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تجهم الصور ..
تصحيحات العدسة للحصول على صور أفضل
LENS CORRECTIONS FOR BETTER PICTURES
Unfortunately , only a theoretically perfect lens will produce the accurate picture we have just described . In actual practice , a simple lens of this type does not produce a perfect image ; instead , it produces a badly blurred one . The reason for this difference is that a simple lens is subject to a number of shortcomings or faults , which are known as aberrations . These are :
1. Chromatic aberration .
2. Spherical aberration .
3. Curvature of field .
4. Distortion ( or Curvilinear Distortion ) .
5. Coma .
6. Astigmatism .
As a student of photography , you should know how these aberrations prevent a simple lens from producing a sharp image . Unfortunately , a thorough understanding of this subject requires a rather deep knowledge of mathematics , but we will do our best to give you a simple explanation so you will have at least a working knowledge of photographic lenses . You may not understand all of it at first unless you have already had some experience in photography , but we strongly recommend that you use this lesson as a reference source and go back to it from time to time as you learn more about lenses from practical experience .
CHROMATIC ABERRATION . If we pass a narrow beam of white light through a prism ( Figure 4 ) , we find that it emerges as a band of colors . This occurs because each color is refracted or bent a different amount . The band of projected colors is called a spectrum . A rainbow is nothing more than a giant spectrum produced by sunlight being refracted as it passes through drops of rain .
Note that in Figure 4 the red light is refracted least , and the blue - violet light is refracted most . This separation of white light into its component colors is called dispersion .
LIGHTS BLUE LIGHT Fig . 4 A beam of white light passing through a colorless prism is separated into its component colors .
When a beam of light passes through an ordinary convex lens , as shown in Figure 5 , the blue rays are refracted to a greater degree than the green and still more than the red . As a result , there is no single plane or distance where all colors come to a focus . At plane R , for example , there is a sharp red image , but overlapping it are blurred blue and green images . Similarly , the plane G has al sharp green image but blurred red and blue images , and so on . All these images overlap so you do not see them as separate colors but as a blurred white image .
Fig . 5 CHROMATIC ABERRATION . In a simple lens , colors do not all converge at same point , thus producing a blurred image .
This lens defect is called chromatic aberration . Refraction ( bending of light ) and dispersion ( spreading of colors ) always go together but not always to an equal degree . It is thus possible , by using different kinds of glass , to combine a positive lens having high refraction and low dispersion with a negative lens having low refraction and low dispersion in such a way that the two dispersions cancel out ( because they are in opposite directions ) . Such a combination lens will focus all colors in one plane . A lens of this type is said to be chromatically corrected or color corrected . All photographic lenses are corrected in this way .
In actual practice , it isn't possible to correct perfectly for all colors with just two lenses , although the correction can be close enough for most purposes . For very exacting color work , three lenses are used . Such lenses are called apochromats .
SPHERICAL ABERRATION . In a simple lens with spherical surfaces , such as our reading glass , the rays of light through the center of the lens come to a focus at a point farther away than the rays from the outer edges of the lens ( see Figure 6 ) . As a result the image is blurred .
It is possible to correct this to a limited extent by stopping down the lens . and using only its center . This reduces the speed of the lens , however , and increases exposure . There are many occasions when this might not be practical .
Fig . 6 SPHERICAL ABERRATION . Rays of light around the periphery of lens come to focus at a different point than central rays .
Spherical aberration can be corrected to a large extent by combining negative and positive lenses of different curvatures , as in chromatic aberration . When this is done , the positive aberration of one lens cancels out the negative aber ration of the other .
Fig . 7 CURVATURE OF FIELD . Plane of sharpest focus with a simple lens is curved .
CURVATURE OF FIELD . In Figure 7 , we see a simple lens forming an image of the arrow ABC . Let us assume that point B of the arrow comes to a sharp focus at point b . Point A , however , is farther from the lens and therefore comes to focus at point a , which is closer to the lens than point b . Similarly , point C focuses at c which also closer to the lens than point b . In other words , in a simple lens , the plane of sharpest focus ( focal - plane ) is not flat but curved or dished . You can focus sharply for points A and C , or for the central zone B , but not for both . This fault is called of field . Again , is possible to orrect for curvature of field by combining positive and negative lenses with different indexes of refraction ( glass density ) and different surface curvatures .
IMAGE APPEARS THIS WAY IF STOP IS IN FRONT OF MENISCUS LENS
OUTLINES OF IMAGE APPEARS THIS WAY IF STOP IS BEHIND MENISCUS LENS
Fig . 8 CURVILINEAR DISTORTION . This distortion is corrected by placing diaphragm between the lens elements as shown in Fig . 9 .
DISTORTION OR CURVILINEAR DISTORTION . Distortion as a lens aberration means that straight lines near the edges of the image are produced as curves . The location of the lens diaphragm determines the direction of the curve . Generally speaking , when the diaphragm is in front of the lens , the lines curve out , and the result is a barrel - shaped image , as in Figure 8. This is called barrel - shaped distortion . When the diaphragm is in back of the lens , the lines curve in , and the result is pincushion distortion . The remedy for distortion is obvious . Two or more lenses are combined and the diaphragm is placed between the front and back elements , as shown in Figure 9. In this way , the two types of distortion neutralize each other .
COMA . Coma is a sort of spherical aberration for rays of light reaching the lens from an angle . A lens that is not corrected for coma will cause bright points of light to be reproduced as cone - shaped blurs pointed toward the center of the picture area . Coma is more pronounced as you go toward the marginal areas of the field of view . It is also corrected by combining positive and negative lenses made of different types of glass .
ASTIGMATISM . Astigmatism in a camera lens is not at all the same as astigmatism in the human eye . The two are frequently confused , however , because they both show up as an inability to focus vertical and horizontal lines on the same plane . If a subject having both vertical and horizontal lines is focused , it will be impossible to focus them both sharply . The photographer has the choice of getting the vertical lines sharp and the horizontal lines blurred , or vice - versa - unless he compromises by focusing in between and getting both only slightly blurred .
Astigmatism was the last of the aberrations to be corrected . It required the development of new types of optical glass . Generally speaking , an anastigmat lens has not only been corrected for astigmatism , as its name implies , but for all the other aberrations listed here .
تجهم الصور ..
تصحيحات العدسة للحصول على صور أفضل
LENS CORRECTIONS FOR BETTER PICTURES
Unfortunately , only a theoretically perfect lens will produce the accurate picture we have just described . In actual practice , a simple lens of this type does not produce a perfect image ; instead , it produces a badly blurred one . The reason for this difference is that a simple lens is subject to a number of shortcomings or faults , which are known as aberrations . These are :
1. Chromatic aberration .
2. Spherical aberration .
3. Curvature of field .
4. Distortion ( or Curvilinear Distortion ) .
5. Coma .
6. Astigmatism .
As a student of photography , you should know how these aberrations prevent a simple lens from producing a sharp image . Unfortunately , a thorough understanding of this subject requires a rather deep knowledge of mathematics , but we will do our best to give you a simple explanation so you will have at least a working knowledge of photographic lenses . You may not understand all of it at first unless you have already had some experience in photography , but we strongly recommend that you use this lesson as a reference source and go back to it from time to time as you learn more about lenses from practical experience .
CHROMATIC ABERRATION . If we pass a narrow beam of white light through a prism ( Figure 4 ) , we find that it emerges as a band of colors . This occurs because each color is refracted or bent a different amount . The band of projected colors is called a spectrum . A rainbow is nothing more than a giant spectrum produced by sunlight being refracted as it passes through drops of rain .
Note that in Figure 4 the red light is refracted least , and the blue - violet light is refracted most . This separation of white light into its component colors is called dispersion .
LIGHTS BLUE LIGHT Fig . 4 A beam of white light passing through a colorless prism is separated into its component colors .
When a beam of light passes through an ordinary convex lens , as shown in Figure 5 , the blue rays are refracted to a greater degree than the green and still more than the red . As a result , there is no single plane or distance where all colors come to a focus . At plane R , for example , there is a sharp red image , but overlapping it are blurred blue and green images . Similarly , the plane G has al sharp green image but blurred red and blue images , and so on . All these images overlap so you do not see them as separate colors but as a blurred white image .
Fig . 5 CHROMATIC ABERRATION . In a simple lens , colors do not all converge at same point , thus producing a blurred image .
This lens defect is called chromatic aberration . Refraction ( bending of light ) and dispersion ( spreading of colors ) always go together but not always to an equal degree . It is thus possible , by using different kinds of glass , to combine a positive lens having high refraction and low dispersion with a negative lens having low refraction and low dispersion in such a way that the two dispersions cancel out ( because they are in opposite directions ) . Such a combination lens will focus all colors in one plane . A lens of this type is said to be chromatically corrected or color corrected . All photographic lenses are corrected in this way .
In actual practice , it isn't possible to correct perfectly for all colors with just two lenses , although the correction can be close enough for most purposes . For very exacting color work , three lenses are used . Such lenses are called apochromats .
SPHERICAL ABERRATION . In a simple lens with spherical surfaces , such as our reading glass , the rays of light through the center of the lens come to a focus at a point farther away than the rays from the outer edges of the lens ( see Figure 6 ) . As a result the image is blurred .
It is possible to correct this to a limited extent by stopping down the lens . and using only its center . This reduces the speed of the lens , however , and increases exposure . There are many occasions when this might not be practical .
Fig . 6 SPHERICAL ABERRATION . Rays of light around the periphery of lens come to focus at a different point than central rays .
Spherical aberration can be corrected to a large extent by combining negative and positive lenses of different curvatures , as in chromatic aberration . When this is done , the positive aberration of one lens cancels out the negative aber ration of the other .
Fig . 7 CURVATURE OF FIELD . Plane of sharpest focus with a simple lens is curved .
CURVATURE OF FIELD . In Figure 7 , we see a simple lens forming an image of the arrow ABC . Let us assume that point B of the arrow comes to a sharp focus at point b . Point A , however , is farther from the lens and therefore comes to focus at point a , which is closer to the lens than point b . Similarly , point C focuses at c which also closer to the lens than point b . In other words , in a simple lens , the plane of sharpest focus ( focal - plane ) is not flat but curved or dished . You can focus sharply for points A and C , or for the central zone B , but not for both . This fault is called of field . Again , is possible to orrect for curvature of field by combining positive and negative lenses with different indexes of refraction ( glass density ) and different surface curvatures .
IMAGE APPEARS THIS WAY IF STOP IS IN FRONT OF MENISCUS LENS
OUTLINES OF IMAGE APPEARS THIS WAY IF STOP IS BEHIND MENISCUS LENS
Fig . 8 CURVILINEAR DISTORTION . This distortion is corrected by placing diaphragm between the lens elements as shown in Fig . 9 .
DISTORTION OR CURVILINEAR DISTORTION . Distortion as a lens aberration means that straight lines near the edges of the image are produced as curves . The location of the lens diaphragm determines the direction of the curve . Generally speaking , when the diaphragm is in front of the lens , the lines curve out , and the result is a barrel - shaped image , as in Figure 8. This is called barrel - shaped distortion . When the diaphragm is in back of the lens , the lines curve in , and the result is pincushion distortion . The remedy for distortion is obvious . Two or more lenses are combined and the diaphragm is placed between the front and back elements , as shown in Figure 9. In this way , the two types of distortion neutralize each other .
COMA . Coma is a sort of spherical aberration for rays of light reaching the lens from an angle . A lens that is not corrected for coma will cause bright points of light to be reproduced as cone - shaped blurs pointed toward the center of the picture area . Coma is more pronounced as you go toward the marginal areas of the field of view . It is also corrected by combining positive and negative lenses made of different types of glass .
ASTIGMATISM . Astigmatism in a camera lens is not at all the same as astigmatism in the human eye . The two are frequently confused , however , because they both show up as an inability to focus vertical and horizontal lines on the same plane . If a subject having both vertical and horizontal lines is focused , it will be impossible to focus them both sharply . The photographer has the choice of getting the vertical lines sharp and the horizontal lines blurred , or vice - versa - unless he compromises by focusing in between and getting both only slightly blurred .
Astigmatism was the last of the aberrations to be corrected . It required the development of new types of optical glass . Generally speaking , an anastigmat lens has not only been corrected for astigmatism , as its name implies , but for all the other aberrations listed here .
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