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فتحة العدسة .. تقنية الكاميرا .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
Aperture
An adjustable camera controls the amount of light reaching the film in two ways . It can make the image darker or lighter by a vari- able aperture , positioned behind the lens , and control the length of time that the light reaches the film , by a timed shutter .
The lens aperture consists of overlapping movable leaves which form a diaphragm . This can be set to a range of diameters , so that the quantity of light admitted is con- trolled in the same way that the width of a funnel controls the quantity of water flow- ing into a container . When photographing a dimly lit subject you use a wide lens aperture to admit as much light as possible ; for a bright subject you can change to a small aperture to reduce the amount of light . In this way the film still receives the same amount of light .
Since doubling the diameter of a circle increases its area by four times , as you double the diameter of the aperture the light admitted becomes four times brighter . The aperture control ring on a camera includes intermediate settings , creating a series which progressively doubles ( or halves ) the light that is admitted . Each setting is given an " f number " ( f stop ) , see above right .
Apart from controlling light the aperture also increases or reduces the " depth of field " the zone of sharp focus in front of and behind the focused subject ( see pp . 32-3 ) .
Aperture settings and focal length :
Each aperture setting has an " f number " which is the ratio of the diameter of the aperture ( at that setting ) to the focal length of the lens . So f4 really means that the aperture is one - quarter of the lens focal length , as shown by the dotted circles , right ; at f16 the aperture is one - sixteenth of the focal length , and so on .
Using this ratio system , rather than the aperture diameters , means that all camera lenses set to the same f number will photo- graph the same subject at equal image brightness . ( Aperture diameter alone would not give a constant setting because short focal length lenses give brighter images than lenses of long focus . ) As a result , using the same f number , you can change cameras or lenses and know that the ex- posure effect will remain the same .
Using the aperture control :
The aperture is usually adjusted by turning a narrow ring near the focusing control . The ring is marked with the internationally agreed series of f numbers shown at the top of this page . The widest aperture ( lowest f number ) varies according to lens design and price . Lenses of good quality with wide apertures are expensive to make . Many good standard lenses have a maximum aperture of f2 , others may only open to f2.8 or f4 .
Most single lens reflex cameras focus at maximum aperture- whatever the f number chosen , the aperture only changes to its true setting at the moment of exposure . With some cameras , however , altering the setting directly changes the aperture , and so affects the brightness and depth of field of the image seen through the viewfinder . With these it is best to focus at maximum aperture first , so that the image is brightest , then set your f number .
Aperture symbols :
Simple cameras use weather symbols instead of f numbers smallest aperture for brightest conditions . To reduce costs a sliding metal strip with holes of different sizes may replace the adjustable diaphragm .
Aperture and zone of sharpness
The effect of the aperture setting on the sharpness of your image can be clearly seen in the three pictures above . They were taken , from left to right , at f4 , f8 , and f16 with the lens focused for infinity .
At the largest aperture ( f4 ) , left , only a small part of the image is sharp , and objects in the fore- ground of the picture are rendered un- clear . This sharp zone , known as the " depth of field " , is greater in the center picture , which was taken with a smaller aperture - f8 . Finally , in the right hand picture , which was taken at f16 , almost everything from the foreground to the background has recorded sharply . From this you can see that as the aperture is reduced the depth of field increases , ( see pp . 32–3 ) .
Most simple cameras , such as the 35 mm model on page 26 , have a fixed lens aperture ( about f11 ) which enables the fixed focus lens to record detail from infinity down to about 6 ft ( 2 m ) .
Aperture
An adjustable camera controls the amount of light reaching the film in two ways . It can make the image darker or lighter by a vari- able aperture , positioned behind the lens , and control the length of time that the light reaches the film , by a timed shutter .
The lens aperture consists of overlapping movable leaves which form a diaphragm . This can be set to a range of diameters , so that the quantity of light admitted is con- trolled in the same way that the width of a funnel controls the quantity of water flow- ing into a container . When photographing a dimly lit subject you use a wide lens aperture to admit as much light as possible ; for a bright subject you can change to a small aperture to reduce the amount of light . In this way the film still receives the same amount of light .
Since doubling the diameter of a circle increases its area by four times , as you double the diameter of the aperture the light admitted becomes four times brighter . The aperture control ring on a camera includes intermediate settings , creating a series which progressively doubles ( or halves ) the light that is admitted . Each setting is given an " f number " ( f stop ) , see above right .
Apart from controlling light the aperture also increases or reduces the " depth of field " the zone of sharp focus in front of and behind the focused subject ( see pp . 32-3 ) .
Aperture settings and focal length :
Each aperture setting has an " f number " which is the ratio of the diameter of the aperture ( at that setting ) to the focal length of the lens . So f4 really means that the aperture is one - quarter of the lens focal length , as shown by the dotted circles , right ; at f16 the aperture is one - sixteenth of the focal length , and so on .
Using this ratio system , rather than the aperture diameters , means that all camera lenses set to the same f number will photo- graph the same subject at equal image brightness . ( Aperture diameter alone would not give a constant setting because short focal length lenses give brighter images than lenses of long focus . ) As a result , using the same f number , you can change cameras or lenses and know that the ex- posure effect will remain the same .
Using the aperture control :
The aperture is usually adjusted by turning a narrow ring near the focusing control . The ring is marked with the internationally agreed series of f numbers shown at the top of this page . The widest aperture ( lowest f number ) varies according to lens design and price . Lenses of good quality with wide apertures are expensive to make . Many good standard lenses have a maximum aperture of f2 , others may only open to f2.8 or f4 .
Most single lens reflex cameras focus at maximum aperture- whatever the f number chosen , the aperture only changes to its true setting at the moment of exposure . With some cameras , however , altering the setting directly changes the aperture , and so affects the brightness and depth of field of the image seen through the viewfinder . With these it is best to focus at maximum aperture first , so that the image is brightest , then set your f number .
Aperture symbols :
Simple cameras use weather symbols instead of f numbers smallest aperture for brightest conditions . To reduce costs a sliding metal strip with holes of different sizes may replace the adjustable diaphragm .
Aperture and zone of sharpness
The effect of the aperture setting on the sharpness of your image can be clearly seen in the three pictures above . They were taken , from left to right , at f4 , f8 , and f16 with the lens focused for infinity .
At the largest aperture ( f4 ) , left , only a small part of the image is sharp , and objects in the fore- ground of the picture are rendered un- clear . This sharp zone , known as the " depth of field " , is greater in the center picture , which was taken with a smaller aperture - f8 . Finally , in the right hand picture , which was taken at f16 , almost everything from the foreground to the background has recorded sharply . From this you can see that as the aperture is reduced the depth of field increases , ( see pp . 32–3 ) .
Most simple cameras , such as the 35 mm model on page 26 , have a fixed lens aperture ( about f11 ) which enables the fixed focus lens to record detail from infinity down to about 6 ft ( 2 m ) .
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