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Characteristics of imaging lenses.. Depth of field

Depth of field

Let's recall the experiment of projecting an image of an object with a regular magnifying glass or a reading lens. We have noticed how distant and close objects project their clear image onto different distances from the lens, and when the lens focuses on an object or subject at a certain distance, the image of the objects before it and the things after it becomes cloudy.
Fortunately, there is room for tolerance in this rule. In the sense that if we focus the lens on an object that is 15 feet away from it, for example, the objects located between 11 feet and 22 feet will appear clear enough (a foot equals approximately 30 centimeters). This distance (11 feet) (between 11 and 22 feet) is the depth of field for this lens when it focuses at a distance of 15 feet.
Depth of field can be very important for some types of shots, such as group photos and street shots. The depth of field can be increased by adjusting the lens.

The depth of field of a lens depends on four factors:

1- Aperture:
The smaller the aperture (that is, the larger the f-number), the greater the depth of field. When we focus, for example, a 100 mm lens on a subject 15 feet away and with an f / 8 aperture, its depth of field is 1/2 7 feet (from 12 1/2 to 20 feet). If we reduce its aperture to f/32, its depth of field increases to 91 1/2 feet (from 8 1/2 to 100 feet).

Photo No. 13, taken with a wide aperture lens (2.8 / f). Notice how only the leaves in the middle are clear, while the distal and close ones are feathery.
As for photo 14, it was taken with the same lens, at the same distance, but with a smaller aperture (f / 22). Note the clarity of all leaves and the expansion of the field where the image is clear.
The narrowness of the field may be used sometimes to show, and to focus on a specific topic, that is, we do not necessarily seek to deepen the field always. Picture 15 shows how the subject in the middle of the picture was given a value by making it clear, while the elements in front and behind it were left covered (outside the clear field), so they did not compete with the value of the clear central element.

2- The focal length of the lens:
The depth of field of a lens decreases as its focal length increases. This is why wide-angle lenses have a deep field.
These lenses are also known as short focal lenses.

3- The distance of the object or subject from the lens:
The depth of field of a lens increases with the distance of the subject to be photographed. For example, a 100mm lens, which has a depth of field of 1/2 7 feet (from 1/2 12 to 20 feet) when focused on a subject 15 feet away, has a depth of field of 1 foot when focused on a subject eight feet away.

4- The degree of clarity required:
The eye cannot distinguish between lines the distance between which is less than 1/100th of an inch at a distance of 14 inches (about 36 centimeters).
This means that an image whose degree of blur is less than 1/100 of an inch (an inch equals approximately 2.5 centimeters), appears perfectly clear to the normal eye.
(The degree of blurring is called: the noise circuit). Pictures whose clarity is less than this ratio, i.e. up to 1/50 of an inch, are still sufficiently clear.
We point out here that this acceptable degree of clarity is for the final image, that is, after enlarging and printing, so the negative crystal must be more accurate if we want to enlarge it when printing.
Depth of field tables are available for almost all lenses. These tables give the degree of blurring, or the distortion circle, in fractions of an inch or mm, for multiple focus distances of the lens.
For example, the depth of field for a 200 mm lens is from 11 to 22 feet when focusing on a subject 15 feet away, and the degree of blur at that time is 1/200 of an inch.
If we want a greater degree of clarity, or a smaller degree of blur (1/500 of an inch), the depth of field must be reduced to 4 feet (from 13 to 17 feet).