العوامل المؤثرة في عمق الميدان .. كتاب آلة التصوير

Factors affecting depth of field

First: the effect of the focal number on the depth of field:

If we set the photographic lens on an object that is 200 cm away from the machine, and use two focal numbers sequentially, they are f-22 and then f-3.5, and balance between the two results, we will find the following:

(a) In the case of shooting with a lens whose focal length is 22 f, the area of ​​sharp details extends from the point that is 140 cm away from the lens and ends at a distance of 380 cm from it, for example. Hence, it becomes clear that the narrow opening leads to a large depth of field, as it extends in an area of ​​length = 380 - 140 = 240 cm. For example.

(b) In the case of shooting with a lens whose focal length is f3.5, the area of ​​sharp details extends from the point that is 190 cm away from the lens to 210 cm, for example, meaning that the depth of field is 210 - 190 = 20 cm only.
It is understood from this that the wide aperture of the diaphragm (meaning the numerically small focal number) does not lead to the same degree of depth of field that we noticed in the first case. We see in (Fig. 107) an explanation of the reasons for this phenomenon.

Does the narrowness of the diaphragm aperture or its widening affect the sharpness of the image of the surface on which the distance is set?

As a result of the observations and results that we have seen, which prove the increase in the depth of field when the diaphragm aperture is narrowed, it is believed that the narrowing of the diaphragm aperture has an effect on the sharpness of the image of the point or plane on which the distance is set. Here we draw attention to the fact that although the narrowing of the diaphragm aperture results in an increase in the depth of field, i.e. an increase in the sharpness of the details of the objects closest or furthest away from this plane (or this point), the Sharpness of the image of this plane will not increase as a result of narrowing this aperture. Evidence is that if we photographed a flat written paper and placed it perpendicular to the axis of the lens and set the distance between the film and the lens with complete accuracy in proportion to the distance of the paper from it, then we photographed the paper twice, the first with a wide aperture (2 aperture, for example), and the second with a narrower aperture (8 aperture, for example), then it is not It is expected that the sharpness of the image will increase in the case of shooting with a result of (f8) than it is in the case of shooting with an aperture of (2f), but rather the opposite is possible (1). But it is certain that we will see a difference between the two images if there is another object in front of or behind this flat piece of paper and its distance from the lens differs from the distance of this piece of paper, as the depth of field decreases if the imaging is done with an aperture. The wide lens (22) than this depth would be if the shooting was at the narrow aperture (f8).

Second: The effect of the distance between the lens and the middle point on which the distance is set on the depth of field:

Suppose that we made two images in a row using one lens and a fixed aperture, with the distance between the lens and the point on which the distance was set to be 150 cm in the first image, and 300 cm in the second image. If we compare the two images in terms of depth of field, we find the following.

(a) In the first case: the depth of the field is less and may extend only to a distance not exceeding, for example, 15 cm.

(b) In the second case: the depth of the field increases and extends a greater distance, which may be 60 cm, for example.
It is clear from the previous two examples that the farther the lens of the camera is from the point at which the distance is set, the greater the depth of field, while it decreases if the distance of the object from the lens decreases (assuming that the rest of the factors remain constant in both cases). Experiments will also prove to us that the depth of field always increases behind the point on which the distance is set. The depth is in front of this point, and the percentage of this increase is quite clear if the middle point on which the distance was set was far from the lens, while this percentage gradually decreases as the middle point gets closer to it. The front depth is almost equal to the rear depth, and we see in Figure (109) an explanation of the reason for which the depth of field is increased when the objects are far from the lens, and it decreases when these objects are close to it.