Exposure Calibration
by Bob McBride
Obtaining a correct exposure is one of the most difficult challenges faced by photographers at all levels. This critical function is made infinitely more difficult if your exposure meter is not calibrated correctly.
For some reason, many photographers want to believe that just because they purchased an expensive camera with a built-in exposure meter and an array of metering patterns, they can simply trust the cameras exposure meter to magically read their mind and provide a correct exposure setting every time. They just need to turn their camera on, point at any subject, push the button, and the perfect exposure will be at hand.
The truth is, camera meters can’t think. No matter what some manufactures want you to believe camera meters don’t know what you’re photographing and more importantly, they don’t know how you want your images to turn out. In truth, if you allow the camera to make all the decisions about what shutter speed and aperture to use, you will only be guaranteed that you will get an exposure, you will not necessarily be guaranteed that you will get a correct one?
To get a correct exposure, the photographer must either estimate the light falling on a subject or take a meter reading and then make any necessary adjustments. For the nature photographer the problems associated with trying to estimate the light falling on a subject should be obvious: shooting out-of-doors and constantly changing light levels go hand-in-hand. Even if you could control light levels you would need to run a series of tests to first determine correct exposure. I guess you could shoot every f-stop/shutter-speed combination possible to make sure that you captured at least one correct exposure. While the process of trial and error might be an options for some it does not seem to be the most practical or cost effective.
The only estimated exposure system that works is based on a known, consistent, light level: bright sunlight. From about two hours after sunrise until two hours before sunset, proper exposure for any film on a clear, bright day (no haze, no dust in the air, no pollution) for a front lit subject bigger than a camera bag, and of average tonality, is expressed by the sunny f/16 principle. It only works in bright sunlight on a clear day with the sun up above the horizon. The light must be coming over your shoulder and striking the front of the subject. And your subject has to be of average tonality. A subject of average tonality is called a medium-toned subject; it is neither light nor dark but halfway in between. If all these conditions are met, sunny f/16 says that the correct exposure is the shutter-speed number closest to the film’s ISO number at f/16, or any equivalent exposure value. Sunny f/16 = 1/ISO at f/16.
As an example, if you’re shooting Fuji 50 (ISO 50) your correct exposure for this film in bright sunlight would be 1/50 sec. (the shutter speed closest to the film’s ISO number of 50) at f/16 or any equivalent. An exposure of 1/50 sec. at f/16 is the same as 1/100 sec. At f/11, which is the same as 1/250 sec. At f/8, or 1/500 sec. At f/5.6, and so on.
Usually, it is a good idea to take a meter reading and work from it; however, you need to determine a starting point for your camera’s metering. You must learn what your exposure meter does in order to control exposure. The way to do this is to verify what ISO setting on the camera’s meter makes a medium-toned subject appear as a medium-toned image on slide film, which means checking your meter’s calibration.
Checking the calibration of a meter is not difficult: meter a target of known midtone reflectance using an exposure based on the Sunny 16 Rule and compare that exposure with the indicated exposure for your equipment.
According to the Sunny f/16 Rule, a midtoned subject, when exposed in full sunlight, will be correctly exposed at an aperture of f/16and a shutter speed of the reciprocal of the film ISO (1/ISO). In other words, a film with a speed of ISO 50 should correctly expose a midtoned subject at 1/60 second at f/16 in full sunlight. If the camera meter suggests an exposure of 1/30 seconds at f/16, then the meter is reading one stop too high and the exposure must be corrected accordingly. The shutter speed or aperture can be set one stop lower than the meter’s suggested reading or the film ISO setting of the camera can be set to a one stop lower value (ISO 50 film should be used at a camera ISO setting of 100), or, taking the approach of least confusion, take the camera to a repair shop and have the meter adjusted.
Use the camera in the Manual Mode (not an Autoexposure mode); use a fixed focus lens at a moderate distance (a 50mm lens with an 8X10 gray card works well). In addition to checking the meter reading with a gray card, it is also helpful to photograph a Macbeth ColorChecker ® chart with the film or films of choice to verify the color rendition at that exposure setting.
The key concepts in this procedure are “full sunlight” and “midtoned subject”. Full sunlight occurs on a day with a cloudless sky, between the hours of 10:00 AM and 2:00PM (or, if midwinter, around noon). Midtone is a bit more difficult to define. By convention, a Kodak 18 % gray card is considered the standard midtone. Green grass, most foliage, many tree trunks and the north sky on a clear day are also midtones.