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تجهم الصور ..
خصائص العدسة ..
سرعة العدسة ..
LENS SPEED
Lens speed should not be confused with shutter speed . Lens speed indicates the relative lens aperture . A fast lens can take satisfactory pictures in very dim light , because it has a large aperture . A slow lens , on the other hand , requires more light . Shutter speed simply indicates the length of time the shutter is open to expose the film .
The speed of the lens is determined by two things :
1 . The size of the lens opening and
2 . The size of the image over which this light is spread , which in turn is determined by its focal length .
In other words , to indicate the speed of any lens , you need to mention only two things the focal length of the lens and the diameter of its opening . This is exactly what the f / numbers on a photographic lens do . The f / number is simply a ratio between the focal length and the diameter of the lens opening , and it is obtained by dividing the focal length by the diameter . For example , a lens of 8 - inch focal length used with a lens diaphragm opening 1 inch in diameter has a speed of f / 8 , and so does a 4 - inch lens with an aperture ½ inch in diameter . The interesting thing to note at this point is that here we have two lenses , one with an opening of 1 inch , and the other with an opening of ½ inch , yet both have the same speed and require the same exposure . However , if you remember our discussion about focal lengths , you can see why this makes sense . The 4 - inch lens has exactly half the focal length of the 8 - inch lens , so that it projects an image that is only half as high and half as wide ( that is , ¼ the area ) . To spread the same value of light over this quarter - size area , the lens should let in only 4 the total amount of light . If you reduce the diameter of a circle by half , you also reduce it to one - quarter of the area .
F 11 This photograph was taken with a 35mm lens . On a 35mm camera this is considered to be a wide angle lens . On this photograph are outlined the areas of coverage of a Somm lens , 85mm lens , 135mm lens , and a 270mm lents .
Fig . 12 This photograph was taken with a 50mm lens from the same viewpoint as Fig . 11 .
Fig . 13 This photograph was taken with an 85mm lens from the same . viewpoint as Fig . 11 .
Fig . 14 This photograph was taken with a 135mm lens from the same viewpoint as Fig . 11
Fig . 15 This photograph was taken with a 270mm lens from the same viewpoint as Fig . 11. Note that throughout this series of pictures , the image size of any object in the picture . changes in exact ratio to the focal length of the lens .
If you are not very good at arithmetic , all you need to do is to remember one thing the f / number of a lens determines its speed - and it makes no difference whether the lens is large or whether it is small . The larger lens lets in more light , but this light is spread over a larger area , so you are right back to the same exposure .
The faster the lens , or the larger the lens opening , the smaller is the f / number . For example , f / 4 is faster than f / 8 . Let us see just why this is so .
As we saw a little while ago , the f / number is obtained by dividing the focal length by the lens opening diameter . This can be written :
f / number = focal length diameter of diaphragm
If we have a lens with an 8 - inch focal length , and a diameter of 1 inch , our equation becomes :
f / number = 8/1 or 8
Now let us double the diaphragm diameter so that it is 2 inches . Our equation now becomes :
f / number = 8/2 or 4
This little exercise in arithmetic illustrates two things about f / numbers ; ( 1 ) The smaller the f / number , the larger is the lens openings ; ( 2 ) The f / number is really the focal length divided by the effective diameter of the diaphragm or lens stop .
The next question is : How does the speed of a lens change with the f / number ? To answer this , we can go back to the equations we just set up . The 8 - inch lens with the 2 - inch diameter had a speed of f / 4 . By doubling the diameter , we halved the f / number . The area of a circle is proportional to the square of the diameter , so that if we multiply the diameter of a circle by two , we increase the area by the square of two , or by four . In other words , a lens with a 2 - inch diameter lets in four times as much light as the same lens with a 1 - inch diameter . This , of course , means that the f / 4 lens opening lets in four times as much light and requires only the exposure of the f / 8 lens opening .
If you want to put this in the form of a mathematical expression , you can say that the speed of a lens is inversely proportional to the square of the f / number .
SYSTEMS OF LENS MARKINGS IN COMMON USE . The f / numbers on the diaphragm scale of a lens are generally so chosen that each stop requires just half the exposure of the next smaller one . This holds true for all lens stops except the very largest on any particular lens . For example , in a typically fast lens , the f / numbers may be : f / 1.8 , f / 2 , f / 4 , f / 5.6 , f / 8 , f / 11 , f / 16 , f / 22 . In this series f / 1.8 is only about 80 % faster than f / 2 , but after that each lens stop is exactly twice as fast as the next larger f / number .
If your exposure meter shows that for a certain picture the exposure should be 1/100 second at f / 8 , the same exposure would be obtained with 1/50 second at f / 11 , or 1/25 second at f / 16 .
There are two systems of f / numbers in general use . The one commonly used in the United States is : f / 1.4 , f / 2 , f / 2.8 , f / 4 , f / 5.6 , f / 8 , f / 11 , f / 16 , f / 22 , f / 32 , f / 45 , f / 64 . Another series which is used on certain European cameras is : f / 2.2 , f / 3.2 , f / 4.5 , f / 6.3 , f / 9 , f / 12.5 , f / 18 , and so on .
The better lenses used for serious photography are invariably anastigmats , and their widest apertures depend upon the work they are designed to do . For example , 35mm cameras never use lenses slower than about f / 2 for their normal lenses except in their least expensive models , while they may go as high as f / 1.4 . For all - around work with larger negative sizes ( 24 x 24 up to 8 x 10 inch ) , the lens may be f / 2.8 in the shorter focal lengths , and f / 4.5 or even f / 6.3 in the longer focal lengths . Convertible anastigmats , which give you a choice of two or three focal lengths , generally operate at maximum speeds of from f / 5.6 to f / 6.8 . Highly corrected apochromats for very exacting color work generally are rated at f / 10 or
تجهم الصور ..
خصائص العدسة ..
سرعة العدسة ..
LENS SPEED
Lens speed should not be confused with shutter speed . Lens speed indicates the relative lens aperture . A fast lens can take satisfactory pictures in very dim light , because it has a large aperture . A slow lens , on the other hand , requires more light . Shutter speed simply indicates the length of time the shutter is open to expose the film .
The speed of the lens is determined by two things :
1 . The size of the lens opening and
2 . The size of the image over which this light is spread , which in turn is determined by its focal length .
In other words , to indicate the speed of any lens , you need to mention only two things the focal length of the lens and the diameter of its opening . This is exactly what the f / numbers on a photographic lens do . The f / number is simply a ratio between the focal length and the diameter of the lens opening , and it is obtained by dividing the focal length by the diameter . For example , a lens of 8 - inch focal length used with a lens diaphragm opening 1 inch in diameter has a speed of f / 8 , and so does a 4 - inch lens with an aperture ½ inch in diameter . The interesting thing to note at this point is that here we have two lenses , one with an opening of 1 inch , and the other with an opening of ½ inch , yet both have the same speed and require the same exposure . However , if you remember our discussion about focal lengths , you can see why this makes sense . The 4 - inch lens has exactly half the focal length of the 8 - inch lens , so that it projects an image that is only half as high and half as wide ( that is , ¼ the area ) . To spread the same value of light over this quarter - size area , the lens should let in only 4 the total amount of light . If you reduce the diameter of a circle by half , you also reduce it to one - quarter of the area .
F 11 This photograph was taken with a 35mm lens . On a 35mm camera this is considered to be a wide angle lens . On this photograph are outlined the areas of coverage of a Somm lens , 85mm lens , 135mm lens , and a 270mm lents .
Fig . 12 This photograph was taken with a 50mm lens from the same viewpoint as Fig . 11 .
Fig . 13 This photograph was taken with an 85mm lens from the same . viewpoint as Fig . 11 .
Fig . 14 This photograph was taken with a 135mm lens from the same viewpoint as Fig . 11
Fig . 15 This photograph was taken with a 270mm lens from the same viewpoint as Fig . 11. Note that throughout this series of pictures , the image size of any object in the picture . changes in exact ratio to the focal length of the lens .
If you are not very good at arithmetic , all you need to do is to remember one thing the f / number of a lens determines its speed - and it makes no difference whether the lens is large or whether it is small . The larger lens lets in more light , but this light is spread over a larger area , so you are right back to the same exposure .
The faster the lens , or the larger the lens opening , the smaller is the f / number . For example , f / 4 is faster than f / 8 . Let us see just why this is so .
As we saw a little while ago , the f / number is obtained by dividing the focal length by the lens opening diameter . This can be written :
f / number = focal length diameter of diaphragm
If we have a lens with an 8 - inch focal length , and a diameter of 1 inch , our equation becomes :
f / number = 8/1 or 8
Now let us double the diaphragm diameter so that it is 2 inches . Our equation now becomes :
f / number = 8/2 or 4
This little exercise in arithmetic illustrates two things about f / numbers ; ( 1 ) The smaller the f / number , the larger is the lens openings ; ( 2 ) The f / number is really the focal length divided by the effective diameter of the diaphragm or lens stop .
The next question is : How does the speed of a lens change with the f / number ? To answer this , we can go back to the equations we just set up . The 8 - inch lens with the 2 - inch diameter had a speed of f / 4 . By doubling the diameter , we halved the f / number . The area of a circle is proportional to the square of the diameter , so that if we multiply the diameter of a circle by two , we increase the area by the square of two , or by four . In other words , a lens with a 2 - inch diameter lets in four times as much light as the same lens with a 1 - inch diameter . This , of course , means that the f / 4 lens opening lets in four times as much light and requires only the exposure of the f / 8 lens opening .
If you want to put this in the form of a mathematical expression , you can say that the speed of a lens is inversely proportional to the square of the f / number .
SYSTEMS OF LENS MARKINGS IN COMMON USE . The f / numbers on the diaphragm scale of a lens are generally so chosen that each stop requires just half the exposure of the next smaller one . This holds true for all lens stops except the very largest on any particular lens . For example , in a typically fast lens , the f / numbers may be : f / 1.8 , f / 2 , f / 4 , f / 5.6 , f / 8 , f / 11 , f / 16 , f / 22 . In this series f / 1.8 is only about 80 % faster than f / 2 , but after that each lens stop is exactly twice as fast as the next larger f / number .
If your exposure meter shows that for a certain picture the exposure should be 1/100 second at f / 8 , the same exposure would be obtained with 1/50 second at f / 11 , or 1/25 second at f / 16 .
There are two systems of f / numbers in general use . The one commonly used in the United States is : f / 1.4 , f / 2 , f / 2.8 , f / 4 , f / 5.6 , f / 8 , f / 11 , f / 16 , f / 22 , f / 32 , f / 45 , f / 64 . Another series which is used on certain European cameras is : f / 2.2 , f / 3.2 , f / 4.5 , f / 6.3 , f / 9 , f / 12.5 , f / 18 , and so on .
The better lenses used for serious photography are invariably anastigmats , and their widest apertures depend upon the work they are designed to do . For example , 35mm cameras never use lenses slower than about f / 2 for their normal lenses except in their least expensive models , while they may go as high as f / 1.4 . For all - around work with larger negative sizes ( 24 x 24 up to 8 x 10 inch ) , the lens may be f / 2.8 in the shorter focal lengths , and f / 4.5 or even f / 6.3 in the longer focal lengths . Convertible anastigmats , which give you a choice of two or three focal lengths , generally operate at maximum speeds of from f / 5.6 to f / 6.8 . Highly corrected apochromats for very exacting color work generally are rated at f / 10 or
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