Back focus
Imagine a terrible picture: you bought a reflex camera, and the pictures from it come out fuzzy. However, if you look closely at the pictures, you will find that the…

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RAW or not RAW
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Depth of field
Depth of sharply depicted space (depth of field) is the distance between the closest and farthest object in the scene, which are perceived in the photo as sharp. Everything that…

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Exposure: Theory

A good exposure is critical for quality photography. However, the essence of the exposure is extremely simple. Exposure is just the amount of light entering the photosensor. The process of shooting a frame is sometimes called exposure.
Exposure can be reduced, but can be increased. That, in fact, is all that you can influence. A smaller exposure makes the frame darker, a larger exposure makes it brighter. The lack of exposure is called underexposure, excess – overexposure.

Exposure is measured in exposure numbers or exposure levels (EV – exposure value). Changing the exposure by one step means changing the illumination of the frame by half.

Exposure can be controlled by varying two parameters – shutter speed and aperture. Aperture, i.e. the magnitude of the relative aperture of the lens determines the intensity of the light flux, while shutter speed controls the duration of exposure.

Aperture is a device that allows you to change the size of the hole through which light enters the camera. The larger the hole, the more light, and vice versa. The aperture value (aperture value) is defined as the ratio of the focal length of the lens to the diameter of the aperture. For example, recording f / 4 means that the diameter of the aperture is four times smaller than the focal length of the lens.

Aperture numbers form the following row:

f / 1; f / 1.4; f / 2; f / 2.8; f / 4, f / 5.6; f / 8; f / 11; f / 16; f / 22; f / 32; f / 45; f / 64.

The larger the aperture value, the smaller the relative aperture. Each step means a change in illumination by half, i.e. f / 11 aperture transmits two times less light than f / 8, and f / 2.8 – twice as much as f / 4.

With an increase in the number of aperture by one step, the area of ​​the effective aperture is halved, which means that it also passes half the light.
In addition to exposure, aperture also affects depth of field and overall image quality.

Shutter speed is the time during which the shutter of the camera is in the open state, passing light to the matrix. The longer the shutter speed, the longer the shutter is open, the more light enters the camera. As with the aperture, the standard shutter speeds differ by half. Here they are:

30 s .; 15 s .; 8 s .; 4 s .; 2 s .; 1 s .; 1/2; 1/4; 1/8; 1/15; 1/30; 1/60; 1/125; 1/250; 1/500; 1/1000; 1/2000; 1/4000; 1/8000.

A short shutter speed is able to stop the movement in the picture, while a slow shutter speed emphasizes the motion, blurring moving objects (for more details, see the “Shutter speed” article).

Expopara and the law of interchangeability

The combination of aperture and shutter speed required for frame exposure is called an exposure coupler. Both shutter speed and aperture allow you to independently control the amount of light entering the camera. Increasing shutter speed or aperture by one step doubles the amount of light, i.e. Adds one exposure level. In contrast, a lower shutter speed or aperture reduces exposure. For example, the f / 5.6 * 1/30 expopair gives two exposures a larger exposure (i.e., transmits four times more light) than f / 8 * 1/60.

Imagine that you are shooting a certain landscape, and the exposure meter recommends that you use a shutter speed of 1/125 s at f / 8 aperture. However, in order for all the landscape plans to appear sharp in the photo, you decide to cover the aperture from f / 8 to f / 16. Thus, you reduce the exposure by two steps, and now, if you decide to keep the shutter speed 1/125 s, the frame will be very underexposed. For correct exposure, you need to increase the shutter speed by the same two steps, i.e. up to 1/30 s.

Thus, the same exposure can be obtained using various combinations of shutter speed and aperture. This phenomenon is called the law of interchangeability (or the Bunsen-Roscoe law). For example, the combination f / 11 * 1/15 will let in as much light as f / 4 * 1/125. The aperture decreased by three steps, and shutter speed, in contrast, increased by three steps.

Modern cameras allow you to change the shutter speed and aperture not only by whole steps, but also by intermediate values ​​- by half or a third step, which is necessary for more accurate exposure. Therefore, a combination of the form f / 6.3 * 1/80 has a right to exist.

ISO sensitivity
In addition to shutter speed and aperture, to determine the correct exposure, one more parameter must be taken into account – the photosensitivity of the photo material. Photosensitivity is measured in arbitrary units of ISO (ISO – International Organization for Standardization). All film and sensors with the same ISO sensitivity at the same level of illumination require the same exposure.

As in the case of shutter speed and aperture, ISO values ​​form a logarithmic series: 100, 200, 400, 800, 1600, etc. Changing the sensitivity by half requires a twofold change in exposure.

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