تويت
التصوير الفوتوغرافي الحديث
تجهم الصور ..
خصائص العدسة ..
عمق الميدان ..
DEPTH OF FIELD
We learned from our experiment with the reading glass that near and distant objects come to a sharp focus at different distances from a lens . When a lens is sharply focused for one distance , objects that are closer or farther away are more or less out of focus and appear blurred .
Fortunately , there is a certain amount of leeway in such matters . For example , if , in shooting a landscape or outdoor scene , we focus our lens sharply on a subject at about 15 feet away , objects from 11 to 22 feet may all focus sharply enough so that the slight blur will not be noticeable . This distance of 11 feet ( from 11 to 22 feet ) , is the depth of field of this lens when it is focused at 15 feet . It is easy to see that for certain kinds of pictures the depth of field could be very important . Fortunately , it is possible to increase depth of field where necessary , as for group shots , street scenes , etc.
The depth of field of a lens depends primarily on four factors :
1. THE LENS APERTURE . The smaller the aperture ( the larger the f / number ) , the greater the depth of field . For example , when focused at 15 feet , a 100mm lens has a depth of field of 7½ feet ( from 12 % to 20 feet ) at f / 8 . If the lens is stopped down to f / 32 , the depth of field is increased to 91½ feet ( 8½ to 100 feet ) . Figure 16 was taken with a wide open lens ( f / 2.8 ) . Note that both foreground and background are out of focus . Only the middle distance is sharp . Figure 17 , the same subject , was taken with the lens stopped down to ƒ / 22 . Note the greater range of sharpness . We can often gain emphasis by taking advantage of the shallow depth of field of a wide open lens . Figure 18 shows how the important central subject is given major emphasis by being focused sharply while both background and foreground are thrown out of focus so they don't compete for importance .
2. THE FOCAL LENGTH OF THE LENS . The longer the focal length , the shallower the depth of field . That is why wide - angle lenses which have short focal length have so much depth of field . Wide - angle lenses are known as short - focus lenses .
3. THE DISTANCE OF THE OBJECT FROM THE LENS . The greater the distance , the greater the depth of field . For example , the 100mm lens discussed above , which has a depth of field of 7½ feet ( from 12 % to 20 feet ) when focused at 15 feet , has a depth of field of only 1 foot 10 inches when focused at 8 feet .
Fig . 18 SELECTIVE FOCUS . Lens is focused sharply on the cups , but the foreground and background are thrown out of focus to give greater impact to the crisply defined main subject . Courtesy of The West Bend Company
4. REQUIRED SHARPNESS . At normal viewing distance of about 14 inches , the eye cannot separate lines which are closer than 1/100 inch . This means that a photograph with a blur ( technically referred to as circle - of - confusion ) smaller than 1/100 inch is perfectly sharp to the normal eye . For most purposes a photograph with twice that degree of blur ( 1/50 inch ) is still satisfactory .
This , of course , is for the final print . If the negative is to be enlarged , it must be sharper to compensate for the degree of enlargement . Depth of field tables are available for most lenses . They indicate the blur , or circle - of - confusion in fractions of an inch or millimeters at various focusing distances . For example , the depth of field of a 100mm lens shows the range of sharp focus is 11 to 22 feet at a circle - of - confusion of 1/200 inch when focused at 15 feet . But if a circle - of - confusion of 1/500 inch ( greater sharpness ) is desired , the range of sharp focus , or depth of field must be cut down to 4 feet ( from 13 to 17 feet ) .
Because depth of field is so important , most hand camera lenses have a set of calibrations which enable the user to determine the depth of field quite simply and easily without referring to tables . Such a depth of field scale is shown on the camera lens picture in Figure 8 of Assignment 1. The use of this scale is explained in your camera instruction book , but it will be gone into much more fully in a later assignment in this course of study .
تجهم الصور ..
خصائص العدسة ..
عمق الميدان ..
DEPTH OF FIELD
We learned from our experiment with the reading glass that near and distant objects come to a sharp focus at different distances from a lens . When a lens is sharply focused for one distance , objects that are closer or farther away are more or less out of focus and appear blurred .
Fortunately , there is a certain amount of leeway in such matters . For example , if , in shooting a landscape or outdoor scene , we focus our lens sharply on a subject at about 15 feet away , objects from 11 to 22 feet may all focus sharply enough so that the slight blur will not be noticeable . This distance of 11 feet ( from 11 to 22 feet ) , is the depth of field of this lens when it is focused at 15 feet . It is easy to see that for certain kinds of pictures the depth of field could be very important . Fortunately , it is possible to increase depth of field where necessary , as for group shots , street scenes , etc.
The depth of field of a lens depends primarily on four factors :
1. THE LENS APERTURE . The smaller the aperture ( the larger the f / number ) , the greater the depth of field . For example , when focused at 15 feet , a 100mm lens has a depth of field of 7½ feet ( from 12 % to 20 feet ) at f / 8 . If the lens is stopped down to f / 32 , the depth of field is increased to 91½ feet ( 8½ to 100 feet ) . Figure 16 was taken with a wide open lens ( f / 2.8 ) . Note that both foreground and background are out of focus . Only the middle distance is sharp . Figure 17 , the same subject , was taken with the lens stopped down to ƒ / 22 . Note the greater range of sharpness . We can often gain emphasis by taking advantage of the shallow depth of field of a wide open lens . Figure 18 shows how the important central subject is given major emphasis by being focused sharply while both background and foreground are thrown out of focus so they don't compete for importance .
2. THE FOCAL LENGTH OF THE LENS . The longer the focal length , the shallower the depth of field . That is why wide - angle lenses which have short focal length have so much depth of field . Wide - angle lenses are known as short - focus lenses .
3. THE DISTANCE OF THE OBJECT FROM THE LENS . The greater the distance , the greater the depth of field . For example , the 100mm lens discussed above , which has a depth of field of 7½ feet ( from 12 % to 20 feet ) when focused at 15 feet , has a depth of field of only 1 foot 10 inches when focused at 8 feet .
Fig . 18 SELECTIVE FOCUS . Lens is focused sharply on the cups , but the foreground and background are thrown out of focus to give greater impact to the crisply defined main subject . Courtesy of The West Bend Company
4. REQUIRED SHARPNESS . At normal viewing distance of about 14 inches , the eye cannot separate lines which are closer than 1/100 inch . This means that a photograph with a blur ( technically referred to as circle - of - confusion ) smaller than 1/100 inch is perfectly sharp to the normal eye . For most purposes a photograph with twice that degree of blur ( 1/50 inch ) is still satisfactory .
This , of course , is for the final print . If the negative is to be enlarged , it must be sharper to compensate for the degree of enlargement . Depth of field tables are available for most lenses . They indicate the blur , or circle - of - confusion in fractions of an inch or millimeters at various focusing distances . For example , the depth of field of a 100mm lens shows the range of sharp focus is 11 to 22 feet at a circle - of - confusion of 1/200 inch when focused at 15 feet . But if a circle - of - confusion of 1/500 inch ( greater sharpness ) is desired , the range of sharp focus , or depth of field must be cut down to 4 feet ( from 13 to 17 feet ) .
Because depth of field is so important , most hand camera lenses have a set of calibrations which enable the user to determine the depth of field quite simply and easily without referring to tables . Such a depth of field scale is shown on the camera lens picture in Figure 8 of Assignment 1. The use of this scale is explained in your camera instruction book , but it will be gone into much more fully in a later assignment in this course of study .
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