تويت
التصوير الفوتوغرافي الحديث
تجهم الصور ..
تأثيرات العدسة بسبب وضع الكاميرا ..
قوة التغطية ..
COVERING POWER
The term " covering power " should be thoroughly understood . The covering power of a lens is the angle through which the lens will produce a well defined and uniformly illuminated image . As a rule , only part of the image produced by the lens is used . This is the part that is bounded by the edges of the focusing screen but the entire image that the lens will produce is circular because the lens itself is circular . This circular image is commonly called the " circle of illumination . "
This circular field is modified by the construction of the lens and sometimes by the lens mount or tube in which the lens elements are fitted . The circle of illumination produced by the lens is greater than its covering power , but since the definition ( sharpness ) almost invariably falls off towards the edges of the circle , the covering power only extends to the point where the definition begins to fall off appreciably .
It is generally desirable to have a lens which will cover a larger circle than is sufficient to include the size of the film that is used . This is particularly true for commercial work with a view or similar camera in which the lens or camera back positions are adjustable . Some lenses , especially those of medium speed , will cover a larger area when stopped down . Others , owing to their construction or to the way they are mounted , do not cover a larger circle when stopped down .
Here is a practical example of the advantage of extra covering power obtained by stopping down . A photographer has a lens of about 7 - inches focal length that works at a maximum aperture of f / 6.8 . This photographer generally uses the lens on a 4 x 5 camera . This lens has a fairly long focal length for this size film , so it gives him excellent perspective for portraits over the entire negative area .
Because of the exceptional covering power of this lens , it will actually cover a 5 x 7 film at full aperture , and when stopped down to about f / 11 or f / 16 , it will cover an 8 x 10 film perfectly , even to the corners . This lens is , therefore , very useful for interiors , for groups in a limited space , or for any work calling for a wide - angle lens .
A very sharp lens with excellent covering power can , in this way , be used for many purposes . On a 4 x 5 camera , it is a normal lens for all - around work ; on a 5 x 7 camera , it is a semi - wide angle lens for taking pictures in city streets , or in somewhat cramped quarters ; on an 8 x 10 camera , it is a wide - angle lens for photographing interiors .
There is no perfect or universal lens . If a lens designer wants to build the fastest possible lens , he cannot at the same time make it the sharpest . If he wants to cover a very wide angle , he must sacrifice speed . If he wants to correct it as perfectly as possible for color , he must reduce the covering power as well as the speed . In other words , if you must have a lens that is particularly outstanding for one type of work , it will probably not be as good for another type of work .
تجهم الصور ..
تأثيرات العدسة بسبب وضع الكاميرا ..
قوة التغطية ..
COVERING POWER
The term " covering power " should be thoroughly understood . The covering power of a lens is the angle through which the lens will produce a well defined and uniformly illuminated image . As a rule , only part of the image produced by the lens is used . This is the part that is bounded by the edges of the focusing screen but the entire image that the lens will produce is circular because the lens itself is circular . This circular image is commonly called the " circle of illumination . "
This circular field is modified by the construction of the lens and sometimes by the lens mount or tube in which the lens elements are fitted . The circle of illumination produced by the lens is greater than its covering power , but since the definition ( sharpness ) almost invariably falls off towards the edges of the circle , the covering power only extends to the point where the definition begins to fall off appreciably .
It is generally desirable to have a lens which will cover a larger circle than is sufficient to include the size of the film that is used . This is particularly true for commercial work with a view or similar camera in which the lens or camera back positions are adjustable . Some lenses , especially those of medium speed , will cover a larger area when stopped down . Others , owing to their construction or to the way they are mounted , do not cover a larger circle when stopped down .
Here is a practical example of the advantage of extra covering power obtained by stopping down . A photographer has a lens of about 7 - inches focal length that works at a maximum aperture of f / 6.8 . This photographer generally uses the lens on a 4 x 5 camera . This lens has a fairly long focal length for this size film , so it gives him excellent perspective for portraits over the entire negative area .
Because of the exceptional covering power of this lens , it will actually cover a 5 x 7 film at full aperture , and when stopped down to about f / 11 or f / 16 , it will cover an 8 x 10 film perfectly , even to the corners . This lens is , therefore , very useful for interiors , for groups in a limited space , or for any work calling for a wide - angle lens .
A very sharp lens with excellent covering power can , in this way , be used for many purposes . On a 4 x 5 camera , it is a normal lens for all - around work ; on a 5 x 7 camera , it is a semi - wide angle lens for taking pictures in city streets , or in somewhat cramped quarters ; on an 8 x 10 camera , it is a wide - angle lens for photographing interiors .
There is no perfect or universal lens . If a lens designer wants to build the fastest possible lens , he cannot at the same time make it the sharpest . If he wants to cover a very wide angle , he must sacrifice speed . If he wants to correct it as perfectly as possible for color , he must reduce the covering power as well as the speed . In other words , if you must have a lens that is particularly outstanding for one type of work , it will probably not be as good for another type of work .
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