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التصوير الفوتوغرافي الحديث
تجهم الصور ..
العدسات ذات الأغراض الخاصة ..
عدسات كونفيرتر ..
عدسات قابلة للتحويل ..
العدسات العملية ..
العدسات البورتريه ..
CONVERTER LENSES
Converter lenses are not complete photographic lenses but are used in combination with such lenses to change their focal length , thus con verting them to telephoto or wide angle use . Of the many types of converter lenses which have been marketed from time to time , the only one which has achieved any . degree of popularity is the telephoto converter ( often called tele - converter ) designed for use with the 35mm single lens reflex camera . When mounted between the camera body and the regular lens it multiplies the focal length of the lens by a fixed amount . A typical tele - converter lens ( Figure 29 ) shows a 35mm lens equipped with a tele - converter . Beside it are two separate tele - converters to give you an idea of their appearance .
Fig . 29 A typical 35mm camera equipped with lens and converter lens . Beside it are 2X and 3X converter lenses .
Since the size of the image produced by a lens is proportional to the focal length , a telephoto converter , or tele - converter , in effect magnifies the image . It also multiplies the f / number by the same amount . For example , when a 2X tele - converter is added to an f / 2.8 , 100 millimeter lens , the result is an f / 5.6 , 200 millimeter lens . All lens aperture numbers marked on the lens must be multiplied by the magnification factor to obtain the true aperture of the combination .
The image produced by a lens and tele - converter combination is not as sharp as the image produced by a regular telephoto lens of the same focal length . However , if the tele - converter is used properly , the loss of sharpness may be relatively small . Maximum image sharpness will be obtained if the following facts are kept in mind :
1. Tele - converters vary widely in performance . Some produce much sharper images than others . If the lens you are considering is not made by one of the better known lens manufacturers , better try it out first .
2. In general , the greater the magnification , the greater the loss of sharpness . A 2X tele - converter produces a sharper picture than a 3X tele - converter .
3. Best results are obtained if the lens is stopped down at least two full stops from maximum aperture .
4. Tele - converters work best with lenses of long focal length . Loss of picture quality may be quite pronounced with a 50mm lens , not too bad with an 85mm lens , and relatively minor with a 135mm lens .
A very important feature of the tele - converter is that the focusing is not affected . The lens is , therefore , focused in the normal manner and the footage markings on the lens barrel remain unchanged . The depth of field markings , however , no longer apply since the depth of field of the new arrangement is reduced in accordance with the increase in focal length .
It is seldom practical to use a tele - converter without a tripod for two reasons . First , the lens is quite slow because of the added magnification and the need for stopping down , and second , because the extremely long focal length of the combination results in greatly magnified camera movement .
CONVERTIBLE LENSES
Convertible lenses must not be confused with converter lenses . Convertible lenses are complete lenses and are generally made only in relatively long focal lengths for use in commercial photography with larger film size . A convertible lens consists of two separate components , each of which is a complete lens that can be used separately . When these two components of the lens are similar and are of equal focal length , the lens is called a symmetrical convertible lens and each separate component has usually about double the focal length of the combined lenses .
Sometimes the two components are not both of the same focal length , and then the lens is known as an unsymmetrical convertible or a triple convertible . When the two separate com ponents are similar , there is a choice of only two focal lengths , namely , the focal length of the combined lens and that of either of the . components . But when the lens is unsymmetrical , there is a choice of three different focal lengths .
The Goerz Dagor lens ( Figure 30 ) is a symmetrical convertible lens . Each of its single elements can be used alone , and each has a focal length of about 1-3 / times that of the combination . Another well - known Goerz lens , the Goerz Dogmar , is an unsymmetrical convertible . The front element has a focal length very nearly double that of the combination , and the rear element has a length about 1½ times that of the combined lenses .
The advantage of the convertible lens is that it offers a choice of two or three different focal lengths . Its disadvantage is that the separate components are necessarily considerably slower than the combined lens .
Fig . 30 The Goerz Dagor symmetrical convertible lens .
PROCESS LENSES
Process lenses are designed primarily for photo - engraving and similar copying processes in which the lens is used to copy material at fairly close distances . The corrections of the various lens faults or aberrations must be done in a slightly different manner for close - up work than for distant work , and so process lenses are especially corrected for copying at close distances . Practically all process lenses are also apochromats -that is , they are color - corrected for three colors instead of only two .
Because of the exceptionally high degree to which process lenses are corrected , they usually have maximum speed of about f / 9 .
Fig . 31 A lens designed for photo - engraving and copy work . Maximum speed of this lens is f / 9 .
Process lenses make exceptionally fine enlarging lenses where maximum sharpness is an important consideration . However , they are quite expensive in spite of their slow speed . Figure 31 shows a top quality process lens .
PORTRAIT LENSES
In talking about portrait lenses , you should not confuse them with portrait attachments . A portrait lens , as its name implies , is simply a lens that is specifically designed for portrait work . Originally , when all portrait prints were contact prints , portrait lenses were quite different in their characteristics . Now that distinction has been largely lost , and any good lens is used . However , there are still many true portrait lenses in use .
A portrait lens of the old - fashioned type was primarily a fast , partially corrected lens which covered only a narrow angle . Speed was hecessary because emulsions were very slow in those days , but needle - sharpness was a hindrance rather than a help . As a result , portrait lenses were not completely corrected for the various lens aberrations . Some lenses actually provided adjustments to introduce a controlled amount of diffusion .
Diffused - focus portraits are no longer popular . In addition , most portraits are enlarged from smaller negatives . As a result , the old - fashioned soft - portrait lenses are no longer widely used . Instead , the general practice is to use an ordinary lens . If the lens is too sharp , a diffusion disk is used with it to soften its harshness .
تجهم الصور ..
العدسات ذات الأغراض الخاصة ..
عدسات كونفيرتر ..
عدسات قابلة للتحويل ..
العدسات العملية ..
العدسات البورتريه ..
CONVERTER LENSES
Converter lenses are not complete photographic lenses but are used in combination with such lenses to change their focal length , thus con verting them to telephoto or wide angle use . Of the many types of converter lenses which have been marketed from time to time , the only one which has achieved any . degree of popularity is the telephoto converter ( often called tele - converter ) designed for use with the 35mm single lens reflex camera . When mounted between the camera body and the regular lens it multiplies the focal length of the lens by a fixed amount . A typical tele - converter lens ( Figure 29 ) shows a 35mm lens equipped with a tele - converter . Beside it are two separate tele - converters to give you an idea of their appearance .
Fig . 29 A typical 35mm camera equipped with lens and converter lens . Beside it are 2X and 3X converter lenses .
Since the size of the image produced by a lens is proportional to the focal length , a telephoto converter , or tele - converter , in effect magnifies the image . It also multiplies the f / number by the same amount . For example , when a 2X tele - converter is added to an f / 2.8 , 100 millimeter lens , the result is an f / 5.6 , 200 millimeter lens . All lens aperture numbers marked on the lens must be multiplied by the magnification factor to obtain the true aperture of the combination .
The image produced by a lens and tele - converter combination is not as sharp as the image produced by a regular telephoto lens of the same focal length . However , if the tele - converter is used properly , the loss of sharpness may be relatively small . Maximum image sharpness will be obtained if the following facts are kept in mind :
1. Tele - converters vary widely in performance . Some produce much sharper images than others . If the lens you are considering is not made by one of the better known lens manufacturers , better try it out first .
2. In general , the greater the magnification , the greater the loss of sharpness . A 2X tele - converter produces a sharper picture than a 3X tele - converter .
3. Best results are obtained if the lens is stopped down at least two full stops from maximum aperture .
4. Tele - converters work best with lenses of long focal length . Loss of picture quality may be quite pronounced with a 50mm lens , not too bad with an 85mm lens , and relatively minor with a 135mm lens .
A very important feature of the tele - converter is that the focusing is not affected . The lens is , therefore , focused in the normal manner and the footage markings on the lens barrel remain unchanged . The depth of field markings , however , no longer apply since the depth of field of the new arrangement is reduced in accordance with the increase in focal length .
It is seldom practical to use a tele - converter without a tripod for two reasons . First , the lens is quite slow because of the added magnification and the need for stopping down , and second , because the extremely long focal length of the combination results in greatly magnified camera movement .
CONVERTIBLE LENSES
Convertible lenses must not be confused with converter lenses . Convertible lenses are complete lenses and are generally made only in relatively long focal lengths for use in commercial photography with larger film size . A convertible lens consists of two separate components , each of which is a complete lens that can be used separately . When these two components of the lens are similar and are of equal focal length , the lens is called a symmetrical convertible lens and each separate component has usually about double the focal length of the combined lenses .
Sometimes the two components are not both of the same focal length , and then the lens is known as an unsymmetrical convertible or a triple convertible . When the two separate com ponents are similar , there is a choice of only two focal lengths , namely , the focal length of the combined lens and that of either of the . components . But when the lens is unsymmetrical , there is a choice of three different focal lengths .
The Goerz Dagor lens ( Figure 30 ) is a symmetrical convertible lens . Each of its single elements can be used alone , and each has a focal length of about 1-3 / times that of the combination . Another well - known Goerz lens , the Goerz Dogmar , is an unsymmetrical convertible . The front element has a focal length very nearly double that of the combination , and the rear element has a length about 1½ times that of the combined lenses .
The advantage of the convertible lens is that it offers a choice of two or three different focal lengths . Its disadvantage is that the separate components are necessarily considerably slower than the combined lens .
Fig . 30 The Goerz Dagor symmetrical convertible lens .
PROCESS LENSES
Process lenses are designed primarily for photo - engraving and similar copying processes in which the lens is used to copy material at fairly close distances . The corrections of the various lens faults or aberrations must be done in a slightly different manner for close - up work than for distant work , and so process lenses are especially corrected for copying at close distances . Practically all process lenses are also apochromats -that is , they are color - corrected for three colors instead of only two .
Because of the exceptionally high degree to which process lenses are corrected , they usually have maximum speed of about f / 9 .
Fig . 31 A lens designed for photo - engraving and copy work . Maximum speed of this lens is f / 9 .
Process lenses make exceptionally fine enlarging lenses where maximum sharpness is an important consideration . However , they are quite expensive in spite of their slow speed . Figure 31 shows a top quality process lens .
PORTRAIT LENSES
In talking about portrait lenses , you should not confuse them with portrait attachments . A portrait lens , as its name implies , is simply a lens that is specifically designed for portrait work . Originally , when all portrait prints were contact prints , portrait lenses were quite different in their characteristics . Now that distinction has been largely lost , and any good lens is used . However , there are still many true portrait lenses in use .
A portrait lens of the old - fashioned type was primarily a fast , partially corrected lens which covered only a narrow angle . Speed was hecessary because emulsions were very slow in those days , but needle - sharpness was a hindrance rather than a help . As a result , portrait lenses were not completely corrected for the various lens aberrations . Some lenses actually provided adjustments to introduce a controlled amount of diffusion .
Diffused - focus portraits are no longer popular . In addition , most portraits are enlarged from smaller negatives . As a result , the old - fashioned soft - portrait lenses are no longer widely used . Instead , the general practice is to use an ordinary lens . If the lens is too sharp , a diffusion disk is used with it to soften its harshness .
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