To put it bluntly, our aperture coefficient is the focal length/aperture diameter of the lens, because the exposure is related to the aperture area, and the exposure series is a multiple relationship between each level (when the exposure time is constant, each level increases or decreases; The area of the light hole is correspondingly increased or decreased by one time); Everyone who knows mathematics knows that the geometric size (such as diameter) will only increase the square root of 2 when the area doubles, so the relationship of aperture coefficient is the square root of 2 (about 1.4), which leads to the current arrangement ... 1, 1.4, 2.8, 4, 5.6 and so on. If you are more careful, you won't find that the arrangement above is almost a multiple of every other number (for example, 2, 4, 8, 18). Do you understand now?

The luminous flux of the lens has the function of controlling the aperture. Generally, the larger the lens aperture (the smaller the F value), the more light passes through. The characteristic of large aperture is that it can obtain a shallow depth of field, that is, the subject is clear and the foreground background is blurred. This method is often used in portrait photography to highlight the theme. Of course, the focus under large aperture must be accurate, otherwise the shallow depth of field can easily be illuminated as the deviation of focus. The smaller the aperture (the larger the f value), the less light passes through it, and the longer the depth of field can be obtained under a small aperture, which is more suitable for representing a wide range of clear landscapes or environments.

AF (autofocus) autofocus?