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ما الذي تتوقف العدسة عن استخدامه
WHAT LENS STOP TO USE
The aperture or lens stop of a lens has a number of very important uses . No photographer can consider himself an experienced operator unless he understands these uses and can select the right lens stop for the job he has to do . Let us list the principal uses to which lens stops are put so we can discuss them more fully .
1. To regulate exposure
2. To increase depth of field
3. To increase over - all sharpness
4. To obtain selective focus
1. USING THE LENS STOP TO REGULATE EXPOSURE . This is the primary . reason for equipping lenses with variable diaphragms . A photographic film will give a satisfactory image only for a rather limited range of light values . If not enough light or if too much light reaches the film , the picture won't be right . The lens diaphragm , together with the shutter , permits you to regulate the amount of light reaching the film .
2. USING THE LENS STOP TO INCREASE DEPTH OF FIELD . If an object a few feet from the camera is sharply focused with a large stop , the background will be blurred because it will be out of focus . Stopping the lens down will permit you to bring both the foreground and background into good focus at the same time . Because depth of field is greater for lenses of short focal lengths , it is possible to use fairly wide open stops with small cameras or with movie cameras and still get good depth of field . For example , when focused at 15 feet , you will get a depth . of field from 12 % to 19 feet at f / 3.5 with a 4 - inch lens , but if you use an 8½ - inch lens , you must stop down to f / 16 to get the same depth of field , and if you use a 12 - inch lens , you must stop down to // 22 .
3. USING A LENS STOP TO INCREASE OVER - ALL SHARPNESS . In discussing lens aberrations , we have seen how they could be corrected or neutralized in practical lens design . However , even the best lenses it isn't possible to correct all aberrations perfectly , and some slight amount remains . Stopping a lens down from maximum aperture , reduces these aberrations and results in a sharper picture , but there are limits beyond which there is no need to go . With the better lenses , stopping the lens down one stop or two at the most is enough ; with lower priced lenses , two or three stops is necessary . Further stopping down does not reduce the aberrations ( but it does increase the depth of field ) . With lenses of short - focal length , it isn't advisable to stop down much below f / 11 for a 50mm lens , because at this point diffraction ( bending of the light rays they pass through a very small opening ) sets in and further stopping down may actually reduce sharpness . However , lenses with longer focal lengths generally used by commercial photographers may be stopped down to f / 22 and even smaller .
4. USING THE LENS STOP TO OBTAIN SELECTIVE FOCUS . It isn't always best to get everything in the field of view of the camera in the sharpest possible focus . Sometimes , as in portraiture or in commercial studio work , you may want to have the principal object sharp and clear but the foreground and background more or less out of focus so that they aren't quite so noticeable . In that case you can use a relatively large stop and focus on the principal object . This is clearly illustrated in Figure 18 .
تجهم الصور ..
ما الذي تتوقف العدسة عن استخدامه
WHAT LENS STOP TO USE
The aperture or lens stop of a lens has a number of very important uses . No photographer can consider himself an experienced operator unless he understands these uses and can select the right lens stop for the job he has to do . Let us list the principal uses to which lens stops are put so we can discuss them more fully .
1. To regulate exposure
2. To increase depth of field
3. To increase over - all sharpness
4. To obtain selective focus
1. USING THE LENS STOP TO REGULATE EXPOSURE . This is the primary . reason for equipping lenses with variable diaphragms . A photographic film will give a satisfactory image only for a rather limited range of light values . If not enough light or if too much light reaches the film , the picture won't be right . The lens diaphragm , together with the shutter , permits you to regulate the amount of light reaching the film .
2. USING THE LENS STOP TO INCREASE DEPTH OF FIELD . If an object a few feet from the camera is sharply focused with a large stop , the background will be blurred because it will be out of focus . Stopping the lens down will permit you to bring both the foreground and background into good focus at the same time . Because depth of field is greater for lenses of short focal lengths , it is possible to use fairly wide open stops with small cameras or with movie cameras and still get good depth of field . For example , when focused at 15 feet , you will get a depth . of field from 12 % to 19 feet at f / 3.5 with a 4 - inch lens , but if you use an 8½ - inch lens , you must stop down to f / 16 to get the same depth of field , and if you use a 12 - inch lens , you must stop down to // 22 .
3. USING A LENS STOP TO INCREASE OVER - ALL SHARPNESS . In discussing lens aberrations , we have seen how they could be corrected or neutralized in practical lens design . However , even the best lenses it isn't possible to correct all aberrations perfectly , and some slight amount remains . Stopping a lens down from maximum aperture , reduces these aberrations and results in a sharper picture , but there are limits beyond which there is no need to go . With the better lenses , stopping the lens down one stop or two at the most is enough ; with lower priced lenses , two or three stops is necessary . Further stopping down does not reduce the aberrations ( but it does increase the depth of field ) . With lenses of short - focal length , it isn't advisable to stop down much below f / 11 for a 50mm lens , because at this point diffraction ( bending of the light rays they pass through a very small opening ) sets in and further stopping down may actually reduce sharpness . However , lenses with longer focal lengths generally used by commercial photographers may be stopped down to f / 22 and even smaller .
4. USING THE LENS STOP TO OBTAIN SELECTIVE FOCUS . It isn't always best to get everything in the field of view of the camera in the sharpest possible focus . Sometimes , as in portraiture or in commercial studio work , you may want to have the principal object sharp and clear but the foreground and background more or less out of focus so that they aren't quite so noticeable . In that case you can use a relatively large stop and focus on the principal object . This is clearly illustrated in Figure 18 .
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