عمق الميدان في أحوال التصوير عن قرب .. كتاب آلة التصوير

Depth of field in close-up conditions:

Both the front and back depth of field are equal in close-up conditions. There is only a very slight difference between the main factors that affect the depth of field in the case of the usual standard images, and those affecting the depth of field in the conditions of close-up photography (1).

We can make these factors as follows:

(a) magnification (c), which depends on two factors:

1- The focal length of the lens (p).

2- The distance of the object from the lens (S), i.e. the point on which the distance is set. The magnification can be calculated according to equation (2) v = h over x - z

(b) The new focal number.f.No:

Whether the focal number change is a result of the bellows extension, the use of extension tubes, or the placement of additional positive lenses in front of the original lens.

Note: For how to calculate the new f-number, refer to pages 172-178).

(c) Diameter of the permissible mixing circle:

In order to achieve accuracy and a desire to increase image sharpness in close-up conditions, it was agreed that this diameter be within approximately 1/2000 of the average focal length (i.e. 1/1000 of an inch in 35 millimeter shooting conditions, 2/1000 of an inch in around Filming with a machine measuring 6 x 9 cm, 3/1000 of an inch if the machine measures 9 x 12 cm, for example... etc.).

Based on the previous factors, an equation was developed to calculate the depth of the foreground or creation, as both are equal in close-up conditions. The following is the text of the equation:

Depth of field = new focal number X diameter of mixing circle X (magnification +1) at (zoom) 2

i.e. = q x x (v + 1) over v2

Based on this equation, the table was placed on the corresponding page to calculate the depth of field in close-up conditions based on our knowledge of both the transmission scale (i.e. the zoom ratio) and the new focal number.

Notes on the corresponding table The following is noted when using this table:

(a) Consider the diameter of the permissible mixing circle equal to 1/1000 of an inch. If he considers the diameter of the mixing circle greater or less than that, then the result must be multiplied by a new factor, which is the amount of increase or decrease from 1/1000.

(B) The reading that we take from the table indicates the depth of field in only one side, the front or the back. Therefore, this value must be multiplied by 2 if we want. Calculate the total depth of field.

Focus depth:

Some people sometimes confuse the depth of field with the depth of focus. To avoid this confusion, we say that the term “depth of field”

That distance in front of the lens is called sharp images because of the objects in it. As for the depth of focus, it is the distance behind the lens in which if the film moves, whether to the front or the back, the image will remain acceptable for novelty. Therefore, knowing the depth of focus is important if we want to know the extent of the error allowed in adjusting the distance and the film so that this error does not have an effect on the sharpness of the image (and the depth of the focus may be less than a part of a millimeter or increase to several millimeters). The focus in the picture and in (Fig. 105 page 183) we see that the distance B`D` represents the depth of field area, while the distance BD represents the depth of focus).

The focal depth can be roughly estimated according to the following equation:

The depth of focus = 2 x the focal length of the lens x the focal number over 1000
If the focal length of the lens is 50 mm.
Its focal number = 2 f

Then. The focal depth = 2 x 50 x 2 by 1000 = approximately 0.2 millimeters.

And if we assume that the focal number = 8 f

Then the focal depth = 2 x 50 x 8 over 1000 =
0.08 mm approximately.

We conclude from the previous two examples that the depth of focus increases as the aperture of the diaphragm narrows (i.e. when the numerical value of the focal number is large) and vice versa, the depth of focus decreases as the aperture widens, which then calls for the necessity of adjusting the focusing distance with extreme precision.