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تحديد الوضع الصحيح ..
عدادات التعرض ..
EXPOSURE METERS
Every photographer should know enough about the principles of correct exposure to be able to estimate exposures with a fair degree of accuracy . That is why we have spent so much time on this subject . In actual practice , however , very few of the better photographers rely entirely on their own judgment - they use exposure meters . These meters actually measure the intensity of the light , so if used properly they eliminate much of the guess - work that often results in wrong exposures .
There are so many different makes of exposure meters on the market that it is impossible to give specific instructions for the use of all of them , but the manufacturers always provide such complete operating instructions that you should have no trouble with any of them . However , it will be time well spent if we discuss rather briefly the type of exposure meters available so that you will understand the general principles of each .
There are three well - recognized types that you should know about - the slide - rule type , the extinction type , and the photoelectric type .
SLIDE - RULE EXPOSURE METERS
These are the simplest and least expensive of them all . Actually , they are not exposure meters in the true sense , but exposure calculators . A slide - rule exposure meter is nothing more than a convenient form of exposure table , generally somewhat more detailed than the one given in this Assignment . It is printed on durable material so that you can slide one section with reference to another and thus set it so that you can read the film speed against the light value and get the correct exposure .
EXTINCTION - TYPE EXPOSURE METERS
Exposure meters of the extinction type were widely used before the introduction of the photoelectric type , but now they are used mainly in very inexpensively designed cameras for amateurs who do not feel justified in buying the more expensive photoelectric meters . To use an extinction type meter , you generally look through it at your subject . You either see the subject or an illuminated number ; you then turn a ring or some similar device which cuts down the intensity of the illuminated number . When the number is just visible , you can read the exposure from a scale attached to the ring you rotated . A modification of the extinction type of meter is built into the simple Polaroid camera designed primarily for use by the novice .
PHOTOELECTRIC EXPOSURE METERS
By far the most accurate exposure meters are the photoelectric type . The following pages will discuss such meters in detail .
Ordinary light can be changed into electricity through a device known as a photoelectric cell . The cells used in exposure meters are of two types - the true photoelectric cell and the photo - resistive cell . The true photoelectric cell , also called the photovoltaic cell , generally uses selenium as its active element and for that reason meters using it are often called selenium type meters . Such a cell . generates an electric voltage when light falls on it . With this type of cell , no external batteries are required - the tiny electric voltage is simply measured by a very sensitive meter . The meter scale , instead of being marked off in units of electricity , is marked off in units of light or in units of exposure . A typical exposure meter of this kind is shown in Figure 6 .
The photo - resistive cell generally uses cadmium sulfide as its active element . Meters using it are generally called Cadmium Sulfide ( abbreviated Cd S ) meters . The cadmium sulfide cell does not actually generate an electric voltage . With the cadmium sulfide type of photoelectric cell , you need a small dry cell or battery to operate the meter . The battery current registers on the meter .
Fig . 6 A photoelectric exposure meter using a selenium cell ( left ) and one using a Cd S cell ( right ) as the light sensitive elements ..
The photocell simply changes its own electrical resistance when light falls on it . This change in resistance causes a change in the meter reading and thus indicates the light intensity . A typical Cd S exposure meter is shown in Figure 7 .
The particular advantage of a Cd S meter is that it responds to extremely low light intensities . This makes it very useful for indoor photography , or for photography under very poor light conditions . The principle disadvantage , of course , is that the cell has to be replaced from time to time . A selenium photocell on the other hand will function almost indefinitely without attention .
Fig . 7 A Cd S exposure meter with attachments to make it usable ( 1 ) as spot meter , ( 2 ) with micro scope , ( 3 ) as an enlarging meter .
Because of the much greater sensitivity of the Cd S photocell , it is made very much smaller and is , therefore , used with excellent results in the design of extremely tiny exposure meters . For that reason , it has replaced the selenium type of meter almost entirely for the very compact meter systems that are built into cameras . These tiny meters , however , are still capable of giving excellent results .
Exposure meters may be used to measure light in two ways . They may be used to measure the light that is actually falling on the subject ( this is called incident light ) , or they may be used to measure the light that is reflected from the subject into the camera . ( This is called reflected light . ) Most exposure meters are designed to measure reflected light . A good many such meters , however , also have attachments which permit the measurement of incident light . Let us see how ! these two types of meters are used and what are their respective advantages .
INCIDENT LIGHT METERS . The idea in back of incident light meters is that the strength of the light falling on the subject determines the exposure . If this is true , all you need do is to measure that light . Dark areas of the subject will photograph . dark , light areas will photograph light , and so the subject will be represented as it appears to the eye . The exposure latitude of the film will allow for the proper recording of all the variations of brightness in the picture area . This will ordinarily be true as long as there are no areas of extreme brightness or deep shadow .
The incident light meter is quite simple to use . You stand near the subject and point the meter at the camera . In this way the meter receives the same illumination as the subject . If you cannot stand near the subject , or if it is an open landscape , stand in a spot that has the same illumination as the subject and point the meter in the same direction and at the same angle as you would if you were pointing at the subject .
Where the subject illumination is very uneven , as is usually the case indoors , or where there are areas of extreme highlights or shadows which must be properly recorded , the incident light method is not too successful .
REFLECTED LIGHT METERS . Reflected light meters work on the principle that the camera sees and records the light reflected from the subject . With this system , the best approach is to measure this reflected light , not the light falling on the subject . This automatically takes into account any variation in reflectivity of the subject . It also permits the best exposure for subjects where the range of light intensities may be beyond the exposure latitude of the film .
Since reflected light meters can be used in a wider variety of situations and they are by far the most commonly used meters , we will use them in the following discussion .
Bear in mind that no two manufacturers make their meters exactly alike . As a result , our comments on exposure meters must be of a general nature . If you have an exposure meter , carefully study the instruction book which came with it after you have read this Assignment . The instruction booklet will help you better understand your particular meter .
تحديد الوضع الصحيح ..
عدادات التعرض ..
EXPOSURE METERS
Every photographer should know enough about the principles of correct exposure to be able to estimate exposures with a fair degree of accuracy . That is why we have spent so much time on this subject . In actual practice , however , very few of the better photographers rely entirely on their own judgment - they use exposure meters . These meters actually measure the intensity of the light , so if used properly they eliminate much of the guess - work that often results in wrong exposures .
There are so many different makes of exposure meters on the market that it is impossible to give specific instructions for the use of all of them , but the manufacturers always provide such complete operating instructions that you should have no trouble with any of them . However , it will be time well spent if we discuss rather briefly the type of exposure meters available so that you will understand the general principles of each .
There are three well - recognized types that you should know about - the slide - rule type , the extinction type , and the photoelectric type .
SLIDE - RULE EXPOSURE METERS
These are the simplest and least expensive of them all . Actually , they are not exposure meters in the true sense , but exposure calculators . A slide - rule exposure meter is nothing more than a convenient form of exposure table , generally somewhat more detailed than the one given in this Assignment . It is printed on durable material so that you can slide one section with reference to another and thus set it so that you can read the film speed against the light value and get the correct exposure .
EXTINCTION - TYPE EXPOSURE METERS
Exposure meters of the extinction type were widely used before the introduction of the photoelectric type , but now they are used mainly in very inexpensively designed cameras for amateurs who do not feel justified in buying the more expensive photoelectric meters . To use an extinction type meter , you generally look through it at your subject . You either see the subject or an illuminated number ; you then turn a ring or some similar device which cuts down the intensity of the illuminated number . When the number is just visible , you can read the exposure from a scale attached to the ring you rotated . A modification of the extinction type of meter is built into the simple Polaroid camera designed primarily for use by the novice .
PHOTOELECTRIC EXPOSURE METERS
By far the most accurate exposure meters are the photoelectric type . The following pages will discuss such meters in detail .
Ordinary light can be changed into electricity through a device known as a photoelectric cell . The cells used in exposure meters are of two types - the true photoelectric cell and the photo - resistive cell . The true photoelectric cell , also called the photovoltaic cell , generally uses selenium as its active element and for that reason meters using it are often called selenium type meters . Such a cell . generates an electric voltage when light falls on it . With this type of cell , no external batteries are required - the tiny electric voltage is simply measured by a very sensitive meter . The meter scale , instead of being marked off in units of electricity , is marked off in units of light or in units of exposure . A typical exposure meter of this kind is shown in Figure 6 .
The photo - resistive cell generally uses cadmium sulfide as its active element . Meters using it are generally called Cadmium Sulfide ( abbreviated Cd S ) meters . The cadmium sulfide cell does not actually generate an electric voltage . With the cadmium sulfide type of photoelectric cell , you need a small dry cell or battery to operate the meter . The battery current registers on the meter .
Fig . 6 A photoelectric exposure meter using a selenium cell ( left ) and one using a Cd S cell ( right ) as the light sensitive elements ..
The photocell simply changes its own electrical resistance when light falls on it . This change in resistance causes a change in the meter reading and thus indicates the light intensity . A typical Cd S exposure meter is shown in Figure 7 .
The particular advantage of a Cd S meter is that it responds to extremely low light intensities . This makes it very useful for indoor photography , or for photography under very poor light conditions . The principle disadvantage , of course , is that the cell has to be replaced from time to time . A selenium photocell on the other hand will function almost indefinitely without attention .
Fig . 7 A Cd S exposure meter with attachments to make it usable ( 1 ) as spot meter , ( 2 ) with micro scope , ( 3 ) as an enlarging meter .
Because of the much greater sensitivity of the Cd S photocell , it is made very much smaller and is , therefore , used with excellent results in the design of extremely tiny exposure meters . For that reason , it has replaced the selenium type of meter almost entirely for the very compact meter systems that are built into cameras . These tiny meters , however , are still capable of giving excellent results .
Exposure meters may be used to measure light in two ways . They may be used to measure the light that is actually falling on the subject ( this is called incident light ) , or they may be used to measure the light that is reflected from the subject into the camera . ( This is called reflected light . ) Most exposure meters are designed to measure reflected light . A good many such meters , however , also have attachments which permit the measurement of incident light . Let us see how ! these two types of meters are used and what are their respective advantages .
INCIDENT LIGHT METERS . The idea in back of incident light meters is that the strength of the light falling on the subject determines the exposure . If this is true , all you need do is to measure that light . Dark areas of the subject will photograph . dark , light areas will photograph light , and so the subject will be represented as it appears to the eye . The exposure latitude of the film will allow for the proper recording of all the variations of brightness in the picture area . This will ordinarily be true as long as there are no areas of extreme brightness or deep shadow .
The incident light meter is quite simple to use . You stand near the subject and point the meter at the camera . In this way the meter receives the same illumination as the subject . If you cannot stand near the subject , or if it is an open landscape , stand in a spot that has the same illumination as the subject and point the meter in the same direction and at the same angle as you would if you were pointing at the subject .
Where the subject illumination is very uneven , as is usually the case indoors , or where there are areas of extreme highlights or shadows which must be properly recorded , the incident light method is not too successful .
REFLECTED LIGHT METERS . Reflected light meters work on the principle that the camera sees and records the light reflected from the subject . With this system , the best approach is to measure this reflected light , not the light falling on the subject . This automatically takes into account any variation in reflectivity of the subject . It also permits the best exposure for subjects where the range of light intensities may be beyond the exposure latitude of the film .
Since reflected light meters can be used in a wider variety of situations and they are by far the most commonly used meters , we will use them in the following discussion .
Bear in mind that no two manufacturers make their meters exactly alike . As a result , our comments on exposure meters must be of a general nature . If you have an exposure meter , carefully study the instruction book which came with it after you have read this Assignment . The instruction booklet will help you better understand your particular meter .
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