What are the two prerequisites and two decisive conditions of photographic exposure?

In order to get the correct exposure, the correct combination of shutter and aperture is needed. When the shutter is fast, the aperture will be larger; The aperture is smaller when the shutter is slow. Shutter priority refers to the exposure value calculated by the automatic metering system of the machine, and then automatically determines how much aperture to use according to the shutter speed you choose. Aperture priority refers to the exposure value calculated by the automatic metering system of the machine, and then automatically decides how many shutters to use according to the aperture size you choose. When shooting, users should balance the exposure and shutter according to the actual environment, which complement each other. The larger the aperture, the more light passes through per unit time, and vice versa. Aperture is generally expressed as the letter "F+ value", such as F5.6, F4, etc. It should be noted that the smaller the value, the larger the aperture. For example, F4 is larger than the aperture of F5.6, and the difference between two adjacent apertures is twice, which means F4 is twice as big as that passing through F5.6. Relatively speaking, the definition of shutter is very simple, that is, the time allowed for light to pass through the aperture is expressed by numerical values, such as 1/30 seconds, 1/60 seconds, etc. Similarly, the difference between two adjacent shutters is twice, and the combination of aperture and shutter forms exposure. Under certain exposure conditions, this combination is not unique. For example, the current normal exposure combination is F5.6 and 1/30 seconds. If the aperture is increased by one step, that is F4, the shutter value at this time will become 1/60, and this combination can also achieve normal exposure. Although different combinations can achieve the same exposure, the pictures taken have different effects. Shutter priority is to get the aperture value through camera metering when the shutter is defined manually. For example, shutter priority is mostly used to shoot moving objects, especially sports shooting. Many friends find that when shooting moving objects, the subject is often blurred, mostly because the shutter speed is not fast enough. In this case, you can use the shutter priority mode, roughly determine a shutter value, and then shoot. Because the shutter is fast, the amount of light entering may be reduced and the color is light, so it is necessary to increase the exposure to enhance the brightness of the picture. The motion of the object is generally regular, so the shutter value can also be roughly estimated. For example, when shooting pedestrians, the shutter speed is almost1125 seconds, while it takes11000 seconds to shoot falling water droplets.