عمق الميدان .. كتاب آلة التصوير

Chapter Seven

DEPTH OF FIELD

Depth of field is the distance in front of the lens in which sharp image objects appear, despite the difference in the distance of these objects from the lens.

If we suppose that the lens of the camera has been set (1) Focused on an object that is 200 cm away from it, and there are other objects in front of it and behind it whose dimensions differ from 175 to 250 cm, for example, then we express this by saying, “The image would have had a depth of field if Sharp images of these objects near and far from the lens appeared in it, and then the depth of field is estimated numerically by the length of the distance between the two nearest and farthest points located in front of the lens, and the lens has transmitted sharp images of the objects located between them. If the distance of the lens from the nearest point = 175 cm, and its distance from the farthest point = 250 cm, then this means that:
Depth of field = 250 - 175 cm = 75 cm

Here we wonder about the basis on which our description of the image was built as “sharp” or our description of it as not sharp. To answer that, we say that this characteristic depends first and foremost on what we call circles of mixing.

Mixing circles:

According to the primary theoretical rules of lenses, the object in front of the lens has only one sharp image located at a specific distance from the lens, or in other words, the light points emanating from the object are not what appears as a point in the image except what is located at this specific distance. As for the light points emanating from other bodies that lie in front of or behind this body, their images are recorded on the surface of the film, sensitive plate, or frosted glass in the form of mixed circles of confusion (Fig. 15). Our description of the image as sharp or not sharp depends on the diameter of these mixed optical circles. If their diameter is small, we describe the image as sharp, and if the circles are large, then the image is not sharp. This requires us to answer another question: When is the mixing circle considered large and when is it considered small? Or in other words, what is the standard for image sharpness?