العدسة والحدقة - ١ - طريقة عمل آلة التصوير .. كتاب التصوير الضوئي

Lens and pupil - 1 - How the camera works .. Book of photography

Lens and pupil

Aperture

When the pupil value increases, the latter closes and accepts twice as much light. Therefore, the duration of taking the picture must double so that the picture remains stable and does not change.

The three main calibrations of the machine are the shutter speed, the pupil aperture value, which indicates the opening, and the distance scale for calibration. The speed calibration is usually on the box and sometimes on the lens. In general, the pupil and calibration are controlled by rings that are attached to the lens, and each of these rings can be placed between precise limits that can be identified by an index on the values ​​engraved in the ring.

The image is the result of the amount of light hitting the film
(this amount is monitored by the pupil) and the exposure time (this time is monitored by the shutter speed). In order for the captured image to remain stable, the change in the aperture must be compensated for by adjusting the shutter speed and vice versa. Shutter speeds are indicated in fractions of a second in the following order: 1 - 2/1 - 4/1 - 8/1 - 15/1 30/1 - 60/1 - 125/1 - -250/1 - 500/1 - 1000/1, i.e. each speed is twice the speed before it. Apertures are calculated using the following values: 1/F F/22 F/16 F/11 F/8 F/5.6 F/4 F/2.8 F/2 F/1.4 32/F/4/45 F This series forms a geometric increase such that each increase in value reduces the area of ​​the aperture by half, which reduces the amount of light passing through it by half. If the maximum aperture is not included in this series - for example, 1.8/F or 3.5/F - then the series increases naturally for the rest of the apertures.

The speed and aperture scales are closely related to each other. When changing the aperture value, the amount of light reaching the film is doubled or halved. The duration of light on the film is adjusted in the same way, i.e. by adjusting the shutter speed.

Let's take an example: the lighting conditions allow us to take a correct image at 1/125 sec and 16/F, but a fast-moving subject requires a speed of 1/250 sec: so to maintain a correct image at this speed, the aperture must be doubled, i.e. decreased by one value, i.e. 11/F.

The distance scale allows the lens to be calibrated to obtain a clear image of 50 cm. This scale most of the time starts from and gradually increases until it reaches infinity, which is symbolized by a sign at each end of the distance scale mark. There are often series of values ​​engraved corresponding to the pupil apertures, and this scale indicates the depth of field obtained for a given aperture and distance.

Depth of field scale

A depth of field scale is located at each end of the distance scale. It indicates the distance between the first and last visible surfaces depending on the selected aperture and the focal length.

Expression:

A rotating frame with a distance ring moves the lens closer or further away from the film to focus subjects that are different distances away: this distance increases as the subject approaches.

Pupil aperture value

All lenses calibrated to the same aperture admit an equal amount of light to the same subject. Distance The diameter of the pupil aperture is directly related to the focal length, for example an 80mm lens with a pupil aperture of 5 cm is set at 16/F. The focal length is the focal length divided by the diameter of the pupil aperture.