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and circle of confusion — remain constant. The other two may vary, and therefore influence each other.
Suppose we have a 2-inch lens used at f:2.3. Assuming the circle of confusion as the usual .002 inch, we have "H" equals 2x2 divided by 2.3 x .002. This evolves to 4 divided by .0046, and gives us 896 inches or approximately 72 feet. If the lens is focused at 72 feet, everything from a point about 36 feet from the lens on to infinity will be reasonably sharp.
But if we take the same lens and stop it down to f : I I , we will have "H" equal to 2 x 2 divided by I I x .002. This works out to 4 divided by .022 and in turn gives us a hyperfocal distance of 181.8 inches or about 15 feet; everything from about 7I/2 feet to infinity will be reasonably well defined at this setting.
Depth of Field
Depth of field is often confused with depth of focus. The latter actually is the distance which the lens may be moved in or out before a given object in sharp focus goes out of focus. Depth of Field is the distance between the nearest plane in sharp focus and the farthest plane in sharp focus. More simply, it is the distance between the nearest and farthest objects in sharp focus at any given time.
These points are determined by use of the two following formulasTo determine nearest plane:
Near
U x H
H + D
To determine farthest plane:
In this, D represents the distance of the object, and H represents the hyperfocal distance under the conditions of stop, focal length and circle of confusion applying to that particular shot.
For example, suppose we use the two-inch lens previously mentioned, at its maximum aperture of f:2.3. The hyperfocal distance is the same as in the previous example — 72 feet. Assume our object is 40 feet from the camera. Then:
Far =
D x H Ft I
Near =
40 x 72 40 + 72
2880 I 12
25.9 feet.
Far =
40 x 72 72-40
2880
32
= 90 feet.
But suppose the object distance is 72 feet, which is the same as the hyperfocal distance. In that case we find that:
Near =
72 x 72 72 + 72
Far
72 x 72 72-72
5184
144
5184
0
= 36 feet.
= which is mathematically infinity!
On the other hand, when the object distance is greater than the hyperfocal distance, the resulting answer for the far plane becomes a negative or imaginary number, and as such has no practical value.
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