For our troublesome backlit portrait shot, we can use the spot meter to take a reading from the subjectâ€™s face. We know that the lightmeter will give a reading that would make the face mid-tone grey, which is zone five. However from the zone chart we know that average Caucasian skin should be zone six, so we need to increase the spot metered exposure by one stop, in this case from 1/30th at f5.6 to 1/30th at f4. In the resulting shot the background details are all burned out, but the subject is correctly exposed.
For the over-exposed photo of the car on the black background, we can use a similar but different approach. Spot metering the background gives an exposure setting of 1/3rd sec at f5.6 to render it as zone five mid-grey. By reducing that exposure by four stops to 1/3rd at f22 we can make the background come out as it should be, zone one black.
The zone exposure system can help you with all difficult exposures, but it can also vastly improve even your general photography. Learning to think of images in terms of the zone system will encourage you to take more care with your shots, and also to become more creative. Control over exposure is the primary creative tool of the photographer, and learning how to use it will make the difference between mere snapshots and artistic photographs. This system has been used by professional photographers for over 60 years. Used properly, it can help to improve your photography immensely, probably more than any other single technique.
Note: Iâ€™ve made a card that can be printed out at 10x15cm which has the zone system chart on it. You can download this reference card by clicking here.