تويت
الخطوة 2: عناصر التحكم في الكاميرا / التركيز
.. تقنية الكاميرا .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
STEP 2 : CAMERA CONTROLS / Focusing
As we have seen , a fixed position lens on a simple camera is placed so as to give an equally sharp image from about 6 ft ( 2 m ) to infinity . At closer distances , the nearer you are to your subject the more the image will be out of focus . This can be avoided if you have a lens on your camera which can be focused : that is , moved away from the film plane . As shown in the diagram . right , the nearer the subject is to a lens . the farther behind the lens its image is formed . Thus for close subjects the lens must move away from the film plane to focus the image .
Being able to focus the lens gives you two important advantages : you can get sharp pictures when you are quite close to your subject , and you can give emphasis to one part of the subject by focusing on it and allowing everything nearer or further away to appear progressively unsharp . In both cases it is vital to know which part of the scene is being sharply rendered . An adjust- able focusing lens body therefore has a scale ( or at least a series of symbols ) showing the correct lens positions for focusing on various distances . To set this you have to estimate the subject distance , unless you have a camera with a focusing aid coupled with the viewfinder or through - the - lens focusing , as shown opposite .
Subject distance and focus :
When a convex lens is focused for very distant subjects it is positioned at its own focal length from the film , for example , when the film is 50 mm away from a 50 mm lens . If the subject is closer , its rays are much less parallel when they reach the lens , and the " bending power " of the glass will only bring them into focus , to form a larger image , at a greater distance from the lens , as shown in the diagram , right . On many cameras , the lens moves forward to compensate for this , so that the image still focuses on the film plane . The nearer the subject , the greater the distance required between lens and film .
Using the focus control :
Most lenses are focused by turning the largest control ring on the lens barrel . This will slowly extend or retract the lens . At the same time a scale of subject distances moves past a fixed mark . One end of this scale is marked " inf . " or.co , which denotes infinity . On this setting the lens is at its closest position to the film , and will give sharp images of distant subjects - in practice about 50 ft ( 15.2 m ) or beyond . At the other end of the focusing scale , which may read 3 ft ( 0.9 m ) , the lens will have moved out to its furthest distance from the film . Some lenses have a greater focus- ing movement ( and therefore allow closer photography ) than others . The limiting factor is usually image quality , since a lens designed for general distance photography may not perform so well when used extremely close up . Attachments for close - up work are discussed on pages 102-3 .
Focusing symbols :
Some simple cameras use symbols on the focusing scale , like those above , which focus the lens for close - up , middle distance , and distant shots .
The effect of focusing :
The two pictures above differ only in the camera focus setting and show how this control can be used to pick out parts of the subject . In the picture , left , the lens setting was about the same as for a fixed focus lens 30 ft ( 9 m ) . Almost all the picture , except for the fore- ground , is sharp .
In the picture , right , the lens was focused for about 3 ft ( 0-9 m ) so that the foreground appears sharp while detail in the background is unsharp . In this way you can use the focus to concentrate interest on areas of your subject , in the same way as subconsciously you focus your eyes on selected parts of your field of vision .
.. تقنية الكاميرا .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
STEP 2 : CAMERA CONTROLS / Focusing
As we have seen , a fixed position lens on a simple camera is placed so as to give an equally sharp image from about 6 ft ( 2 m ) to infinity . At closer distances , the nearer you are to your subject the more the image will be out of focus . This can be avoided if you have a lens on your camera which can be focused : that is , moved away from the film plane . As shown in the diagram . right , the nearer the subject is to a lens . the farther behind the lens its image is formed . Thus for close subjects the lens must move away from the film plane to focus the image .
Being able to focus the lens gives you two important advantages : you can get sharp pictures when you are quite close to your subject , and you can give emphasis to one part of the subject by focusing on it and allowing everything nearer or further away to appear progressively unsharp . In both cases it is vital to know which part of the scene is being sharply rendered . An adjust- able focusing lens body therefore has a scale ( or at least a series of symbols ) showing the correct lens positions for focusing on various distances . To set this you have to estimate the subject distance , unless you have a camera with a focusing aid coupled with the viewfinder or through - the - lens focusing , as shown opposite .
Subject distance and focus :
When a convex lens is focused for very distant subjects it is positioned at its own focal length from the film , for example , when the film is 50 mm away from a 50 mm lens . If the subject is closer , its rays are much less parallel when they reach the lens , and the " bending power " of the glass will only bring them into focus , to form a larger image , at a greater distance from the lens , as shown in the diagram , right . On many cameras , the lens moves forward to compensate for this , so that the image still focuses on the film plane . The nearer the subject , the greater the distance required between lens and film .
Using the focus control :
Most lenses are focused by turning the largest control ring on the lens barrel . This will slowly extend or retract the lens . At the same time a scale of subject distances moves past a fixed mark . One end of this scale is marked " inf . " or.co , which denotes infinity . On this setting the lens is at its closest position to the film , and will give sharp images of distant subjects - in practice about 50 ft ( 15.2 m ) or beyond . At the other end of the focusing scale , which may read 3 ft ( 0.9 m ) , the lens will have moved out to its furthest distance from the film . Some lenses have a greater focus- ing movement ( and therefore allow closer photography ) than others . The limiting factor is usually image quality , since a lens designed for general distance photography may not perform so well when used extremely close up . Attachments for close - up work are discussed on pages 102-3 .
Focusing symbols :
Some simple cameras use symbols on the focusing scale , like those above , which focus the lens for close - up , middle distance , and distant shots .
The effect of focusing :
The two pictures above differ only in the camera focus setting and show how this control can be used to pick out parts of the subject . In the picture , left , the lens setting was about the same as for a fixed focus lens 30 ft ( 9 m ) . Almost all the picture , except for the fore- ground , is sharp .
In the picture , right , the lens was focused for about 3 ft ( 0-9 m ) so that the foreground appears sharp while detail in the background is unsharp . In this way you can use the focus to concentrate interest on areas of your subject , in the same way as subconsciously you focus your eyes on selected parts of your field of vision .
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