. Suppose you are looking through a wooded area with deep shade into an area of bright sun. You want detail in the sunny area but your spot meter tells you that the much of the shaded area will fall in zone I or II (perhaps even off the scale). If you expose for the shadows the bright area could require extensive burning in or may even block up to the point that there is no detail.
A solution is to pre-expose the film with the gray card held in front of the lens at a distance which will be completely out of focus and will cover the entire field of view. Make sure the lighting on the card is even and it is best to use your tripod. Meter off the gray card to give an exposure that will record at the level you want your shadow detail (Zone II or III) then make the first exposure. Cock the shutter again without winding the film ahead and shoot the scene at an exposure that will record the highlight detail in the appropriate Zone.
You might expect that this would do little but make the shadow areas a lighter gray but exposure is a cumulative process. The light from the gray card was even all over the frame but the light from the shadow area of the scene varies with the depth of the shadows and that variance is recorded on top of the pre-exposure. The deepest shadows will still be without detail but the less deep areas will add their light to the pre-exposure and raise the value by the combination of their Zones.
The highlights will show no appreciable effect. This is because the exposure for each Zone is double (or half) the neighboring Zone. A Zone II exposure for example, represents only 2 units of exposure while a Zone VIII exposure is 128 units of exposure. A pre-exposure of Zone II combined with a second exposure of an area that would otherwise fall in Zone II will then have 4 units of exposure moving that area into Zone III. The highlight area will have 130 units (2 from the gray card and 128 from the scene). The additional 2 units in the highlight area is an insignificant increase and much too small to push the highlight into Zone IX which is 256 units.
You can then process the roll normally. Your first reaction to the resulting negative will probably be that it looks as if it has been "fogged" because there are no completely clear areas and you'd be right. It has been fogged. However if you were careful about having even light on the gray card it won't be detectable in the print. Moreover it will be easier to print both the shadows and highlight areas with satisfactory detail. There is a corresponding process for printing that is called 'flashing' (no relation the '70s craze of running through public places naked). I will cover that in a future darkroom lesson. Now if only we could find a way to expand the contrast in the camera.
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"Don't let life discourage you. Everyone who got where he is had to begin where he was." Sydney J. Harris
Updated Dec. 26, 2000 © All rights reserved. James F. Bullard