The principle of camera

20 10-08- 18 10:55:56

Aperture, also known as aperture stop, is an important mechanical device in lens. Its function is to control the ability of light to pass through the lens by changing the effective aperture of the optical lens, so that the photosensitive element or film can be accurately exposed, thus controlling the depth of field or adjusting the imaging quality of the lens. Aperture diaphragm is located inside the lens and usually consists of several movable metal blades (called diaphragm blades), which can be adjusted steplessly. The aperture mechanism can be driven by mechanical or electric or electromagnetic devices, and can also be adjusted manually.

Because different lenses have different diaphragm positions, different focal lengths and different entrance pupil diameters. It is impossible to compare different lenses by using aperture to describe their light transmission ability. The concept of "relative aperture" is adopted in order to calculate the exposure in actual photography and measure the actual effect of different lens apertures with a unified standard.

Relative aperture = [lens focal length]/[entrance pupil diameter] = f/d

Usually, [f/] or f is added before the relative aperture to indicate the relative aperture, such as f/ 1.4, f/2, f/2.8, etc. The smaller the value after f/ or f, the greater the light transmittance; The larger the number, the smaller the light transmission. Because of this standardization, for different lenses, under the condition of constant shutter speed, as long as the f value is the same, the exposure is the same.

Shutter is a mechanical device used to control the exposure of camera photosensitive element or film. Louvers are made of metal, fabric or other synthetic materials. The opening time of the shutter is controlled by a mechanical or electronic mechanism and driven by mechanical or electrical energy. According to the camera type, the shutter is divided into inter-mirror shutter and focal plane shutter. Inter-mirror shutter, also known as back mirror shutter and aperture integrated shutter, is mainly used for portable cameras and some 120 lenses. Focal plane shutter, also known as curtain shutter, is mainly used for SLR cameras. According to the driving mode, it can be divided into electronic shutter and mechanical shutter. The electronic shutter controls the shutter opening time by controlling the shutter coil magnet through the circuit, and the gears and linkage parts are mostly made of plastic. At present, most digital cameras and AF optical cameras are electronic shutters. The mechanical shutter is driven by a gear to control the opening time of the shutter. Connecting rods and gears are mostly made of copper and iron. In the past, manual cameras basically used mechanical shutters. The former is easy to be destroyed by wind and sand, and the latter is also afraid of wind and sand erosion, but it is clean and convenient.

The working principle of the shutter is as follows: in order to protect the photosensitive element or film in the camera from exposure, the shutter is always closed; After the shutter speed is set, as long as the shutter release button of the camera is pressed, the camera will let the light passing through the lens make the photosensitive element or film in the camera get the correct exposure within the time of shutter opening and closing.

The viewfinder of a SLR camera is generally a pentaprism viewfinder (some low-end digital SLR cameras use pentaprism), which is used to correct the left and right reflections on the focusing screen, so that the image seen by the viewfinder is exactly the same as the direction of the scene directly seen. This viewfinder uses the same lens for framing and photography, so there is no parallax, so framing is more accurate. The image passing through the pentaprism is sent to the viewfinder eyepiece. The schematic diagram of the light path through the pentaprism is shown below.

SLR cameras have two main features. First, they can change lenses of different specifications, but look through the photographic lens. Most of the traditional camera lenses with the same bayonet can also be used in digital SLR cameras. Digital SLR cameras are more expensive than ordinary household digital cameras, and SLR cameras are more suitable for professionals and photographers. The body and lens of most SLR cameras are sold separately.

SLR cameras are also called single-lens reflex cameras, which can also be abbreviated as "SLR" (single-lens reflex), and digital SLR cameras are called "DSLR".

In this system, the unique design of the mirror and pentaprism enables the photographer to directly observe the image passing through the lens from the viewfinder. It can be seen from the structural diagram of a single-lens reflex camera that the light passes through the lens and reaches the reflector, and then refracts to the focusing screen above to form an image. Through the eyepiece and pentaprism, we can see the scenery outside in the observation window.

When shooting, press the shutter button, the reflector will pop up, and the shutter curtain in front of the film will be opened at the same time. The light (image) passing through the lens will be projected on the film to make the film sensitive, and then the reflector will immediately return to its original state, and the image can be seen in the viewfinder. This structure of the single-lens reflex camera ensures that it is shot through the lens, so that the image seen in the observation window is always consistent with the image on the film, and its framing range is basically consistent with the actual shooting range, eliminating the parallax phenomenon of the paraxial head-up framing camera. From the perspective of learning photography, it is very conducive to intuitive framing and composition. Another major feature of a single-lens reflex camera is that it can replace lenses with different specifications.

What is a 135 camera?

A camera using 135 film is called a 135 camera or a 35mm camera.

A camera with an imaging area of 24×36mm is also called "Quan Huafu 135 camera", and a digital SLR camera with the same film size as 135 is called "Quan Huafu Digital Camera". A camera with an imaging area of 24× 18mm is called a "half-grid 135 camera", while a camera with an imaging range greater than 36mm is usually called a "wide-angle camera", such as Hasselblad XPAN.

A camera that uses 120 film is called a 120 camera. Cameras that specialize in 220 film are rare now. Most of them use 220 film by replacing the back of 220 film on 120 camera. The following photos were taken with a wide-format camera:

For 120 cameras, there is a similar saying of "wide width", such as 6× 12 and 6× 17, which all belong to wide width.

Comparison between 135 frame and 120 frame

135 film: because its width is 35mm, it is also called "35mm film", and it is packed by cassette.

120 film: the width is 6 1.5mm, without cassette packaging, but with backing paper;

220 film: the width is 6 1.5mm, and the length is twice that of 120 film. There is no backing paper, but there are protective papers at the beginning and end.

Frame detailed specification table

Actual area comparison table

The standard imaging area of 135 film is: 24×36mm, and the length unit is mm; 120/220 film has various nominal imaging areas: 6×4.5, 6×6, 6 ×7, 6×8 and 6×9cm, and the length unit is cm. The imaging area is square or rectangular. In addition to the side length, there is also an important indicator, which is the diagonal length.