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Accurate Exposure with Your Meter
Accurate exposure is fundamental
to creating a high-quality photograph. With an exposure meter, you can
venture into a wide variety of lighting conditions and be confident that you
will consistently get good results. You can use an exposure meter to make selective
light readings, to handle unusual and difficult lighting, or to make special
creative effects.
Exposure meters, or light meters,
are light sensing devices that are good at doing what the human eye
can't--quantifying light. With an exposure meter you can relate the
brightness of light reflected from a subject toward the camera, or the
brightness of the light illuminating the subject, to the sensitivity of the
film. The meter expresses this relationship in terms of lens openings and
shutter speeds.
Most of today's 35 mm cameras have
exposure meters built into them. In a single-lens-reflex (SLR) camera, the
built-in meter's photocell measures the intensity of the light passing
through the lens. In a non-SLR camera, the photocell is on the front of the
camera body or lens. Some in-camera meters set exposure controls for you
automatically, but with the exception of non-SLR auto-focus cameras, most
models have overrides that enable you to change the camera's aperture and
shutter speed. The desire for added flexibility and control is why some
photographers still prefer to use a separate handheld meter. Whether you use
a camera with a built-in meter or a separate handheld meter, the following
information will help you to use your meter to its fullest potential to give
you exciting and well-exposed pictures.
The first step in getting good
results is to set your meter or camera for the correct speed of the film you
are using. Many cameras automatically set the film speed when you load
DX-encoded film. DX-encoded films can also tell the camera the exposure
latitude and the number of exposures. If your camera has this feature, the
camera instructions will explain how it works.
The film ISO speed is a number
that indicates the relative sensitivity of the film to light. The higher the
number, the more sensitive or "faster" the film; the lower the
number, the less sensitive or "slower" the film. For example, a
film with a speed of ISO 200 is twice as light-sensitive or fast as a film
with a speed of ISO 100. Similarly, and ISO 200 film is only half as fast as
a film with a speed of ISO 400. Your film carton (and the film cartridge)
will tell you the speed of your film.
Correct exposure is necessary for
getting the best possible picture quality from your film. However, because of
the exposure latitude inherent in most films, exposure can be off slightly
and you'll still get good pictures.
Some types of films have more
exposure latitude than others. Color slide films, for example, usually
produce the best results when they are within a 1/2 stop over or under the
correct exposure. But you will have somewhat more latitude--often two-stops
or more--with black-and-white and color negative (print) films. The
printmaking process is the difference in exposure latitude for these films.
Because it is possible to
compensate for variations in the camera exposure when the negative is
printed, the prints made from incorrectly exposed negatives can often appear
properly exposed. Slides, on the other hand, don't offer the opportunity for
exposure correction that is available through a printing step--the finished
slide that you project is the same film that went through the camera. The end
result is that slide films have less exposure latitude.
The kind of meter you have
determines not only how it measures light, but how you should use it. There
are two basic kinds of meters: reflected-light (they measure the light
reflecting from a scene) and incident-light (they measure the light falling
onto a scene). Each type has its advantages and its disadvantages. Some
meters have accessories that enable you to use them to read either reflected
light or incident light. Reflected-light meters that read a wide angle of
view are also called averaging meters. They account for most of the
reflected-light meters in use, including those built into cameras. Another
type of reflected-light meter, the spot meter, is designed to take readings
from only a small part of the scene.
Virtually all in-camera meters are
the reflected type--they measure the average brightness of the light within
the lens' field of view. As you look through the lens of a 35 mm SLR camera,
you can see what the meter sees. Change the lens and you change the area
being metered. Most handheld meters are also reflected-light meters. Many
photographers prefer reflected-light meters because they can use them to take
light readings from the camera position. To take a light reading with a
handheld reflected-light exposure meter, you usually stand at the camera
position and aim the meter at your subject. To use an incident-light meter,
on the other hand, you usually position the meter as near the subject as
possible, in the same light that is illuminating the subject, and aim the
meter back at the camera (unless the meter instruction book recommends a
different technique).
A spot meter has a unique and
helpful feature--an extremely narrow angle of coverage. Instead of measuring
the brightness of an entire scene, a spot meter measures the brightness of a
small area or "spot" within the scene, usually only a few degrees
or less. Handheld spot meters have a built-in viewing lens so you can see
precisely what part of a scene you're metering. As you'll see, this feature
can be very helpful.
Most through-the-lens SLR camera
meters are the averaging type, reading the entire picture area. Many read the
entire area but are biased for the central part of the viewfinder. The center
has the greatest influence over the exposure reading, although the remaining
picture area still has some effect on exposure. Other cameras can be switched
to modes similar in principle to spot meters; they read only a small segment
of the scene. With the advent of advanced electronics, it is possible to make
several spot readings that are automatically integrated for optimum exposure.
Another type of system offers
multi-pattern metering. Also called matrix or segmented metering, these
systems work by dividing the viewing area into a set pattern or grid of
several segments. The meter reads each segment and makes a comparative
analysis of things like subject size and the brightness of each zone. The
camera compares this data with programmed information it contains to
calculate exposure. This type of metering system can adjust automatically for
backlighting, snow and other unusual scenes to give better exposure. Check
your camera manual for specific instructions and information about the area
of coverage fore your camera's meter.
Both reflected-light and
incident-light meters are made to "see" the world as a medium gray.
The assumption is that most subjects, most of the time, are of average tone
and reflectance. So long as there is an even distribution of light and dark
subjects in the scene, correct exposure is usually as easy as pointing the
meter or camera at the scene and using the reading you get. But the real
world does not always present subjects to you in such a straightforward way.
For example, with either a reflected-light meter or an incident-light meter,
if the main subject is very dark or very light, the indicated exposure will
make the subject appear as a medium tone in the picture. The result will be
incorrect exposure unless you apply your own judgement to the information the
meter gives you.
Use a meter reading as a guideline
rather than a dictate for correct exposure. This makes it important that you
understand how your particular meter works so you can consistently get good
results no matter what the lighting. The place to begin this understanding is
the instruction manual that came with your meter or camera. The instructions
should familiarize you with the meter's specific features, its flexibility,
and its limitations. Most camera and exposure meter instructions provide the
basic techniques of light measurement and mention some of the situations that
may "fool" the meter. If you can't find the instructions, write to
the manufacture for them.
Once you have set the proper film
speed on your camera or meter, you are ready to make the exposure-meter
reading. With a reflected-light meter (in camera or handheld), point the
camera or meter at the subject. The meter will measure the average brightness
of the light reflected from the various parts of the scene. With an in-camera
meter, a needle or diode display in the viewfinder or an LCD display on top
of the camera will tell you when you have achieved the proper combination of
lens and shutter-speed settings. If the camera is fully manual. you must set
both the aperture and shutter speed. Automatic cameras may set both shutter
speed and aperture; or they may set just one of the controls, leaving you to
set the other.
If you're using a handheld meter,
read the information on your meter and set the camera controls accordingly.
An overall exposure reading taken from the camera position will give good
results for and average scene with an even distribution of light and dark
areas. For many subjects, then, exposure-meter operation is mostly
mechanical; all you do is point the meter (or camera) at the scene and set
the aperture and shutter speed as indicated. But your meter does not know if
you need a fast shutter speed to stop action or a small aperture to extend
depth of field. You will have to select the appropriate aperture and shutter
combination for the effect you want. There will be other situations where
either the lighting conditions or the reflective properties of the subject
will require you to make additional judgements about the exposure information
the meter provides, and you may have to adjust the camera controls
accordingly.
A reflected-light meter reading is
influenced by both how much light there is in the scene and how reflective
the subject is. The meter will indicate less exposure for a subject that reflects
little light, even if the two subject are in the same scene and in the same
light. Because reflected-light meters are designed to make all subjects
appear average in brightness, the brightness equivalent to medium gray, they
suggest camera settings that will overexpose (make too light) very dark
subjects and underexpose (make too dark) very light subjects.
Although reflected-light meters
are influenced more by the largest areas of the scene, the results will be
acceptable even when the main subject fills the picture but it's still of
average reflectance (neither very light nor very dark). However, what happens
if a relatively small subject is set against a large dark or light
background? The meter will indicate a setting accurate for the large area, not
for the smaller, but important, main subject. Therefore, when the area from
which you take a reflected-light reading is very light or very dark, and you
want to expose it properly, you should modify the meter's exposure
recommendation as follows:
To determine the correct exposure
for contrasty scenes with large areas that are much darker or much lighter
than the principle subject, take a selective meter reading of only the
subject itself. How do you do this? Move the meter or camera close to the
subject. Exclude unimportant dark or light areas that will give misleading
readings. In making close-up readings, also be careful not to measure your
own shadow or the meter's shadow.
Selective meter readings are
useful for dark subjects against a bright background like snow or light sand,
or for subjects in shade against a bright sunlit background. There is also
the reverse of this: The subject is in bright sun and the background is in
deep shade. In all these situations, your camera has no way of knowing which
part of the scene is the most important and requires the most accurate
exposure, so you must move in close so the meter will read only the key
subject area. For example, if you want to photograph a skier posed on a snowy
slope on a bright, sunny day, taking an average reading of the overall scene will
result in underexposure. The very bright snow will overly influence the meter
and the reading will be too high. The solution is to take a close-up reading
from the skier's face (or a piece of medium-toned clothing) and then step
back the desired distance to shoot the picture. Some cameras with built-in
meters have an exposure-hold button or switch to lock the exposure setting
when you do this.This technique is useful anytime the surroundings are much
brighter or darker than your subjects.
Landscapes and other scenes with
large areas of open sky can also fool the meter. The sky is usually much
brighter than other parts of the scene, so an unadjusted meter reading will
indicate too little exposure for the darker parts of the picture. One way to
adjust for this bias without having to move in close is to tilt your lens or
meter down to exclude the sky while taking your meter reading. The sky will
probably end up slightly overexposed, but the alternative would be to find a
different shooting position excluding most or all of the sky. There are also
graduated neutral density floaters that work well in such situations. A
neutral density filter absorbs all colors of visible light evenly, and you
can position a graduated filter so that the darker portion is at the top of
the image where it will darken the sky without affecting the ground below.
Incidentally, some built-in meters are bottom-weighted to automatically
compensate for situations like this, so check your manual.
Bright backlighting with the
subject in silhouette can also present a challenge. With the light shining
directly into the lens or meter, aiming the meter into the light can cause
too high a reading. If you don't want to underexpose the subject, take a
close-up reading, being especially careful to shade the lens or meter so that
no extraneous light influences the reading.
What if you can't walk up to your
subject to take a meter reading? For instance, suppose that you're trying to
photograph a deer in sunlight at the edge of a wood. If the background is
dark, a meter reading of the overall scene will give you an incorrect
exposure for the deer. Obviously, if you try to take a close-up reading of
the deer, you're going to lose your subject before you ever get the picture.
One answer is to make a substitute reading off the palm of your hand,
providing that your hand is illuminated by the same light as your subject,
then use a lens opening 1 stop larger than the meter indicates. For example,
if the reading off your hand is f/16, open up one stop to f/11
to get the correct exposure. The exposure increase is necessary because the
meter overreacts to the brightness of your palm which is about twice as
bright as an average subject. When you take the reading, be sure that the
lighting on your palm is the same as on the subject. Don't shade your palm.
Another subject from which you can
take more accurate and more consistent meter readings is a KODAK Gray Card,
sold by photo dealers. These sturdy cards are manufactured specifically for
photographic use. They are neutral gray on one side and white on the other.
The gray side reflects 18% of the light falling on it (similar to that of an
average scene), and the white side reflects 90%. You can use a gray card for
both black-and-white and color balance. Complete instructions are included in
the package with the cards.
How do you determine the correct
exposure for a high-contrast scene, one that has both large light and dark
areas? If the highlight of shadow areas are more important, take a close-up
reading of the important area to set the exposure. With color slide film,
keep in mind that you will get more acceptable results if you bias the
exposure for the highlights, losing the detail in the shadows. In a slide,
the lack of detail in the shadows is not as distracting as overexposed
highlights that project as washed-out color and bright spots on the screen.
If you are working with black-and-white film, you can adjust the development
for better reproduction of the scene contrast, particularly in highlights.
But what if the very light and
very dark areas are the same size and they are equally important to the
scene? One solution is to take selective meter readings from each of the
areas and use a f-number that is midway between the two indicated
readings. For instance, if your meter indicates an exposure of 1/125 second
at f/22 for brightest area and 1/125 second at f/2.8 for the
darkest area--a range of six stops--set your camera 1/125 second f/8.
This is a compromise solution, but sometimes it is your only choice short of
coming back another day or changing your viewpoint, and the composition of
the picture, to eliminate the contrast problem.
Perhaps the best solution when you
need a selective meter reading is offered by the spot meter. Handheld
averaging meters generally cover about 30º, while handheld spot meters
typically read a 1º angle The angle of spot meters built into the camera are
usually wider, about 3 to 12º. The biggest advantage of a spot meter is that
is allows you to measure the brightness of small areas in a scene form the
camera position without walking in to make a close-up reading. Since a spot
meter measures only the specific area you point it at, the reading is not
influenced by large light or dark surroundings. This makes a spot meter
especially useful when the principal subject is a relatively small part of
the overall scene and the background is either much lighter or darker than
the subject. Spot meters are also helpful for determining the scene
brightness range.
A spot meter can take more time to
use since it usually requires more than one reading of the scene. This is
particularly true when the scene includes many different bright or dark
areas. To determine the best exposure in such a situation, use the same
technique described previously for high-contrast subjects: Select the
exposure halfway between the reading for the lightest important area in the
scene and that for the darkest important area in the scene. Bear in mind,
though, all films have inherent limits on the range of contrast they can
accurately record. Remember too, you can sometimes create more dramatic
pictures by intentionally exposing for one small area, such as a bright spot
of sunlight on a mountain peak, and letting the dark areas fall into black
shadow without detail. Spot meters are ideal for such creative applications.
To use an incident-light meter,
hold it at or near the subject and aim the meter's light-sensitive cell back
toward the camera. The meter reads the amount of light illuminating the
subject, not light reflected from the subject, so the meter ignores the
subject and background characteristics. As with a reflected reading, an
incident reading provides exposure information for rendering average subjects
correctly, making incident readings most accurate when the subject is not
extremely bright or dark.
When taking an incident-light
reading, be sure you measure the light illuminating the side of the subject
you want to photograph, and be careful that your shadow isn't falling on the
meter. If the meter isn't actually at the subject, you can get a workable
reading by holding the meter in the same kind of light the subject is in.
Because the meter is aimed toward the camera and away from the background
light, an incident reading is helpful with backlighted subjects. This is also
the case when the main subject is small and surrounded by a dominant
background that is either much lighter or darker.
The exposure determined by an incident-light
meter should be the same as reading a gray card with a reflected-light meter.
Fortunately, many scenes have average reflectance with an even mix of light
and dark areas, so the exposure indicated is good for many picture-taking
situations. However, if the main subject is very light or very dark, and you
want to record detail in this area, you must modify the meter's exposure
recommendations as follows:
You will notice that these
adjustments are just the opposite from those required for a reflected-light
meter. An incident meter does not work well when photographing light sources
because it cannot meter light directly. In such situations you will be better
off using a reflected-light meter or an exposure table.
If the scene is unevenly
illuminated and you want the best overall exposure, make incident-light
readings in the brightest and darkest areas that are important to your
picture. Aim the meter in the direction of the camera position for each
reading. Set the exposure by splitting the difference between the two
extremes.
If your results are consistently
too light or too dark, your meter may be providing erroneous readings because
of age, extensive use, or malfunction. The first thing to suspect, when a
meter fails or readings become erratic, is the battery. Handheld and
in-camera meters require fresh batteries to provide accurate readings; you
should replace batteries at least once a year. Cold weather is particularly
hard on batteries, so if you're shooting in winter, carry backup batteries in
a warm inside pocket where they'll retain their charge. If new batteries
don't cure the problem, have your camera or meter checked by a professional
repair technician or by the manufacturer.
To check the accuracy of your
meter, you can compare its readings with a guide such as the daylight
exposure table found inside a Kodak film carton. On a sunny day, aim your
camera meter at frontlighted, average subject. The exposure indicated by the
meter should match the film carton recommendation within a 1/2 stop. For
example, the suggested exposure for KODACHROME 64 Film is 1/60 at f/16
or an equivalent shutter-aperture combination. If your meter indicates
another setting, it may need repair or adjustment. If your meter indicates
the correct exposure but your pictures are still too light or too dark, then
your camera's shutter or lens aperture may be at fault.
This simple daylight exposure
guideline works because it is based on the relatively constant light from the
sun plus the sky on sunny days. Sometimes called the "f/16"
rule, the exposure for a frontlighted, average subject in bright sun
(distinct shadows) is f/16 at a shutter speed that matches the film
speed. For example, if you are using KODAK GOLD 200 Film (ISO 200), the
exposure should be 1/250 second at f/16, the same as the film carton
recommendation. Incidentally, even though most cameras do not have a 1/200
second shutter speed setting, the exposure latitude of the film will easily
cover the slight underexposure at 1/250. If you are using a slide film with
less exposure latitude, you can slightly adjust the aperture setting as
needed when the shutter speed does not exactly match the film speed.
As with a meter reading, the
exposure is adjusted for changes in the lighting level, the direction of the
light and the reflectivity of the light, and the reflectivity of the subject.
For example, you should decrease exposure by one stop for bright sun
on light snow or sand, and increase exposure by one stop for an
average subject in weak or hazy sun (soft shadows). There is an additional
stop increase for cloudy bright conditions (no shadows), and another for
heavy overcast or open shade. Using the previous example with GOLD 200 Film, the
change from bright sun to heavy overcast or open shade for an average subject
is three stops --a suggested exposure for f/5.6 at 1/250. Increase
exposure by one stop (either a slower shutter step or larger lens opening)
from the normal for sidelighting, and two stops for backlighting.
If your pictures are consistently
too light or too dark under a variety of lighting conditions, your lens
diaphragm or shutter may be sticking or your meter may be slightly off. You
can temporarily compensate for minor meter or shutter/aperture errors by
adjusting the ISO setting on your camera or exposure meter (or your camera's
exposure-compensation control) based on the following test procedure. For
this test, you will probably find it easier to use a slide film rather than a
print film. Color-slide films work best because they have more critical
exposure requirements, and there won't be any corrections made during
printing that would make it more difficult to judge the accuracy of the
initial exposure. Accurately evaluating results from print film requires an
examination of the negatives--something that isn't as easy as judging slides.
To begin, set the appropriate film
speed on your camera or meter and make a light reading of an average frontlighted
subject. Then, using this reading as a starting point, make a series of test
exposures beginning with the recommended exposure. If your pictures have been
consistently too dark, make a second exposure with a lens opening 1/2 stop
larger than the meter indicates and a third exposure with a lens opening that
is 1 stop larger. For example, if the meter indicates 1/250 at f/8,
use these settings for the first exposure, then shoot the second at an
aperture midway between f/8 and f/5.6 (without changing the
shutter speed); shoot the third at 1/250 at f/5.6.
If your pictures have been
consistently too light using the recommended exposure, make these exposures
with lens openings 1/2 stop and 1 full stop smaller than the one the meter
recommends. Using the example of 1/250 at f/8, make your first test
exposure at 1/250 second at f/8. Shoot the second at a lens opening
halfway between f/8 and f/11, and make the third at 1/250 at f/11.
Be sure to write down the meter reading, exposure, and frame number for each test
picture. One sure way to remember the exposure settings is to include them
right in the picture. You can write the lens opening and shutter speed on a
large piece of paper and include it in the scene you're photographing. (Don't
include it when you're metering.)
View the processed slides on a
light box or under your usual projection conditions and select the best
exposure. Then use your exposure notes for that slide to find the best
film-speed number for that particular meter and film combination. For
example, if your best exposure is the one that was a full stop less exposure,
the correct ISO speed for the meter/film combination would be twice the
recommended speed (ISO 200 instead of ISO 100, for instance), since this
would give you a stop less exposure for all future shots. If the best test
exposure was the one that was shot at a full stop more than the meter
indicated, you should halve the ISO number (ISO 50 instead of ISO 100) or set
the exposure-compensation control to +1. Each doubling or halving of the film
speed is equal to a 1-stop adjustment. If you want to increase exposure, use
a lower number; if you want to decrease exposure, use a higher number.
As a result of your exposure tests
with color-slide film, you can change the film-speed number of other films by
the same percentage.
When you are faced with a scene
that challenges your normal metering techniques, a good solution is to
bracket your exposures.
Bracketing doesn't simply mean
guessing. It is a calculated technique to obtain a few additional insurance
exposures that are based on your best estimate for correct results. The key
is to recognize those unusual situations or unorthodox lighting conditions
where a single exposure may not assure you of success. Because slide films
have less exposure latitude than negative films, bracketing is a technique
that is more useful for slide films.
Bracketing is a simple procedure.
First take a reflected-light meter reading of the subject from the camera
position or use the recommendation in an exposure table. This will give you
an approximate exposure and provide a starting point for making any
subsequent adjustments. Take one picture using this exposure. Then bracket
your exposure by using one stop larger and then one stop smaller than the
indicated exposure. For extra assurance, you can use a bracketing range of ±
2 stops. For more subtle control, particularly with slide films, you can
bracket in 1/2-stop increments.
Negative films for either
black-and-white or color prints, are generally more tolerant of overexposure
than underexposure, so bracket toward more rather than less exposure unless
there is a significant risk of badly overexposing important elements of the
scene. Usually, one-stop increments are enough because of the film's exposure
latitude. Whatever you do, it's a good idea to write down the meter reading
and the settings used for each exposure so you can compare them with your
results to help you judge similar situations in the future.
Kodak,
Gold, and Kodachrome are trademarks.
AF-9
October 1998
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