تويت
التصوير الفوتوغرافي الحديث
تجهم الصور ..
العدسات ذات الأغراض الخاصة ..
عدسات عالية السرعة ..
عدسات بزاوية عريضة ..
عدسات تليفوتوغرافي ..
عدسات زووم ..
العدسات التكميلية ..
SPECIAL PURPOSE LENSES
Earlier in this assignment we discussed the fact that aberrations in a simple lens must be corrected before the lens can be used for photography . You will recall too that each correction required a difference in lens design . Sometimes in order to correct one aberration most effectively , another aberration may be only partially corrected . Because of this , lens design is necessarily a compromise between many conflicting requirements . The lens designer must , therefore , decide in advance what the lens will be used for so he can make the best compromise . Our review of special purpose lenses should be approached with this in mind .
HIGH SPEED LENSES
Speed in lenses is a relative term . Not so very long ago , an f / 4.5 lens was considered fast . Now it is considered quite slow . Depth of field puts a practical limit on the usable speed of photographic lenses . Since depth of field is greater for short - focus lenses , it follows that the 35mm and motion picture camera can make use of extremely fast lenses . A lens with a speed of f / 1.4 is entirely practical for such cameras .
Although an f / 2.8 lens is now considered slow by many photographers , it is nevertheless quite a fast lens . For focal length appreciably greater than 100mm , this speed is about as fast as you can use if you want to retain any depth of field .
You often hear the statement that one should buy a fast lens because the speed is always there if you need it . This statement overlooks two important points . First , the price of a lens goes up very rapidly with the speed , particularly in the longer focal lengths . Second , the ultra - speed lenses are generally not so sharp as the slower ones , even when stopped down to the same aperture . A good f / 2 lens wide open , gives a sharper and more brilliant picture than an f / 1.4 lens stopped down to f / 2 if the overall quality of the lenses are equal .
However , high speed lenses may be necessary when there is very little light available .
The high speed lens has another advantage which is particularly important with single lens reflex cameras . Under poor light conditions , it is sometimes difficult to focus such cameras accurately . The faster lenses help focusing in two ways : First , they provide brighter images , and second , the image jumps into focus and out again more positively as the lens is focused because of the shallower depth of field . This more positive in and out of focus action makes focusing easier .
WIDE ANGLE LENSES
Wide - angle lenses are designed to give reasonably sharp , well - defined images over a relatively wide angle of view . The normal angle of a photographic lens is between 45 and 60 degrees . A wide - angle lens gives good coverage for as much as 90 degrees , and there are extreme wide - angle lenses which cover as much as 135 degrees or more . A very unusual wide - angle lens for the 35mm camera is popularly called the " Fisheye " lens and it covers a complete half circle or 180 degrees .
TELEPHOTO LENSES
You will recall that the size of an image produced by a lens is determined by its focal length . When it is necessary to take pictures of very distant objects , or to get large images of objects at normal photographing distances , you must use lenses with long focal lengths for the film size you are using . Such lenses are called telephoto lenses .
The advantage of being able to take close - up shots without being right on top of the subject , or normal shots while at a distance from the subject are obvious .
A 35mm still camera may be equipped with a number of telephoto lenses ranging from 85mm to as much as 500mm and some special types of telephotos with much longer focal lengths are available . A 24 x 24 inch camera may have telephoto lenses of 120mm , 180mm , and 300mm focal lengths .
Since the picture made by a telephoto lens is enlarged a great deal , the telephoto lens must be sharper than a normal lens of the same focal length . The much narrower angle of view of a telephoto lens makes it possible for the lens designer to give added sharpness to the smaller angle , and still keep the speed fairly high and the cost reasonable .
ZOOM LENSES
Undoubtedly , the most interesting and certainly the most complicated of all common photographic lenses is the zoom lens . In a zoom lens , the focal length can be varied so it can serve as a wide - angle , normal angle , and a telephoto lens simply by pulling a lever or rotating the lens barrel . It is extremely useful in movie and television work where frequent changes in focal length are needed to zoom in for a close - up and then back away to broaden the field of view .
The zoom lens is called upon to do an extremely complicated and difficult job . While its focal length is being varied by as much as three or four to one , its over - all length cannot change a great deal and the plane on which it is focused must remain constant . Because of these requirements , a zoom lens cannot produce as sharp an image as a top performing conventional lens of equal focal length . The complicated design of the zoom lens imposes some very severe restrictions on the manufacturer . As a result , zoom lenses are not best suited for still photography where maximum sharpness is needed . However , press
photographers find zoom lenses extremely valuable for quick work when they do not have time to switch lenses .
SUPPLEMENTARY LENSES
An inexpensive means of changing the focal length of any lens is to slip a supplementary lens in front of it . If lenses on your camera are not readily interchangeable , or if cost is an important factor , supplementary lenses may be the best answer . However , you must remember that adding an extra lens to a very carefully balanced optical system always disturbs the balance . In other words , if you use supplementary lenses , your pictures will not be as sharp , and will not stand enlargement or projection as well as pictures taken without them . This fact applies even if the supplementary lenses are produced by the manufacturer of the main lens .
Supplementary lenses are of two types - positive and negative . The positive lenses reduce focal length . They are used for close - up work with cameras that cannot focus on close - up subjects . Since they reduce the focal length , they make it possible to work closer to the subject . They also increase the angle of view , and so have some use as wide - angle lenses .
Portrait attachments are also positive supplementary lenses . If the camera does not permit focusing closer than 6 feet , a supplementary portrait attachment can be used to shorten the focal length . The camera can then be moved closer to the subject , providing a larger image . However , this practice is not recommended , since it gives exaggerated perspective .
Negative supplementary lenses increase the focal length . They give larger images , but require additional extension of the camera lens . If the camera has sufficient lens extension , or permits the use of an extension tube , it is much better to use a supplementary lens of this type for portrait work , rather than the usual portrait attachment . Primarily , however , negative supplementary lenses are intended to be used as telephoto lenses .
تجهم الصور ..
العدسات ذات الأغراض الخاصة ..
عدسات عالية السرعة ..
عدسات بزاوية عريضة ..
عدسات تليفوتوغرافي ..
عدسات زووم ..
العدسات التكميلية ..
SPECIAL PURPOSE LENSES
Earlier in this assignment we discussed the fact that aberrations in a simple lens must be corrected before the lens can be used for photography . You will recall too that each correction required a difference in lens design . Sometimes in order to correct one aberration most effectively , another aberration may be only partially corrected . Because of this , lens design is necessarily a compromise between many conflicting requirements . The lens designer must , therefore , decide in advance what the lens will be used for so he can make the best compromise . Our review of special purpose lenses should be approached with this in mind .
HIGH SPEED LENSES
Speed in lenses is a relative term . Not so very long ago , an f / 4.5 lens was considered fast . Now it is considered quite slow . Depth of field puts a practical limit on the usable speed of photographic lenses . Since depth of field is greater for short - focus lenses , it follows that the 35mm and motion picture camera can make use of extremely fast lenses . A lens with a speed of f / 1.4 is entirely practical for such cameras .
Although an f / 2.8 lens is now considered slow by many photographers , it is nevertheless quite a fast lens . For focal length appreciably greater than 100mm , this speed is about as fast as you can use if you want to retain any depth of field .
You often hear the statement that one should buy a fast lens because the speed is always there if you need it . This statement overlooks two important points . First , the price of a lens goes up very rapidly with the speed , particularly in the longer focal lengths . Second , the ultra - speed lenses are generally not so sharp as the slower ones , even when stopped down to the same aperture . A good f / 2 lens wide open , gives a sharper and more brilliant picture than an f / 1.4 lens stopped down to f / 2 if the overall quality of the lenses are equal .
However , high speed lenses may be necessary when there is very little light available .
The high speed lens has another advantage which is particularly important with single lens reflex cameras . Under poor light conditions , it is sometimes difficult to focus such cameras accurately . The faster lenses help focusing in two ways : First , they provide brighter images , and second , the image jumps into focus and out again more positively as the lens is focused because of the shallower depth of field . This more positive in and out of focus action makes focusing easier .
WIDE ANGLE LENSES
Wide - angle lenses are designed to give reasonably sharp , well - defined images over a relatively wide angle of view . The normal angle of a photographic lens is between 45 and 60 degrees . A wide - angle lens gives good coverage for as much as 90 degrees , and there are extreme wide - angle lenses which cover as much as 135 degrees or more . A very unusual wide - angle lens for the 35mm camera is popularly called the " Fisheye " lens and it covers a complete half circle or 180 degrees .
TELEPHOTO LENSES
You will recall that the size of an image produced by a lens is determined by its focal length . When it is necessary to take pictures of very distant objects , or to get large images of objects at normal photographing distances , you must use lenses with long focal lengths for the film size you are using . Such lenses are called telephoto lenses .
The advantage of being able to take close - up shots without being right on top of the subject , or normal shots while at a distance from the subject are obvious .
A 35mm still camera may be equipped with a number of telephoto lenses ranging from 85mm to as much as 500mm and some special types of telephotos with much longer focal lengths are available . A 24 x 24 inch camera may have telephoto lenses of 120mm , 180mm , and 300mm focal lengths .
Since the picture made by a telephoto lens is enlarged a great deal , the telephoto lens must be sharper than a normal lens of the same focal length . The much narrower angle of view of a telephoto lens makes it possible for the lens designer to give added sharpness to the smaller angle , and still keep the speed fairly high and the cost reasonable .
ZOOM LENSES
Undoubtedly , the most interesting and certainly the most complicated of all common photographic lenses is the zoom lens . In a zoom lens , the focal length can be varied so it can serve as a wide - angle , normal angle , and a telephoto lens simply by pulling a lever or rotating the lens barrel . It is extremely useful in movie and television work where frequent changes in focal length are needed to zoom in for a close - up and then back away to broaden the field of view .
The zoom lens is called upon to do an extremely complicated and difficult job . While its focal length is being varied by as much as three or four to one , its over - all length cannot change a great deal and the plane on which it is focused must remain constant . Because of these requirements , a zoom lens cannot produce as sharp an image as a top performing conventional lens of equal focal length . The complicated design of the zoom lens imposes some very severe restrictions on the manufacturer . As a result , zoom lenses are not best suited for still photography where maximum sharpness is needed . However , press
photographers find zoom lenses extremely valuable for quick work when they do not have time to switch lenses .
SUPPLEMENTARY LENSES
An inexpensive means of changing the focal length of any lens is to slip a supplementary lens in front of it . If lenses on your camera are not readily interchangeable , or if cost is an important factor , supplementary lenses may be the best answer . However , you must remember that adding an extra lens to a very carefully balanced optical system always disturbs the balance . In other words , if you use supplementary lenses , your pictures will not be as sharp , and will not stand enlargement or projection as well as pictures taken without them . This fact applies even if the supplementary lenses are produced by the manufacturer of the main lens .
Supplementary lenses are of two types - positive and negative . The positive lenses reduce focal length . They are used for close - up work with cameras that cannot focus on close - up subjects . Since they reduce the focal length , they make it possible to work closer to the subject . They also increase the angle of view , and so have some use as wide - angle lenses .
Portrait attachments are also positive supplementary lenses . If the camera does not permit focusing closer than 6 feet , a supplementary portrait attachment can be used to shorten the focal length . The camera can then be moved closer to the subject , providing a larger image . However , this practice is not recommended , since it gives exaggerated perspective .
Negative supplementary lenses increase the focal length . They give larger images , but require additional extension of the camera lens . If the camera has sufficient lens extension , or permits the use of an extension tube , it is much better to use a supplementary lens of this type for portrait work , rather than the usual portrait attachment . Primarily , however , negative supplementary lenses are intended to be used as telephoto lenses .
تعليق