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تجهم الصور ..
خصائص العدسة ..
الطول البؤري ..
LENS PROPERTIES
FOCAL LENGTH
A fundamental property of a photographic lens is its focal length - but before we explain its importance , we had better define it . Stated roughly , the focal length is the distance from the lens to the image it forms when focused on a far distant object ( at infinity ) . Stated more exactly , the focal length of a lens is the distance from an imaginary point in the lens known as the second nodal point ( sometimes called the optical center ) to the point at which parallel rays ( or rays from very distant objects ) are brought to a focus . It is difficult to measure this distance accurately because the nodal point is an imaginary point and can be located only by indirect means . It may vary considerably from one type of lens to another , but it is generally somewhere near the center for ordinary photographic lenses . However , it may actually be in back of the lens in some short - focus wide - angle lenses , and considerably out in front in telephoto lenses .
The focal length of a lens determines the size of the image it produces from any given distance . For distant objects , the size of an image ( that is , the height and width ) produced by any lens is proportional to its focal length . As an example , if you compare the image produced by a 6 - inch lens ( lens having a focal length of 6 inches ) with that produced by a 3 - inch lens ( lens having a focal length of 3 inches ) , you will see that the 6 - inch lens image is twice as high and twice as wide , so that it covers four times the area of the image produced by the 3 - inch lens . Remember this fact - to the practical photographer it is one of the most important facts about lenses . The diagram in Figure 10 shows why this is so .
In most cameras , the lens used has a focal length about equal to the diagonal of the negative , but this is not a hard and fast rule . The press photographer , who frequently very close to his subject , generally uses a shorter focal length than this so he can get everything on his film even when he is very close to the subject . Wide - angle lenses may have half or even less than half the normal focal lengths , while telephoto lenses may easily be several times the usual focal length . This subject is treated in more detail later in this assignment .
Fig . 10 Image produced by a lens is in direct proportion to its focal length .
The focal length of a lens may be given in inches or millimeters ( there are approximately 25 millimeters to the inch ) . Figures 11 through 15 show five photographs taken from the same position with five lenses of different focal lengths . Figure 11 was made with a 35mm lens , Figure 12 with a 50mm lens , Figure 13 with an 85mm lens , Figure 14 with a 135mm lens , and Figure 15 with a 270mm lens . If you measure the height of the lettering in the background sign , you would find that in Figure 15 it is 7 2/3 times as large as in Figure 11. This is the ratio between 270mm and 35mm .
تجهم الصور ..
خصائص العدسة ..
الطول البؤري ..
LENS PROPERTIES
FOCAL LENGTH
A fundamental property of a photographic lens is its focal length - but before we explain its importance , we had better define it . Stated roughly , the focal length is the distance from the lens to the image it forms when focused on a far distant object ( at infinity ) . Stated more exactly , the focal length of a lens is the distance from an imaginary point in the lens known as the second nodal point ( sometimes called the optical center ) to the point at which parallel rays ( or rays from very distant objects ) are brought to a focus . It is difficult to measure this distance accurately because the nodal point is an imaginary point and can be located only by indirect means . It may vary considerably from one type of lens to another , but it is generally somewhere near the center for ordinary photographic lenses . However , it may actually be in back of the lens in some short - focus wide - angle lenses , and considerably out in front in telephoto lenses .
The focal length of a lens determines the size of the image it produces from any given distance . For distant objects , the size of an image ( that is , the height and width ) produced by any lens is proportional to its focal length . As an example , if you compare the image produced by a 6 - inch lens ( lens having a focal length of 6 inches ) with that produced by a 3 - inch lens ( lens having a focal length of 3 inches ) , you will see that the 6 - inch lens image is twice as high and twice as wide , so that it covers four times the area of the image produced by the 3 - inch lens . Remember this fact - to the practical photographer it is one of the most important facts about lenses . The diagram in Figure 10 shows why this is so .
In most cameras , the lens used has a focal length about equal to the diagonal of the negative , but this is not a hard and fast rule . The press photographer , who frequently very close to his subject , generally uses a shorter focal length than this so he can get everything on his film even when he is very close to the subject . Wide - angle lenses may have half or even less than half the normal focal lengths , while telephoto lenses may easily be several times the usual focal length . This subject is treated in more detail later in this assignment .
Fig . 10 Image produced by a lens is in direct proportion to its focal length .
The focal length of a lens may be given in inches or millimeters ( there are approximately 25 millimeters to the inch ) . Figures 11 through 15 show five photographs taken from the same position with five lenses of different focal lengths . Figure 11 was made with a 35mm lens , Figure 12 with a 50mm lens , Figure 13 with an 85mm lens , Figure 14 with a 135mm lens , and Figure 15 with a 270mm lens . If you measure the height of the lettering in the background sign , you would find that in Figure 15 it is 7 2/3 times as large as in Figure 11. This is the ratio between 270mm and 35mm .
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