Determining film exposure (This information is largely extracted from Black and White Photography: A Basic Manual by Henry Horenstein [an excellent book])
I suggest that you use two or more of these methods and compare the results. They should be very close. If not, you may be doing something wrong.
1. Take a general reading.
For predominately white or light subjects, add more light-by the equivalent of one f-stop or more-than the meter suggests.
For predominately dark subjects, cut back on the light the meter suggests; close down the aperture or make the shutter speed faster.
2. Use a gray card. Photographers sometimes use a gray card to reflect an average amount of light falling onto the subject rather than the light reflected off specific parts of the subject. To use the gray card, place it in front of the subject, and aim it towards the camera position. Take a meter reading off the card only; take care not to cast a shadow onto the card while reading the light. Use the f-stop and shutter speed combination indicated by the meter without adjustment.
3. Expose off skin. Skin can substitute for a gray card. Take a light reading off the palm of a hand, flattened out and positioned directly in from of the subject, toward the camera position. Or take the reading off the subject’s face. Be careful not to cast a shadow onto the hand or face when taking the reading. For ‘average’ Caucasian skin, add the equivalent of one f-stop more exposure. If the meter reads f16 at 1/250, use instead f11 @ 1/250 or f16 @ 1/125. Dark skin may simulate a gray card perfectly, so use the meter reading without adjustment. Extremely dark skin may need as much as one-half to one f-stop less exposure than the meter suggests.
4. Read incident light. Incident-light meters read light falling onto the subject rather thanlight reflecting off the subject. They do not read specific dark, gray, or light areas of a subject. As such they provide an average exposure for the given subject lighting conditions, much like a gray card.
5. Average the shadows and highlights. Since meters read for middle gray, the correct exposure will be somewhere between the meter readings for the dark and light areas of the subject. Meter a dark area, then a light area, and average the two readings. If the reading off the dark area is f4 at 1/60, use f8 at 1/60.
Most of the time this exposure system works well enough. However, the dark and light areas metered should be approximately equal in darkness and lightness. Do not average meter readings from a jet black car and mildly tanned Caucasian skin.
6. Bracket. Bracketing means taking exposures on either side of the recommendations of the meter. It is a safe way to guarantee good exposure. If the meter says f8 @ 1/250. You might also take a photo at f5.6 @ 1/250 and f11@ 1/250. This produces several exposures of the same image. At least one exposure should be ideal.
7. Expose for shadows and compensate. A more exact system for exposing film is to take the meter reading in the dark shadow area of the subject and then make an adjustment to that reading. The area should be the darkest part of the subject where detail is desired in the final print.
Let’s say a dark sweater represents the deepest shadow area where detail is desired. Read off the sweater only. (A spot meter is helpful.) Suppose the meter indicates f2.8 at 1/60. Use instead an exposure of two f-stops (or the equivalent) less, such as f5.6 at 1/60, f 4 at 1/125, or f2.8 at 1/250.
If the darkest areas of the subject are not particularly dark, the adjustment should be for one stop, rather than two.
8. Sunny 16 rule. On a sunny day set aperture to f/16 and shutter speed to the [reciprocal of the] ISO film speed [or ISO setting] for a subject in direct sunlight. So if you are using 400 speed film on a very sunny day, you would set your aperture at f16 and your shutter speed at 1/400.
As with other light readings, shutter speed can be changed as long as the f-number is altered to compensate, e.g. 1/250 second at f/11 gives equivalent exposure to 1/125 second at f/16.
An elaborated form of the Sunny 16 rule is to set shutter speed nearest to the reciprocal of the ISO film speed / setting and f-number according to this table:
Aperture Lighting Conditions Shadow Detail
f/22 Snow/Sand Dark with sharp edges
f/16 Sunny Distinct
f/11 Slight Overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy Overcast No shadows
f/4 Open Shade/Sunset No shadows
Add One Stop Backlighting n/a
