تويت
التصوير الفوتوغرافي الحديث
تحديد الوضع الصحيح ..
العوامل التي يحكم التعرض لها
FACTORS WHICH GOVERN EXPOSURE
Occasionally you hear students of photography say , " Why worry about understanding exposure when you can get all the information you need from an exposure meter ? " It is true , there is no trick at all to pointing a meter at the subject and reading it . Anybody who spends a few minutes reading instructions can do it . For ordinary , average subjects , that is good enough . But , if you want to become a good photographer , whether amateur or professional , you must understand the basic principles of exposure so you can use good judgment in interpreting the information your exposure meter gives you . It cannot think for you . The four factors which govern correct exposure are :
1. The type of subject .
2. The intensity of the light .
3. The speed of the lens .
4. The speed of the film .
TYPE OF SUBJECT
First , let us think of ourselves in the middle of an ocean with no land in sight - nothing but cloudless sky and water . There is nothing to hold back the sun's light , so there are no shadows . Everything is brilliantly illuminated by the full light of the sun . If we wanted to photograph such a seascape we would obviously need a very short exposure .
Fig . 3 Open seascape in bright sunlight re quires minimum exposure unless a close - up is made which will include important shadow areas .
Fig . 4 Even though sun is shining brightly , much of the light is cut off by trees . Expo sure must be increased accordingly .
Now let us put some boats on the water , not too far away . This gives us . some shadows , as shown in Figure 3. If we want to photograph detail in the boats , parts of which are not in the direct light of the sun , we must increase our exposure somewhat . Now let us suppose we are back on land on an open desert . with the whole sky illuminating it . If we set an object in the middle of this plain , we would require about the same exposure as for the marine scene . If we add anything , such as trees , buildings , or nearby mountains , they will cut off light from part of the sky . As a result , we will have to increase our exposure accordingly . To take a picture like the one in Figure 4 , exposure would have to be increased a great deal . Even though it is a bright sunny day , the trees are blocking out much of the light .
This increase of exposure as objects cut off the light is more important than most beginners in photography realize . Suppose we place a wall on one side of the subject - almost half the light is cut off . Build another wall on the other side , and the subject is illuminated only by a narrow strip of sky . Such a subject would require even more exposure . This explains why snapshots in city streets such as Figure are often underexposed .
Suppose we put a roof over the walls so the only light reaching our subject comes from the side , as through a window in a home . Suddenly we have cut down the volume of light tremendously . Now we might need to increase our exposure by a hundred or more times . It is now obvious why pictures taken under trees , near buildings , or porches , and indoors require much longer exposures than open landscapes . No matter . how . bright it may be somewhere else , it is the amount of light falling on the subject which must be considered in estimating exposure . In a heavily wooded . grove or in a house , only a small amount of light falls on the subject . In other words , the type of subject an open marine view , a portrait in the shade , or an interior view - must be taken into . consideration when determining the exposure .
INTENSITY OF LIGHT
Obviously , the stronger the light , the shorter the exposure needed to produce the required chemical reaction in the film . Unfortunately , the human eye is not a good judge of light intensity . The sun . may be shining full strength , or it may be hazy enough to cut light . intensity in half , yet the eye will see little difference . At night , a brightly illuminated room may appear as dazzingly bright as a sunny scene outdoors , yet it may have only 1 / 50th or 1 / 100th as much actual light intensity . This is why it is necessary to accurately measure light intensity - and this is why we have exposure meters .
Fig . 5 Tall city buildings greatly reduce the amount of light at street level . Such scenes seem brighter than they really are , and longer exposures are required .
SPEED OF THE LENS
You will recall that the speed of a lens is determined by its f / value . A fast lens can be opened wider than a slow lens , and will have a higher f / value ( smaller f / number ) . Naturally a fast lens with larger lens aperture will admit more light , and require less time for exposure . The subject of lens apertures has already been discussed in Assignment 2. We need only add that all exposure meters take the lens aperture into consideration when indicating the shutter speed for correct exposure .
SPEED OF THE FILM
All film emulsions do not react equally to the same amount of light . Some films are much more sensitive than others , and therefore respond to much weaker light . These more sensitive films must be given shorter exposures , either by stopping down the lens , or using a faster shutter speed .
We will say more about film speed later in this Assignment . However , we do want to make this one point clear . You must know the speed of the film you are using before you can estimate exposure correctly . As an example , an average subject in bright summer sun , when photographed on a slow film may require an exposure of 1 / 100th second at f / 8 . Under the same conditions , a medium - speed film may be correctly exposed at 1 / 200th second at f / 16 , and a super - speed film at only 1 / 500th second at f / 22 or less .
One other point should be mentioned . A number of different systems are used for rating the speed of a film . The standard method used in the United States is known as the ASA System . Practically all exposure meters sold for use in the United States use this system of film - speed rating . In the ASA rating system , the speed of a film is proportional to its rating number . For example , a slow film may have an ASA speed of 40 , a medium - speed film may be four times as fast with a rating of 160 , and a fast film may be four times faster still , or ASA 640 .
تحديد الوضع الصحيح ..
العوامل التي يحكم التعرض لها
FACTORS WHICH GOVERN EXPOSURE
Occasionally you hear students of photography say , " Why worry about understanding exposure when you can get all the information you need from an exposure meter ? " It is true , there is no trick at all to pointing a meter at the subject and reading it . Anybody who spends a few minutes reading instructions can do it . For ordinary , average subjects , that is good enough . But , if you want to become a good photographer , whether amateur or professional , you must understand the basic principles of exposure so you can use good judgment in interpreting the information your exposure meter gives you . It cannot think for you . The four factors which govern correct exposure are :
1. The type of subject .
2. The intensity of the light .
3. The speed of the lens .
4. The speed of the film .
TYPE OF SUBJECT
First , let us think of ourselves in the middle of an ocean with no land in sight - nothing but cloudless sky and water . There is nothing to hold back the sun's light , so there are no shadows . Everything is brilliantly illuminated by the full light of the sun . If we wanted to photograph such a seascape we would obviously need a very short exposure .
Fig . 3 Open seascape in bright sunlight re quires minimum exposure unless a close - up is made which will include important shadow areas .
Fig . 4 Even though sun is shining brightly , much of the light is cut off by trees . Expo sure must be increased accordingly .
Now let us put some boats on the water , not too far away . This gives us . some shadows , as shown in Figure 3. If we want to photograph detail in the boats , parts of which are not in the direct light of the sun , we must increase our exposure somewhat . Now let us suppose we are back on land on an open desert . with the whole sky illuminating it . If we set an object in the middle of this plain , we would require about the same exposure as for the marine scene . If we add anything , such as trees , buildings , or nearby mountains , they will cut off light from part of the sky . As a result , we will have to increase our exposure accordingly . To take a picture like the one in Figure 4 , exposure would have to be increased a great deal . Even though it is a bright sunny day , the trees are blocking out much of the light .
This increase of exposure as objects cut off the light is more important than most beginners in photography realize . Suppose we place a wall on one side of the subject - almost half the light is cut off . Build another wall on the other side , and the subject is illuminated only by a narrow strip of sky . Such a subject would require even more exposure . This explains why snapshots in city streets such as Figure are often underexposed .
Suppose we put a roof over the walls so the only light reaching our subject comes from the side , as through a window in a home . Suddenly we have cut down the volume of light tremendously . Now we might need to increase our exposure by a hundred or more times . It is now obvious why pictures taken under trees , near buildings , or porches , and indoors require much longer exposures than open landscapes . No matter . how . bright it may be somewhere else , it is the amount of light falling on the subject which must be considered in estimating exposure . In a heavily wooded . grove or in a house , only a small amount of light falls on the subject . In other words , the type of subject an open marine view , a portrait in the shade , or an interior view - must be taken into . consideration when determining the exposure .
INTENSITY OF LIGHT
Obviously , the stronger the light , the shorter the exposure needed to produce the required chemical reaction in the film . Unfortunately , the human eye is not a good judge of light intensity . The sun . may be shining full strength , or it may be hazy enough to cut light . intensity in half , yet the eye will see little difference . At night , a brightly illuminated room may appear as dazzingly bright as a sunny scene outdoors , yet it may have only 1 / 50th or 1 / 100th as much actual light intensity . This is why it is necessary to accurately measure light intensity - and this is why we have exposure meters .
Fig . 5 Tall city buildings greatly reduce the amount of light at street level . Such scenes seem brighter than they really are , and longer exposures are required .
SPEED OF THE LENS
You will recall that the speed of a lens is determined by its f / value . A fast lens can be opened wider than a slow lens , and will have a higher f / value ( smaller f / number ) . Naturally a fast lens with larger lens aperture will admit more light , and require less time for exposure . The subject of lens apertures has already been discussed in Assignment 2. We need only add that all exposure meters take the lens aperture into consideration when indicating the shutter speed for correct exposure .
SPEED OF THE FILM
All film emulsions do not react equally to the same amount of light . Some films are much more sensitive than others , and therefore respond to much weaker light . These more sensitive films must be given shorter exposures , either by stopping down the lens , or using a faster shutter speed .
We will say more about film speed later in this Assignment . However , we do want to make this one point clear . You must know the speed of the film you are using before you can estimate exposure correctly . As an example , an average subject in bright summer sun , when photographed on a slow film may require an exposure of 1 / 100th second at f / 8 . Under the same conditions , a medium - speed film may be correctly exposed at 1 / 200th second at f / 16 , and a super - speed film at only 1 / 500th second at f / 22 or less .
One other point should be mentioned . A number of different systems are used for rating the speed of a film . The standard method used in the United States is known as the ASA System . Practically all exposure meters sold for use in the United States use this system of film - speed rating . In the ASA rating system , the speed of a film is proportional to its rating number . For example , a slow film may have an ASA speed of 40 , a medium - speed film may be four times as fast with a rating of 160 , and a fast film may be four times faster still , or ASA 640 .
تعليق