تويت
البعد البؤري وعمق المجال .. معدات وتقنيات إضافية .. المرشد خطوة بخطوة للتصوير الفوتوغرافي
Focal length and depth of field
Changing to a shorter focal length lens always increases the depth of field in your pictures , even at the same aperture . This is partly due to the smaller diaphragm on short focal length lenses . On a 28 mm lens , for example , the aperture at f8 is only one- eighth of 28 mm , whereas on a 135 mm lens , f8 is one - eighth of 135 mm ( see p . 30 ) . In addition , short focal length lenses have a stronger bending power on the light . This means that foreground and background elements are brought into sharp focus much closer together within the camera . As a result , at any one aperture a larger area of the image is acceptably sharp than with a lens of longer focal length .
Comparing lenses
As explained on page 33 , the depth of field scale on the lens shows the amount of the subject that is sharp at any given focus and aperture setting . This varies according to the focal length of the lens .
The diagrams and pictures below show the extent of the depth of field in front of and behind the sub- ject on different lenses with the focus and aperture constant . The 28 mm lens , below top , renders objects sharp from about 6 ft 1.8 m ) to infinity at f5.6 . On a standard 50 mm lens , below center the depth field is reduced . Less of the foreground and much less of the background is sharp - from 8 ft ( 2.4 m ) to 13 ft ( 3.9 m ) . The bottom diagram and picture show the narrow depth of field produced by a 135 mm lens , at f5.6- from 9 ft ( 2.9 m ) to 10 ft ( 3.2 m ) .
Because of the extended depth of field , a short focal length lens is useful when the focus settings have to be estimated or made very rap- idly , such as in some action shots .
A long focus lens restricts the detail to the main area of interest . But because of the shallow depth of field it gives there is little latitude , on either side of the subject for focusing errors .
Focal length and depth of field
Changing to a shorter focal length lens always increases the depth of field in your pictures , even at the same aperture . This is partly due to the smaller diaphragm on short focal length lenses . On a 28 mm lens , for example , the aperture at f8 is only one- eighth of 28 mm , whereas on a 135 mm lens , f8 is one - eighth of 135 mm ( see p . 30 ) . In addition , short focal length lenses have a stronger bending power on the light . This means that foreground and background elements are brought into sharp focus much closer together within the camera . As a result , at any one aperture a larger area of the image is acceptably sharp than with a lens of longer focal length .
Comparing lenses
As explained on page 33 , the depth of field scale on the lens shows the amount of the subject that is sharp at any given focus and aperture setting . This varies according to the focal length of the lens .
The diagrams and pictures below show the extent of the depth of field in front of and behind the sub- ject on different lenses with the focus and aperture constant . The 28 mm lens , below top , renders objects sharp from about 6 ft 1.8 m ) to infinity at f5.6 . On a standard 50 mm lens , below center the depth field is reduced . Less of the foreground and much less of the background is sharp - from 8 ft ( 2.4 m ) to 13 ft ( 3.9 m ) . The bottom diagram and picture show the narrow depth of field produced by a 135 mm lens , at f5.6- from 9 ft ( 2.9 m ) to 10 ft ( 3.2 m ) .
Because of the extended depth of field , a short focal length lens is useful when the focus settings have to be estimated or made very rap- idly , such as in some action shots .
A long focus lens restricts the detail to the main area of interest . But because of the shallow depth of field it gives there is little latitude , on either side of the subject for focusing errors .
تعليق