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Lesson Number Four

Exposure

 

     Many of you taking this photography course will have automatic cameras that do not have manual exposure settings. Automatic cameras have made photography easier for the masses but the best automatic cameras also have manual override capabilities. Modern film allows good quality prints to be made from film that is improperly exposed but to get the most out of your film and get the very best prints possible, there will be times that you must override your camera's light meter. This will be explained further in the section on film.

     Before you adjust the camera settings, you must obviously come to some decisions about the correct exposure for a given film under a given set of circumstances. There are three ways to do so: you can read and apply the information supplied by the film manufacturer; you can estimate exposure from knowledge of film characteristics and their response to standard lighting situations; or you can use a photoelectric light meter. Each method has its virtues.

     The first and second methods are fast, and are very useful when you do not have time to make formal exposure readings. Estimation is also useful when you are unable to make a meter reading because of the unusual nature of the subject (e.g., distant night cityscapes, or eclipses). Meter readings are best, of course, when the lighting is nonstandard but the scene is accessible.

     The f:16 rule will apply to exposures taken at midday with uniform lighting. The rule states that using a lens aperture of f:16 at midday will require a shutter speed approximate of that to the ISO speed of the film. For example, a film rated at ISO 100 would require an exposure of f:16 at 1/125th of a second. 1/125th is used since there is no 1/100th setting on the camera.

     Using a light meter is the best way to get the proper exposure. Most 35mm cameras have built in light meters that guide the photographer in setting the exposure controls on the camera (in manual mode). Automatic exposure cameras use the light meter also but automatically tell the camera what adjustments to make to get the proper exposure.

     Light meters are calibrated to give an exposure for the average light of a scene which normally is approximately 18% reflectance. Pure black actually reflects about 3% of the light falling on it while pure white reflects about 90% of the light reflecting on it. The average scene can be exposed properly with a light meter taking an average reading. However, there are times that you must meter parts of the scene to get the proper exposure. This is not always possible with fully automatic cameras.

The Basics of the Zone System

     Ansel Adams invented the "Zone System" which divides the light of a scene into eleven zones, from pure black to pure white. Each zone is numbered Roman numerically from 0 to X. Zone V is what exposure meters read, or 18% reflectance. Sometimes when you meter a certain portion of a scene you may not want it to fall on zone V in the final print, so using the Zone System you can change the exposure to give that certain subject a different zone.

 

The subject could be placed on zone III just as easily by decreasing exposure by two stops which would make the scene darker. Many times a scene will have too much contrast to be recorded fully on the photographic print. In black and white photography the zones can be compressed or expanded to print as less

or more zones. For example, a scene containing zones II through IX can be printed as only having six zones instead of nine. Or a scene containing only three or four zones can be expanded to print five or six zones. This is taught more extensively in our courses on the Zone System and black and white processing and printing. With color films you are pretty much limited to the amount of contrast in that particular film. Kodak does offer a film with more contrast in the 120 and 4X5 sizes, named VCS.

Suggested Trial Exposures for Existing Light (Low Light) Conditions