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Special long focus lenses
Lenses with focal lengths beyond about 250 mm ( for 35 mm cameras ) offer all the characteristics of long lenses in an extreme form . Compared with standard 50 mm lenses they have a narrower angle , a smaller maximum aperture , give extremely shallow depth of field , and are more bulky .
As focal length increases so the perspec- tive is diminished . Long focus lenses enlarge distant objects which would normally appear very small . As a result distant objects almost begin to appear bigger than larger objects in front of them a form of optical illusion . Image magnification is proportional to focal length . For example , if you change from a 100 mm lens to a 500 mm lens the image is enlarged five times .
At one time a long focus lens was essen- tially a weak lens at the end of a long tube A 300 mm type was spaced 300 mm from the film in the camera , and so on . However by incorporating other glass elements the light path can be modified , as shown below This means that the lens barrel can be phy- sically shorter than its true focal length an arrangement known as telephoto design . Today virtually all long focus lenses are telephotos . Normally , the overall length of the lens barrel is only half its focal length . Even so , lenses of 300 mm or longer are awkward to carry and use . They are also unsuitable for use on direct viewfinder cam- eras because the effect of parallax is in-creased by the narrow angle of view they give . The angle of view with a 1000 mm lens , for example , is only 2.5 ° .
Extreme long focus lenses should be used on a tripod . Camera shake with such bulky lenses is particularly visible because they magnify a small area of the subject . So you should always use a fast shutter speed to reduce the possibility of camera shake re- cording . This , combined with the fairly small maximum aperture on long focus lenses , means that you will have to use a fast film , or " uprate " the film speed during pro- cessing ( see pp . 126-7 ) .
Exposure measurement is also a problem when you are taking pictures of very distant subjects . It can be measured most accu rately by a spot reading taken with a hand or through - the - lens meter ( see p . 104 ) .
The longer the lens , the flatter the image contrast tends to be . This is partly due to the amount of light scattered by the atmos- phere between you and the subject . Extra development of the negative will help to compensate for this , and using a lens hood and an ultra - violet filter will assist by reducing unwanted light in the image .
A cheaper alternative to a conventional telephoto lens is a " telephoto - converter " , which attaches to your standard lens , as shown below left . Other long focus lenses include the mirror lens and " zoom " lens , discussed on the opposite page .
Long focus lenses
Focal lengths of very long focus lenses range from 250 mm to 1000 mm , giving angles of view ranging from about 4 ° down to 2.5 ° . Mirror lenses are much more com- pact than long focus lenses . They produce similar results but have some dis- advantages , discussed on the opposite page . A " zoom " or variable focal length lens enables you alter angles of view , magnification , and depth of field without changing lenses .
Telephoto lens construction
The telephoto construction of near- ly all long focus lenses enables the lens barrel to be shorter and more compact than its true focal length . After passing through the main , frontal lens elements , the path of the light is slightly diverged by two weak elements at the back of the barrel , so as to give the effect of a lens positioned further from the film .
1000 mm lens The picture , right , was taken with a 1000 mm lens at f16 . Note how perspective is compressed , owing to the distant viewpoint .
Telephoto - converters
A telephoto converter produces similar effects to a true telephoto lens . It fits between your normal lens and the camera body , as shown right . The converter in- creases the focal length of your regular lens , usually by two or three times . Consequently , lens apertures become proportionally smaller . For example , a 50 mm lens set to 12 with a x3 converter produces the same results as a 150 mm lens set at f5-6 . Image quality is poorer with a converter than with a true long focus lens .
Mirror lens
A mirror lens is a telephoto with " folded up " optics . It uses internal mirrors to reflect the light twice . enabling the lens barrel to be much shorter ( although broader ) . As shown below , light enters through a glass plate and is converged and reflected back by a concave mirror at the camera end of the lens . It then reaches a small backward facing mirrored lens attached to the center of the front element . This reflects the light back through a hole in the concave mirror to focus on the film .
Mirror lenses have two dis- advantages . You cannot alter the aperture and , consequently , the limited depth of field ; and out - of- focus highlights are spread into rings of light . Most mirror lenses work at an aperture of 18 or f11 .
Highlight distortion
When bright highlights are rendered out - of - focus by a regular lens they spread into disk shapes . A mirror lens gives a ring shape to out - of - focus highlights owing to its front mirror . This can be clearly seen in the picture , right , which was taken using a 1000 mm mirror lens .
Zoom lens
A zoom lens has an extra control which allows you to vary its focal length . This requires very complex optics within the lens so that focus and aperture remain constant . The two pictures , right , of the same subject were taken with a zoom lens . The near right picture is equivalent to one taken with a 85 mm lens ; the far right picture is equivalent to one taken with a 200 mm lens .
Altering the focal length during exposure creates interesting blur effects . The picture below was taken with the camera on a tripod and the focal length of the zoom increased during a long exposure . Only the area in the center of the picture remains sharp .
Special long focus lenses
Lenses with focal lengths beyond about 250 mm ( for 35 mm cameras ) offer all the characteristics of long lenses in an extreme form . Compared with standard 50 mm lenses they have a narrower angle , a smaller maximum aperture , give extremely shallow depth of field , and are more bulky .
As focal length increases so the perspec- tive is diminished . Long focus lenses enlarge distant objects which would normally appear very small . As a result distant objects almost begin to appear bigger than larger objects in front of them a form of optical illusion . Image magnification is proportional to focal length . For example , if you change from a 100 mm lens to a 500 mm lens the image is enlarged five times .
At one time a long focus lens was essen- tially a weak lens at the end of a long tube A 300 mm type was spaced 300 mm from the film in the camera , and so on . However by incorporating other glass elements the light path can be modified , as shown below This means that the lens barrel can be phy- sically shorter than its true focal length an arrangement known as telephoto design . Today virtually all long focus lenses are telephotos . Normally , the overall length of the lens barrel is only half its focal length . Even so , lenses of 300 mm or longer are awkward to carry and use . They are also unsuitable for use on direct viewfinder cam- eras because the effect of parallax is in-creased by the narrow angle of view they give . The angle of view with a 1000 mm lens , for example , is only 2.5 ° .
Extreme long focus lenses should be used on a tripod . Camera shake with such bulky lenses is particularly visible because they magnify a small area of the subject . So you should always use a fast shutter speed to reduce the possibility of camera shake re- cording . This , combined with the fairly small maximum aperture on long focus lenses , means that you will have to use a fast film , or " uprate " the film speed during pro- cessing ( see pp . 126-7 ) .
Exposure measurement is also a problem when you are taking pictures of very distant subjects . It can be measured most accu rately by a spot reading taken with a hand or through - the - lens meter ( see p . 104 ) .
The longer the lens , the flatter the image contrast tends to be . This is partly due to the amount of light scattered by the atmos- phere between you and the subject . Extra development of the negative will help to compensate for this , and using a lens hood and an ultra - violet filter will assist by reducing unwanted light in the image .
A cheaper alternative to a conventional telephoto lens is a " telephoto - converter " , which attaches to your standard lens , as shown below left . Other long focus lenses include the mirror lens and " zoom " lens , discussed on the opposite page .
Long focus lenses
Focal lengths of very long focus lenses range from 250 mm to 1000 mm , giving angles of view ranging from about 4 ° down to 2.5 ° . Mirror lenses are much more com- pact than long focus lenses . They produce similar results but have some dis- advantages , discussed on the opposite page . A " zoom " or variable focal length lens enables you alter angles of view , magnification , and depth of field without changing lenses .
Telephoto lens construction
The telephoto construction of near- ly all long focus lenses enables the lens barrel to be shorter and more compact than its true focal length . After passing through the main , frontal lens elements , the path of the light is slightly diverged by two weak elements at the back of the barrel , so as to give the effect of a lens positioned further from the film .
1000 mm lens The picture , right , was taken with a 1000 mm lens at f16 . Note how perspective is compressed , owing to the distant viewpoint .
Telephoto - converters
A telephoto converter produces similar effects to a true telephoto lens . It fits between your normal lens and the camera body , as shown right . The converter in- creases the focal length of your regular lens , usually by two or three times . Consequently , lens apertures become proportionally smaller . For example , a 50 mm lens set to 12 with a x3 converter produces the same results as a 150 mm lens set at f5-6 . Image quality is poorer with a converter than with a true long focus lens .
Mirror lens
A mirror lens is a telephoto with " folded up " optics . It uses internal mirrors to reflect the light twice . enabling the lens barrel to be much shorter ( although broader ) . As shown below , light enters through a glass plate and is converged and reflected back by a concave mirror at the camera end of the lens . It then reaches a small backward facing mirrored lens attached to the center of the front element . This reflects the light back through a hole in the concave mirror to focus on the film .
Mirror lenses have two dis- advantages . You cannot alter the aperture and , consequently , the limited depth of field ; and out - of- focus highlights are spread into rings of light . Most mirror lenses work at an aperture of 18 or f11 .
Highlight distortion
When bright highlights are rendered out - of - focus by a regular lens they spread into disk shapes . A mirror lens gives a ring shape to out - of - focus highlights owing to its front mirror . This can be clearly seen in the picture , right , which was taken using a 1000 mm mirror lens .
Zoom lens
A zoom lens has an extra control which allows you to vary its focal length . This requires very complex optics within the lens so that focus and aperture remain constant . The two pictures , right , of the same subject were taken with a zoom lens . The near right picture is equivalent to one taken with a 85 mm lens ; the far right picture is equivalent to one taken with a 200 mm lens .
Altering the focal length during exposure creates interesting blur effects . The picture below was taken with the camera on a tripod and the focal length of the zoom increased during a long exposure . Only the area in the center of the picture remains sharp .
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