العدسة والحدقة - ٤ - طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

Lens and pupil - 4 - How the camera works .. Book of photography

- Wide angle lens: It has a short focal distance and is usually equal to or less than 35 mm for a size of 24 x 36 mm and gives a wide field of more than 60°. This lens is used especially when it is not possible to go back or when you need a large depth of field as is the case in journalistic reports.

Finally, let us end by explaining that amateurs and professionals are increasingly using lenses with a variable focal length called (zoom) on their 24 x 36 mm machines. Thus, on the same lens, they can choose all the focal lengths from the smallest to the largest, which allow for a very precise frame: for example, we find zoom 35 - 100, 90 - 300, 70 - 220 ... etc. There is also a (macrozoom) with which we can photograph at a distance of 80 mm from the subject.

Clarity

The clarity of the image details depends on the smoothness of each of the points that make up this image. These points are the smallest on the plane of the expressive image. The image planes placed in front or behind this surface are formed from points that grow larger the further they are from the plane on which the instrument is calibrated: from a diameter the points appear as circles. The smallest circle can be likened to a point called (mixing circle).

Depth of the focal point

The lens gives from a point in the subject a point image on a focal plane. In the front and back of the focal plane the image of the point is no longer a point but a circle whose diameter grows larger the further the circle is from the focal plane (the upper image). The circles close to the focal plane are so small that they cannot be distinguished from points, so they are called mixing circles. The depth of the focal point is the distance that separates (from each side of the focal plane) the mixing circles that can be likened to points (the lower image).

Exploiting the adjustment scale

Except for very close subjects, one third of the depth of field extends in the foreground and two thirds behind the level on which the adjustment is made.

A - When the adjustment is made at infinity, it is natural to think that two thirds and beyond infinity are lost.

B - When adjusting the scale so that the rear limits of the clarity are exactly at infinity, we must increase the depth of field forward, and this allows us to obtain a clear, pure image of some parts that precede the scene.

C - Objects close to the instrument only allow us to obtain a limited depth of field.

D - If we adjust the scale so that the depth of field is limited behind the main subject (here the house), the area of ​​clarity extends forward to the standing person.