Good exposure is critical for getting high-quality photos. However, the essence of the exhibition is very simple. Exposure is just the amount of light that hits the photo sensor. The process of shooting a frame is sometimes called exposure.
The exposure can be reduced or increased. That’s all you can influence. A smaller exposure makes the frame darker, a larger exposure makes it lighter. A lack of exposure is called underexposure, and an excess is called overexposure.
Correctly exposed image.
An underexposed snapshot.
Exposure is measured in exposure numbers or exposure steps (EV-exposure value). Changing the exposure by one step means changing the frame’s illumination by half.
Exposure can be controlled by varying two parameters – shutter speed and aperture. The aperture, i.e. the size of the relative aperture of the lens, determines the intensity of the light stream, while the shutter speed regulates the duration of exposure.
A diaphragm 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 number) is defined as the ratio of the lens focal length to the aperture diameter. For example, an f/4 entry means that the aperture diameter is four times less than the focal length of the lens.
The diaphragm numbers form the following series:
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 number, the smaller the relative opening. Each stage means that the illumination changes by half, i.e. the f/11 aperture lets in half as much light as f/8, and f/2.8 lets in twice as much light as f/4.
When the number of apertures increases by one step, the area of the effective aperture is reduced by half, which means that it transmits twice as much light.
In addition to exposure, the aperture also affects the depth of field and overall image quality.
Shutter speed is the time during which the camera’s shutter is open, allowing light to pass 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 can stop movement in the image, while a long shutter speed emphasizes movement, blurring moving objects (for more information, see the article “shutter Speed”).
Expobar and the reciprocity law
The combination of the aperture value and shutter speed required for exposure is called expopark. Both the shutter speed and aperture allow you to independently control the amount of light entering the camera. Increasing the shutter speed or aperture by one step doubles the amount of light, i.e. adds one step of exposure. In contrast, reducing the shutter speed or aperture reduces the exposure. For example, an expopar of f/5,6 * 1/30 gives two steps more exposure (i.e. it lets in four times more light) than f/8*1/60.
Imagine that you are shooting a landscape, and the exposure meter recommends that you use a shutter speed of 1/125 s at an aperture of f / 8. However, in order for all landscape plans to appear sharp in the photos, you decide to cover the aperture from f/8 to f/16. This reduces the exposure by two steps, and now, if you decide to keep the shutter speed at 1/125 s, the frame will be severely 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 different 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 pass as much light as f/4*1/125. The aperture was reduced by three steps, and the shutter speed, on the contrary, 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 of the step, which is necessary for more accurate exposure. Therefore, the combination of the form f/6,3*1/80 has the right to exist.
In addition to the shutter speed and aperture, to determine the correct exposure, another parameter must be taken into account – the photosensitivity of the photo material. Light sensitivity is measured in ISO standard units (ISO – international organization for standardization). All films and sensors with the same ISO sensitivity at the same light level require the same exposure.
As with shutter speed and aperture, ISO values form a logarithmic series: 100, 200, 400, 800, 1600 Changing the sensitivity twice requires changing the exposure twice. For example, if you need f/11*1/30 to shoot a scene at ISO 200, then if you increase the ISO to 400, you should reduce the exposure by half, i.e. take f/11 * 1/60 or f / 16*1/30.
ISO sensitivity, unlike shutter speed or aperture, is not strictly an exposure parameter, and changing ISO does not directly affect exposure. Exposure is the amount of light entering the camera, and the amount of light is controlled solely by the shutter speed and aperture. Increasing the ISO also leads to an increase in the electrical signal generated by the photosensor, which, in turn, makes it possible to proportionally reduce the exposure.
Digital cameras allow you to change the sensitivity of the sensor from frame to frame, which is very convenient. This can be done manually, or you can allow the camera to automatically select the desired ISO value. Higher values allow you to use shorter shutter speeds and shoot with your hands in low light conditions, but at the same time lead to poor image quality, since increasing the sensitivity of the sensor inevitably increases the level of digital noise. The basic ISO value (more often 100, less often 200) always provides the best image quality, and therefore you should avoid over-increasing the ISO if this is not necessary. What does excessive mean? This depends on the characteristics of the particular camera and the preferences of the particular photographer. Experimentally determine the maximum ISO value at which the noise level remains acceptable to you, and do not exceed this value in the future.