تويت
التصوير الفوتوغرافي الحديث
تجهم الصور ..
أهمية العدسة ..
IMPORTANCE OF THE LENS
Some of the light that is reflected from the object we want to photograph strikes the surface of the camera lens . The lens projects this light onto the sensitive film in such a way as to form an image of the original subject . This image , composed of light of various intensities , produces a chemical reaction in the sensitive film which can then be developed in a suitable solution to form a replica of the original subject .
As the lens is the eye of the camera , it is a most important part of the photographer's equipment . A photographer must , therefore , understand something about lenses and how work .
In order to get an idea of how a lens works , take an ordinary reading glass and hold it up facing a brightly lighted window or an electric light bulb . Hold a white card on the opposite side of the reading glass and parallel to it . Then adjust the distance between the card and the reading glass . You will see that an inverted image of the window or the bulb appears quite clearly defined on the card , but the image is clearly defined only when the card is at a specific distance from the glass . Now walk toward the window so you are only a few feet away and produce an image of it on the card again . This time you will have to hold the card farther away from the lens . In other words , you must focus the reading glass for each distance from the subject .
A reading glass , when used in this way to project an image , is acting in exactly the same way as the lens in a camera . It would be quite possible to make a picture in a camera by using a reading glass as the lens . But such a lens , though it is perfectly adequate for the purpose for which it is intended , would give a very blurred and distorted picture .
Simple lenses cannot project accurate pictures . They are subject to quite a number of distortions or faults , which are called " aberrations . " We shall discuss these aberrations in a few moments , but first let us see how a lens projects an image .
When a beam of light passes at an angle from one transparent material to another , it is always bent if there is a difference in density between the two materials . You can demonstrate this very easily by dipping a pencil or a metal rod into a glass of water . The rod will appear to be bent sharply where it enters the water . ( See Figure 2 ) The light reflected from the part of the rod that is in the air comes straight to your eyes , but the light reflected from the part of the rod that is under water travels in one direction as long as it's under water ; then bends as it leaves the water and enters the air . That is why the rod seems bent , although we know it isn't . There is a difference in the density of the water and the air . This property of light - that is , its change in direction when passing from one medium to another - is called " refraction " and is the basic reason why a lens forms an image . This is illustrated more clearly in Figure 3 .
In this illustration , we have a simple lens , L , like the lens in our reading glass . To the left of it is an illuminated subject which is reflecting light in all directions . Let us take just one ray of light coming from the point , H , on the subject and striking the lens at A. When this ray of light enters the lens , it is bent downward ; when it leaves the lens it is bent downward again . Now let us take another single ray from the point H on the subject ; the ray that strikes near the bottom part of the lens at C. This ray is bent upward so that after it leaves the lens it meets our first ray at h . In a similar way , if our lens is a perfect one , every ray of light from H that strikes the lens is bent in such a direction that it meets all the other rays at h . If you stop to think for a moment , you will see that this
means the point h is an image of the point H. In exactly the same way , we get an image of the point F on the subject at f . If we care to go to the trouble , we can show , in this way , that for every point on the subject , there will be a corresponding image point on the opposite side of the lens ; in other words , the lens has actually formed an upside down image of our subject .
Fig . 3 Light reflected from any point on a subject which is intercepted by a simple convex lens is caused to converge to a point after it passes through lens , thus producing an image .
تجهم الصور ..
أهمية العدسة ..
IMPORTANCE OF THE LENS
Some of the light that is reflected from the object we want to photograph strikes the surface of the camera lens . The lens projects this light onto the sensitive film in such a way as to form an image of the original subject . This image , composed of light of various intensities , produces a chemical reaction in the sensitive film which can then be developed in a suitable solution to form a replica of the original subject .
As the lens is the eye of the camera , it is a most important part of the photographer's equipment . A photographer must , therefore , understand something about lenses and how work .
In order to get an idea of how a lens works , take an ordinary reading glass and hold it up facing a brightly lighted window or an electric light bulb . Hold a white card on the opposite side of the reading glass and parallel to it . Then adjust the distance between the card and the reading glass . You will see that an inverted image of the window or the bulb appears quite clearly defined on the card , but the image is clearly defined only when the card is at a specific distance from the glass . Now walk toward the window so you are only a few feet away and produce an image of it on the card again . This time you will have to hold the card farther away from the lens . In other words , you must focus the reading glass for each distance from the subject .
A reading glass , when used in this way to project an image , is acting in exactly the same way as the lens in a camera . It would be quite possible to make a picture in a camera by using a reading glass as the lens . But such a lens , though it is perfectly adequate for the purpose for which it is intended , would give a very blurred and distorted picture .
Simple lenses cannot project accurate pictures . They are subject to quite a number of distortions or faults , which are called " aberrations . " We shall discuss these aberrations in a few moments , but first let us see how a lens projects an image .
When a beam of light passes at an angle from one transparent material to another , it is always bent if there is a difference in density between the two materials . You can demonstrate this very easily by dipping a pencil or a metal rod into a glass of water . The rod will appear to be bent sharply where it enters the water . ( See Figure 2 ) The light reflected from the part of the rod that is in the air comes straight to your eyes , but the light reflected from the part of the rod that is under water travels in one direction as long as it's under water ; then bends as it leaves the water and enters the air . That is why the rod seems bent , although we know it isn't . There is a difference in the density of the water and the air . This property of light - that is , its change in direction when passing from one medium to another - is called " refraction " and is the basic reason why a lens forms an image . This is illustrated more clearly in Figure 3 .
In this illustration , we have a simple lens , L , like the lens in our reading glass . To the left of it is an illuminated subject which is reflecting light in all directions . Let us take just one ray of light coming from the point , H , on the subject and striking the lens at A. When this ray of light enters the lens , it is bent downward ; when it leaves the lens it is bent downward again . Now let us take another single ray from the point H on the subject ; the ray that strikes near the bottom part of the lens at C. This ray is bent upward so that after it leaves the lens it meets our first ray at h . In a similar way , if our lens is a perfect one , every ray of light from H that strikes the lens is bent in such a direction that it meets all the other rays at h . If you stop to think for a moment , you will see that this
means the point h is an image of the point H. In exactly the same way , we get an image of the point F on the subject at f . If we care to go to the trouble , we can show , in this way , that for every point on the subject , there will be a corresponding image point on the opposite side of the lens ; in other words , the lens has actually formed an upside down image of our subject .
Fig . 3 Light reflected from any point on a subject which is intercepted by a simple convex lens is caused to converge to a point after it passes through lens , thus producing an image .
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