How to control exposure in photography

When shooting with a traditional camera, the lens is imaged on the film, and when shooting with a digital camera, the lens is imaged on the image sensor for photoelectric conversion. In order to capture an image with normal tone, the film or image sensor in the camera must be properly exposed. Exposure control is to take various measures to flexibly control exposure during shooting. First, the impact of exposure on image quality When shooting, the amount of light obtained by the film or image sensor is called exposure. Whether the exposure is accurate is one of the important factors that determine the final image quality. Proper exposure, that is, accurate exposure, the final reproduced image has normal tone, appropriate contrast between light and dark, and saturated color; If the exposure is too small, that is, the exposure is insufficient, the developed negative image is bright, the density is too low (commonly known as thin negative), or even there is no image, and the dark part of the scene is transparent and non-hierarchical, and the obtained image is dull, the picture is dim, the contrast is extremely low, and the dark part of the picture is non-hierarchical; If there is too much exposure, that is, overexposure, then the developed negative image will be deep and dense (commonly known as the thickness of negative film), and the positive image (such as a photo) will be white, without hierarchical highlights and color saturation, and the whole picture will look like fading. Second, the factors that affect the exposure. The exposure of film or image sensor depends on the illumination and exposure time when shooting, that is, exposure = illumination × exposure time. The camera uses aperture and shutter to control illumination and exposure time respectively. The illumination intensity is also affected by the brightness of the object. The sensitivity of film or image sensor determines the exposure required to obtain high-quality images. 1. Aperture Aperture is an aperture device that controls the aperture of light in a camera lens. It consists of several pieces of metal, usually located between two lens groups in a lens. The aperture size can be controlled by the aperture adjusting ring on the lens or the adjusting mechanism on the camera body. 1) Aperture coefficient Aperture coefficient refers to the ratio of lens focal length to lens entrance pupil diameter, that is, aperture coefficient = lens focal length ÷ entrance pupil diameter can be roughly understood as the diameter of incident light beam. The aperture coefficient is written as f/ number or f number. The aperture coefficients of the standard series are 1, 1.4, 2, 2.8, 4, 5.6, 8, 1 1, 22, 32, 45, etc. For a lens with a specified focal length, the larger the numerical value of the aperture coefficient, the smaller the corresponding actual aperture (the relationship between the aperture coefficient and the aperture diameter and the incident light). In the standard series of aperture coefficients, the relationship between the diameters of inlet holes of adjacent aperture coefficients is multiple, and the relationship between the areas of inlet holes is multiple. The multiplication relation of the entrance area of any two apertures can be calculated by 2n, where n is the number of different steps between the two apertures. For example, the difference between f/2 and f/22 is 7 steps, and the entrance area of f/2 is 27 times that of f/22. There are two exceptions to the scale of lens aperture coefficient: first, the minimum aperture coefficient of many lenses is not in the standard series of aperture coefficients; Secondly, many zoom lenses have a minimum aperture coefficient range, such as zoom lenses with a focal length of 28-80 mm, and the minimum aperture coefficient is f/3.5-5.6. The first value in this aperture coefficient range is the corresponding aperture coefficient under the shortest focal length, and the last value is the corresponding aperture coefficient under the longest focal length, that is, the aperture coefficient of zoom lens is different under different focal lengths. The zoom lens with the aperture coefficient changing with the focal length increases the difficulty of manual exposure control during photography. A lens whose aperture coefficient does not change with the change of focal length during zooming is called a constant aperture lens, otherwise it is a non-constant aperture lens, or a variable aperture lens. (2) Aperture has three functions: controlling exposure, controlling depth of field and affecting imaging quality. Controlling exposure is the main function of aperture. When the brightness of the subject is constant, the larger the aperture (the smaller the aperture coefficient), the greater the exposure. The influence of aperture on depth of field is as follows: The larger the aperture coefficient, the greater the depth of field. The imaging quality of different apertures of the same lens is not exactly the same, and there is a saying of "optimal aperture", that is, a certain aperture has the best imaging effect (mainly refers to clarity). The aperture coefficient of the optimal aperture of different lenses may be different. From the statistical law, the best aperture is most likely to appear when the maximum aperture of the lens is reduced by 3-4 steps. (3) The adjustment of the aperture is on the camera. There are two ways to adjust and set the aperture. First, for a camera whose aperture coefficient is engraved on the outer circumference of the lens aperture adjusting ring, it is only necessary to rotate the aperture adjusting ring when shooting, so that the aperture coefficient value of this aperture is aligned with the baseline on the lens, and any position between the aperture gears can be aligned with the baseline, so that the aperture changes of 1/2 and 1/3 can be realized. The other is set on the camera body, and the aperture of the aperture in the lens is controlled by the circuit. SLR cameras usually use a preset aperture, that is, when the shutter button is not pressed to release the shutter, the aperture of the aperture is opened to the maximum regardless of the aperture size, so that there is enough illumination for framing, focusing and metering. Only when the shutter button is pressed for exposure, the aperture will shrink to the size corresponding to the selected aperture coefficient, and the aperture will be opened to the maximum again after exposure.