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Does the focal length of the lens affect the depth of field?

Despite what many believe that the focal length of the lens has a direct effect on the depth of field, in reality it is only an indirect factor driven by other factors responsible first and foremost for increasing or decreasing the depth. We discuss this in the following so that it appears to us completely the secondary role that he plays.

Perhaps there is an excuse for those who think that the focal length of the lens has a direct, original effect on the depth of field, because if we used two lenses in one camera, the first with a short focal length, and the second with a long focal length, and the distance between the lens and the object on which the distance was set was 200 cm in In both cases, and the focal number was constant (for example, 4 f) in both cases, we would find, for example, when using a lens with a long focal length, that the depth of field in this case starts from 180 cm from the lens and ends at 220 cm from it (meaning that the depth of field occupies 40 cm ).

When using a lens with a short focal length, the depth starts from a point 150 cm away from the lens and ends at a distance of 250 cm from it, for example. (ie, the depth of field will then occupy 100 cm).

Although it may appear from this observation at first glance that the depth of field depends mainly on the focal length of the lens, so it decreases if the lens has a long focal length, and increases if its focal length is short, but it has been mathematically proven that the focal length of the lens has no direct effect on the depth of field. The correct saying in this regard is that both the focal number of the lens and its focal length are the two factors that together control the depth of field. Since the diameter of the diameter of the diaphragm aperture (i.e. the diameter of the aperture of the focal length of the actual lens through which the light passes is a quotient), the focal number can be said that the diameter of the diaphragm aperture is the direct factor that affects the depth of field. And if the focal length increases in length with the focal number being constant, then the diameter of the diameter of the diaphragm aperture must expand, so the depth of field decreases. It is thought that the length of the focal length is the factor that results in a decrease in the depth of field, and this is a mistaken belief.. The correct view is that the widening of the diameter of the diaphragm aperture is what affects The direct result that led to a decrease in depth, as evidenced by the fact that if we used two lenses in a row in one camera, and the focal length of the first was 12 cm, and the second had a focal length of 8 cm only, and we fixed the focal number in both cases, such as 4a, we would certainly find an increase Depth of field in the image with the shorter focal length lens. Here it comes to mind that this result did not result except from the shortness of the focal length, but if we calculate the diameter of the aperture diameter, then we will find the following:

The diameter of the diaphragm aperture in the first lens = 12 cm / 4 f = 3 cm.

The diameter of the diaphragm aperture in the second lens = 8 cm / 4 f = 2 cm.

0 Finally, it can be said that the narrow diameter of the diaphragm aperture in the second lens is the first reason that increased the depth of field, and its focal length had no effect at all.

In support of this result, we present, for example, the case of photographing a specific subject with two lenses, as follows:

(A) The first one has a focal length of 16 cm and its focal number is 8 f, meaning that the diameter of the diameter of the diameter of the diaphragm in it = 2 cm.

(b) The second one has a focal length of 4 cm and its focal number is 2 f, which means that the diameter of the diameter of the diameter of the diaphragm hole in it = 2 cm | also .

Then the depth of field must be equal in both cases, as the diameter of the diaphragm aperture is the same in both cases, despite the great difference in the focal length of the two lenses.

The effect of focal length on the percentage of enlarging the image of the body on negativity and its effect on field aunts.

We learned previously that the distance between the object and the lens affects the degree of magnification of the image, i.e. it affects the relative area occupied by the image of the object on the film or frosted glass (so the relative area occupied by the image increases over the negative (i.e. the magnification ratio increases) as the object approaches the lens And this in itself is a matter that entails an increase in the diameter of the mixing circle, so the depth of field decreases as the object’s distance from the lens decreases, and conversely, the depth increases whenever the distance from the lens is set to the object.

Here we see that the focal length of the lens. He plays an apparent role. As the focal length would also affect the zoom ratio, that is, it affects the relative space occupied by the image of the object on the film, so the image zoom ratio increases as the focal length lengthens, and according to the increase in the zoom ratio, the diameter of the mixing circles increases, so the depth of field decreases, so it comes to mind that the depth of field decreases. We only come from using a lens with a long focal length. On the other hand, if the imaging was done with a lens with a shorter focal length, it would result in a decrease in the relative area occupied by the image of the object on the negative (i.e. the degree of magnification decreases, so the diameter of the mixing circles decreases, thus the depth of field increases, and then we attribute this increase in the depth of field to that we have We used a lens with a short focal length, or this is a mistake.In support of this statement, we give an example in the case of if we photographed a building whose height is 0.000 meters (by a camera with frosted glass) two consecutive pictures, the first with a short focal length lens, and the second with a long focal length lens.. If the location of the camera is fixed in both cases, then you will see two different images on the frosted glass in both cases:

(a) If we assume that the image of the building will be played on frosted glass, 1 cm in the case of using the short focal length lens, that is, the transmission scale = 5 / 500 = 1: 50

(B) And that his image will be played on 10 cm frosted glass in the case of using the long focal length lens, that is, the transmission scale then = 10 / 500 = 1: 50

By comparing the depth of field in the first case and the depth in the second, an increase in depth is evident in the case of using a lens with a shorter focal length, which results in an increase in depth. It invites many to believe that this depth resulted only from the use of that lens with a shorter focal length... This is wrong, and it is true that this difference in depth of field was only caused by a difference in the transmission scale, with evidence that if we move the camera to the back (nor The lens with a long focal length is removed on it until the length of the image of the building on the frosted glass reaches 5 cm as well (that is, until it becomes the transmission scale. = 1: 100 also). In both cases . Then we conclude from the foregoing that the depth of field increases as the magnification decreases. Since the magnification ratio (or transmission scale) depends on two factors: the focal length of the lens and the distance between the body and the lens, it can be said that the depth of field is directly related to the magnification ratio and indirectly related to both of the previous factors.