ماهي عدسة الكاميرا .. كتاب آلات التصوير

Chapter Six 6 - Lenses

What is the camera lens?

A lens in a camera is a transparent unit usually made up of several different glass elements that focus the image onto the film. The simplest cameras have molded glass or meniscus acrylic lenses. As for the more complex cameras, they contain compound lenses that may total up to seven or eight elements to form a very clear and bright image. In some cameras, it is possible to exchange the standard lens with other lenses in order to provide a wider or narrower field of view, closer focus, or to increase the possibility of light passing. One such replacement camera may contain up to 20 separate components.

What is the focal length?

For a simple lens, the focal length is the distance behind the lens at which a clear image of a distant object (and in technical terms an infinitely distant object) is formed. The focal length also determines the magnification of the image. The longer the focal length, the larger the image. Compound lenses can be focused at a distance greater or less than their focal length, except that the size of the
The image remains the same as that obtained from a simple lens of the same length
focal
What is the standard or standard lens?
The lens is the standard lens that gives an almost natural perspective in the images, and according to custom, the focal length has become approximately equivalent to the diameter and side of the film. This assumes about 23mm for a 110mm camera, 35mm for a 126mm camera, 30mm for a 35mm halftone frame, 43mm for a 35mm camera, 75mm for a 6x6cm camera, and 105mm for a 6x9cm camera. It was common in the past for 35mm cameras to have a slightly longer focal length for standard lenses (50-55mm), but most fixed lens cameras nowadays follow the diameter of the film polygon.

Caliber lenses are designed to be used in all areas of general photography. In practice, these lenses are good for shooting cityscapes, some landscapes, for photographing groups of people, for taking full-length portraits, and for most other casual shots. Many photographers find themselves preferring either focal length taller or wider angles of view for their shots

regular

How is the focus of the lens focused?

Simple camera lenses are set to give sufficient sharpness over normal photographic shots with a range of two to seven meters (6 feet to 23 feet) away. More complex cameras have the ability to move the lens away from the film plane as necessary to focus on
nearby objects. In most cases this process is done by moving the entire lens

By rotating the ring on the focusing helical support. On some cameras the lens is positioned with the bellows behind it so that it forms a tight, opaque connection to the camera. A few modern lenses have internal focusing mechanisms. However, focus is achieved and the focus distance is determined on a scale measured in meters or feet.

Many cameras have a visible focus mark. Viewing cameras show the real image on a ground glass screen, while reflex cameras show an inverted image on their focus screens, and rangefinder models contain a device for fragmented or synchronized image, and this device is usually in the viewfinder. In practice, however, these focus indicators are much more reliable than the focimeter on the lens

What is the amount of light that is focused on the film?

The larger the lens aperture, the greater the amount of light passing through it, and the longer the focal length of the lens, the farther the distance traveled by the light and the more ancient the image. These two factors cancel each other out if the amount of the lens aperture is related to the focal length. Thus, if the lens has the same diameter as the focal length, the same amount of light will reach the film (with the same conditions) regardless of the focal length. Thus, the apertures are usually calibrated on a scale listed with their relative values, and the natural aperture divided by the focal length is called the focal number.

For example, a 50 mm lens with an effective aperture of diameter has a relative aperture of f-number 22. It passes the same amount of light.
A 100 mm lens with a diameter of 50 mm or a lens measuring 24 mm with a diameter of 12 mm .. Thus, the lower the focal number of the relative aperture, the greater the amount of light that reaches the film. The smaller its f-number, the faster the lens

Simple cameras feature lenses with a focal number ranging from 9 to 16. As for the models with the most varied photographic shots, they are characterized by apertures whose f-number ranges from 4 to 8. Ordinary good cameras such as 35 mm types have apertures with an f-number of about 2.8, and most complex 35-mm models have an f-number of about 1.4. Generally, the larger the film format, the longer the focal length, and the longer the lens. Greater is needed to achieve any given number of f-numbers if large-film models feature slow-motion lenses

How is the level of light controlled?

In all types of cameras except for the simple ones, we find that the lens allows a large amount of light to pass through it on bright days. To reduce the amount of light, the aperture of the lens is reduced, and this is done by means of the iris lens. This visor consists of a series of thin plates that can be adjusted to form a nearly circular aperture of variable size. The aperture adjustment mechanism is usually calibrated with 2 numbers, and the graduated scale usually has the following numbers. £/2, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32

We find that each of the successive calibration modes allows the passage of half the amount of light allowed by the previous mode. This meter with the shutter includes a speed chain to make exposure control simple.