Basic Photography Course

ND = near distance

H = hyperfocal distance

D = distance focused upon

F D = far distance

EXAMPLE: What is the depth of field of a 155mm (6.1
inch) lens that is focused on an object 10 feet from the
camera lens using f/2.8? (Note: In a previous example
the hyperfocal distance for the lens was found to be 554

feet.) By the formula, the nearest sharp point is
determined as follows:

ND = 9.8 feet

Thus the nearest point in sharp focus is 9.8 feet from the
lens that is focused on an object at 10 feet, using f/2.8.

Also by the formula, the farthest point in sharp focus

can be determined as follows:

FD = 10.2 feet

Therefore, the far point in sharp focus is 10.2 feet when
focused on an object at 10 feet, using f/2.8.
Consequently, the depth of field in this problem equals
the near distance subtracted from the far distance
(10.2 - 9.8 = 0.4-foot depth of field). Thus all objects
between 9.8 and 10.2 feet are in acceptably sharp focus.
When this depth of field is not great enough to cover the
subject, select a smaller f/stop, find the new hyperfocal
distance, and apply the formula again.

When the only way you have to focus is by

measurement, the problem then becomes one of what
focus distance to set the lens at so depth of field is placed
most effectively. There is a formula to use to solve this
problem.

P = D x d x 2

D + d

Where:

D = distance to farthest point desired in sharp

focus

d = distance to nearest point desired in sharp

focus

p= distance to point at which the lens should be

focused

Substituting the figures from the previous examples,

D= 10.2 feet

d = 9.8 feet

P= lens focus distance

Then:

P = 10 feet

To obtain the desired depth of field at f/2.8, we set the
lens focus distance at 10 feet.

If the preceding explanations and formulas have

confused you, here is some good news! Most cameras
and lenses have depth of field indicators that show the
approximate depth of field at various distances and lens
apertures. Figure 1-30 shows that with the lens set at f/8
and focused at about 12 feet, subjects from about 9 feet
to about 20 feet are in acceptably sharp focus. By
bringing the distance focused upon to a position
opposite the index mark, you can read the depth of field
for various lens openings.

Keep in mind that a depth of field scale, either on

the camera or on the lens, is for a given lens or lens focal
length only. There is no universal depth-of-field scale
that works for all lenses.

In conclusion, the two formulas used to compute

depth of field serve for all distances less than infinity.
When the lens is focused on infinity, the hyperfocal
distance is the nearest point in sharp focus, and there is
no limit for the far point.

CONJUGATE FOCI

Object points and their corresponding image points

formed by a lens are termed conjugate focal points. The

distances from the optical center of the lens to these
points, when the image is in focus, are termed conjugate

focal distances or conjugate foci (fig. 1-31).

1-26

contents

Basic Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> ND = near distance H = hyperfocal distance D = distance focused upon F D = far distance EXAMPLE: What is the depth of field of a 155mm (6.1 inch) lens that is focused on an object 10 feet from the camera lens using f/2.8? (Note: In a previous example the hyperfocal distance for the lens was found to be 554 feet.) By the formula, the nearest sharp point is determined as follows: ND = 9.8 feet Thus the nearest point in sharp focus is 9.8 feet from the lens that is focused on an object at 10 feet, using f/2.8. Also by the formula, the farthest point in sharp focus can be determined as follows: FD = 10.2 feet Therefore, the far point in sharp focus is 10.2 feet when focused on an object at 10 feet, using f/2.8. Consequently, the depth of field in this problem equals the near distance subtracted from the far distance (10.2 - 9.8 = 0.4-foot depth of field). Thus all objects between 9.8 and 10.2 feet are in acceptably sharp focus. When this depth of field is not great enough to cover the subject, select a smaller f/stop, find the new hyperfocal distance, and apply the formula again. When the only way you have to focus is by measurement, the problem then becomes one of what focus distance to set the lens at so depth of field is placed most effectively. There is a formula to use to solve this problem. P = D x d x 2 D + d Where: D = distance to farthest point desired in sharp focus d = distance to nearest point desired in sharp focus p= distance to point at which the lens should be focused Substituting the figures from the previous examples, D= 10.2 feet d = 9.8 feet P= lens focus distance Then: P = 10 feet To obtain the desired depth of field at f/2.8, we set the lens focus distance at 10 feet. If the preceding explanations and formulas have confused you, here is some good news! Most cameras and lenses have depth of field indicators that show the approximate depth of field at various distances and lens apertures. Figure 1-30 shows that with the lens set at f/8 and focused at about 12 feet, subjects from about 9 feet to about 20 feet are in acceptably sharp focus. By bringing the distance focused upon to a position opposite the index mark, you can read the depth of field for various lens openings. Keep in mind that a depth of field scale, either on the camera or on the lens, is for a given lens or lens focal length only. There is no universal depth-of-field scale that works for all lenses. In conclusion, the two formulas used to compute depth of field serve for all distances less than infinity. When the lens is focused on infinity, the hyperfocal distance is the nearest point in sharp focus, and there is no limit for the far point. CONJUGATE FOCI Object points and their corresponding image points formed by a lens are termed conjugate focal points. The distances from the optical center of the lens to these points, when the image is in focus, are termed conjugate focal distances or conjugate foci (fig. 1-31). 1-26 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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