Photographic Exposure

Exposure with or without a light meter

Determining Exposure Without a Meter

Outdoor exposure can be determined without a light meter by using the "Sunny day 16 rule". To use the rule, the ISO of the film is used as the shutter speed and f/16 as the f/stop on a bright sunny day with sharp clear shadows. This is based on the sun being behind the photographer. The following chart shows how you can make appropriate changes to the shutter speed f/stop combinations to achieve the effect you want. The ISO = shutter speed and f/16 combination are used only to establish a correct starting point.

BRIGHT SUN DISTINCT SHADOWS BRIGHT SUBJECTS	HAZY SUN INDISTINCT SHADOWS DISTANT SCENES	BRIGHT OVERCAST NO SHADOWS OPEN SHADE - SKY ABOVE	HEAVY OVERCAST CLOUDY DULL UNDER SHADE OF TREE OR PORCH ROOF
Shutter speed = ISO (or E.I.) Aperture = f /16	Shutter speed = ISO (or E.I.) Aperture = f /11	Shutter speed = ISO (or E.I.) Aperture = f /8	Shutter speed = ISO (or E.I.) Aperture = f /5.6

If the sun is hazy and there are still shadows but they are not distinct, you should open 1 f/stop (i.e.. f/11 ). Bright overcast ( no shadows ) requires opening two stops and heavy overcast or open shade ( the shadow of a building or other large object with open sky above ) requires opening three stops. There is a handy dial version of this rule in the Kodak Master Photoguide which is designed to fit in a pocket or small camera bag and be carried about. The Guide also contains some other useful charts and information that can't be expressed as an easily memorized rule.

Light Meters

Light meters measure the intensity of light. They are calibrated to give a reading which will record on the film at the same tonality as an 18% gray card. This is because the tones of the average photograph average out to an 18% gray. Think of it like a sand painting. If you took all the sand and stirred it up so the dark and light colored grains were uniformly mixed the overall impression would be an 18% gray. Light meters see the scene like that. They don't have a lens to focus the image. Even those built into a camera behind a lens are an array of light sensitive surfaces which combine to average the light they see.

There are exceptions. One that has been around for a while is the spot meter. Instead of a broad area it sees a narrow angle (as little as one degree). They are still calibrated to render the part of the subject that they see as an 18% gray. Another exception is the matrix metering begun by Nikon and now available in other makes of 35mm and for Mamiya 645 Pro. They work with an array of light sensitive cells but instead of just averaging the light, a computer analyzes the patterns and makes adjustments based on parameters which are written into the program.

Using a Light Meter

There are several types of light meters available both handheld and built into the camera. An incident meter measures the light falling on your subject from the position of the subject and pointed toward the camera. Incident meters are the favorite of some portrait photographers. They have a diffusion filter over the light sensitive cell and read the light from a wide angle. Incident meters are usually very accurate but you must be able to walk up to your subject to take a reading. That is not possible with some subjects or practical if light is changing rapidly.

A reflected light meter reads the light reflected off the subject. To use it you point the meter at your subject. Meters built into cameras are all reflected light meters. It is a good idea to know what the meter's angle or area of view is. If the area being read is unusually light or dark and doesn't represent the average for the picture you may end up with the wrong exposure. An example is a snow scene where the snow reflects more light than the average summer scene. The meter will give a reading which is designed to render the snow at the tonality of foliage. Incident meters aren't fooled by snow because they don't read reflected light, only light sources.

Another situation which is problem to a reflected light meter is the backlit subject. Bright light coming into the meter around the subject can fool the meter into thinking that the subject is brighter than it really is. Knowing what area the meter covers helps because you can move in to meter only the area you want. If you can't move in you will need to know how much to compensate under various conditions. For example, opening up the diaphragm will usually compensate for backlight. A good way to get around the conditions that can fool a reflected light meter is to carry a Gray Card. Hold the card so that it is perpendicular to the lens and in the same light as the subject. Take a reading so that the meter sees only the card and you will have the correct exposure.

A spot meter allows the photographer to individually meter specific values within the subject. This allows the photographer to judge the contrast of the scene and make decisions about processing even before tripping the shutter. The processing can either expand or contract the contrast range of the negative. He/she can also decide to render the values darker or lighter than they appear to the eye. Specific information on using a spot meter can be found in the lesson on the Zone System.

Matrix meters have varying numbers of segments which are read and evaluated by a computer program in the camera/meter. Read your instruction book to see how yours works and experiment with various problem conditions.

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