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3-1 Introduction

Light makes photography possible.  It is reflected from the subject, then it enters the camera and exposes the film or falls onto the digital camera’s charg-coupled device (CCD). 

Scientists tell us that light is travels in waves.  In many respects the waves of light can be compared to sound waves.  Sound waves vary in length, and they register as different pitches.  Light waves register as different colors.  This principle will enable you to use light more effectively when photographing. 

In photography, light is the most important basic ingredient and must be measured accurately.  The intensity of light determines the brightness of the subject.  The formula that describes this is the inverse-square law (Figure 3-1).  It states that light decreases as the square of the distance increases.  Being familiar with the light falling upon a subject from a source is called incident light.  When incident light strikes a surface, it will change direction; this change is called reflection.  If the surface is smooth the reflected light is said to be specular; however, if the surface is rough, the reflected light is diffused.  Most objects reflect back both types of light.


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Figure 3-1. Inverse-square law.

Reflection is an important characteristic of light.   It is how our eyes can see objects, how a film acquires a latent image and what the digital camera’s CDD electronically translates. 

In this lesson you will learn about using light meters, shooting photographs using available light, electronic flash and filters.


Light Meters and Metering

Both the 35mm and digital photographer use photoelectric light meters to measure the intensity of light.  These light meters have photosensitive cells that control the current as light strikes them.  This current is measured by a microammeter, and the calibrations of the meter can be interpreted to give the photographer the required camera shutter speed and the lens aperture setting with a given film speed.  Modern 35mm cameras and digital cameras use internal electronics to calculate exposure settings automatically.  There are several types of light meters.


Reflected light meter

In this type of photoelectric light meter, the ammeter is calibrated to permit measurement of the amount of light reflected back into the meter by a particular subject.  It should be aimed at the subject when used, because it will measure all of the light that it is aimed at (including backlight).  This is the type of light meter built into most 35mm and digital cameras.


Through-the-lens meter

This type of reflected light meter reads the light being reflected from the subject into the camera’s lens.  Almost all 35mm single-lens reflex cameras and digital cameras have a built-in, through-the-lens meter.

Some advantages of the built-in meter are that it is always available, compensating for filters (filter factor) is unnecessary and the meter reads only the picture area (if you change lenses the meter's angle of view changes too).

Used correctly, through-the-lens meters are accurate.   But you will still need a handheld meter in situations that call for an incident light meter.

Be especially careful when shooting a subject that has backlighting.  Don't hesitate to move in close to take the meter reading to eliminate the backlight from the camera's field of view, then move back for the shot.

Most through-the-lens meters are "center-weighted."  That  means they will give more attention to the level of light in the center of the viewfinder than the edges of the viewfinder.  You should keep that fact in mind when using them. 


Incident light meter

In this type of photoelectric light meter, the ammeter is similar to the one used in reflected light meters.  In this meter, however, a diffusing device is placed over the meter light window to permit measurement of the amount of light falling on a particular subject.  It should be held in the hand next to the subject and, when used, aimed at the light source. 


Practical Light Metering

Photoelectric meters are valuable tools when used with common sense.   Since every subject reflects various intensities of light with some shadow areas, one general reading does not give a correct measurement of existing light values.

Before measuring light values, determine the type of photograph needed.  If shadow detail is important, measure the shadow area carefully.   If the highlight area is to be emphasized, base exposure on the measurement of the highlight areas alone.  To record both shadow and highlight areas in equal detail, measure both areas and use a compromise setting.

Do not make your light reading at a considerable distance from your subject, because this reading will include distracting areas that are not usually a part of the photograph.  When taking specific readings, hold the meter close enough to the subject to indicate the average reflected light.  The distance recommended is equal to the width of the subject. 

If a strong backlight is likely to throw your meter reading off, then you should move in close enough so that the backlight is not visible to the meter.  With a through-the-lens meter you can also do this by changing to a telephoto lens to get a reading on just the main subject.

Before using an exposure meter, make sure that the light value indicator points to zero while the photo cell unit is covered.  If it does not, turn it in for repair.

Shield exposure meters from shock, strong magnetic influence, dampness, and extreme heat.  Never point a reflection-type meter at the sun. 

Check the calibration of a meter periodically against the quality of resulting photographs.



The substitution method of determining exposure is simply the process of substituting an object from which to take a reflected-light reading when a reading cannot be taken in the usual manner.  This method is useful when it is difficult to approach the subject close enough.  It can be applied to small areas within a scene, either light or dark objects, or to an entire scene.  In normal use, a meter should be held close enough to the principle object or scene to exclude all light except that which is reflected from the area to be included in the picture.  Many times it is impractical to do this.  In such instances, an object which has approximately the same brightness can be substituted.  The back of the hand can be substituted when determining the light value of flesh tones, assuming the lighting where you are is similar to that of your subject. 



If you have any doubts about the proper exposure for a particular scene then you should shoot additional exposures at one f/stop lower and one f/stop higher than the exposure indicated by the built-in meter.  Many advanced 35mm and digital cameras have an "autobracket" feature which, when engaged, automatically brackets each shot.  See your owner’s manual for details pertaining to your camera.


Advanced Exposure Metering Systems

Modern 35mm and professional digital systems incorporate several automated exposure metering options.  These allow the photographer to choose a metering mode appropriate to the lighting conditions affecting not only the subject but also the objects within the frame’s periphery.  Some (Kodak) examples—

Evaluative metering is recommended for general subjects and backlit scenes.  By dividing the viewfinder into 16 metering zones linked with the five focusing points, the camera evaluates subject size, position (based on the focusing point in use), brightness, background, front lighting and back lighting to determine the best exposure setting. 

Partial metering limits the metering area to the center of the viewfinder (approximately 23% of the image area).  Select this mode when the subject is backlit or near a strong light source.  Consult your owner’s manual for instructions on the use of your camera’s options.

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