عمق المجال .. والظروف المؤثرة فيه ، مجلة فن التصوير اللبنانية _ع١٢

Depth of field .. and the conditions affecting it

How do you get a clear depth of field, what are the factors affecting that, and what is the role of each of the focal length or aperture - with regard to the lens - in affecting the depth of field. its dimensions?? Important things to be aware of as one of the most important photography techniques.

The depth of field is the entire distance facing the scene to be photographed, which is in front of the lens directly and at different distances. In it, objects appear sharp or blunt, depending on their reflection, either in the form of points of light or as circles of confusion - or mixing -.

Points of light and circles of confusion?

Each lens has a focal point that serves as a center for gathering and passing rays towards the negative, and this focal point is the point
located on the axis of the circle of the lens, and in it the opposite and reflected rays from the body or bodies located opposite it gather at different dimensions up to the point of infinity... These rays falling on the lens from the opposite bodies are reflected in the form of light points, which produces a sharp image of the body, while the rays emanating from the other are reflected Which is located in front of this object - to be photographed - or behind it or next to it in the form of fuzzy circles of light. which produces images
Distorted and not sharp for these objects, and so we see that the greater the diameter of the optical circle reflected from the opposite body, the more blurred its image, and the smaller the diameter, the sharper the image. This justifies the existence of a clear body and a confused one
One image, according to a system that may have been called the law - circles of confusion. .

Thus, we conclude that each point of light emanating from an object must be represented by a small light circle on the sensitive film in order to obtain a sharp image
It is clear that the closer the lens is to the object to be photographed, the less the depth of field of the lens within the image, but what also contributes to this matter is the effect of aperture. The wider the lens aperture, the lower the depth of field, and vice versa. And we have to control this matter well because we need both cases, as some topics require clearly highlighting the depth of field. But in other cases, it is assumed to focus on the main subject, and here the lack of depth of field helps.

Thus, we can adopt the following equation: an aperture of 3.5 up to f / 2 → a noisy depth of field. P / 11 - 16 - 22 ← Depth of field is wide and clear.

lens effect

Each lens has an angle of view, the degree of which varies according to its focal length. It is known that the angle of view is that confined between the two extreme rays that pass through the axis of the lens, so they form a sharp image with equal light ratio. As for the focal lengths of the circular lenses, they are a long focal length of 135 mm, an angle of 15 degrees.

The average focal length is 50 mm, the angle is 49 degrees.

A short focal length of 28 degrees, an angle of 67 degrees.

Here lies the importance of widening or narrowing the depth of field for the image, according to the different angles of the lenses, without ignoring the role of the lens aperture, which has a basic relationship to the sharpness or blurring of the image.

Diagram

* Point (8) Marmar here to the edge close to the lens, and as we can see, it blurred a small light stroke (8) on the negative of the film.


* Point (F) represents here an object or objects far from the lens and behind the object to be photographed, and its light rays have been reflected, forming two rays that allow them to pass through the lens at any angle (FCC).
* Due to the distance of point (1) from shift (8), the rays reflected from (F) (collected to a point of light (D) before continuing their journey to the film to arrive in the form of a distorted optical circle. Here, the widening or narrowing of this circle depends on the angle of (DMN) ) resulting from the extent of the angle widening or narrowing (BCD) Here we will find that the body that we denoted by (8) will be clear in the image, while its background (F) will be blurred.
Here, the role of the aperture is to control the depth of field. With the top drawing, we see that the opening is wide
The lens - f / 3.5 for example - is the factor affecting the diameter of the optical circle. If we want to achieve the opposite result, such as increasing the depth of field, that is, making both (8) and (F) clear and sharp in the image, we only have to narrow the aperture from f / 3.5 to f / 11, for example, and this will lead to Narrowing the angle (BCD), which in turn will lead to narrowing the angle (DMN) to a point where its reflection is closer to a point of light than to a circle, and this means that the diameter of the circle is reduced to the maximum extent possible, and thus we get more depth of field as shown in the drawing below.

Prepared by Tariq Ibrahim