What are focal length, depth of field and aperture?

Simply put: the focal length is the distance from the lens to the focus of the film (or photosensitive element); Depth of field is the imaging range of a clear image; Aperture is a component that controls the amount of light entering.

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The longer the focal length, the smaller the viewing angle (image from the network):

Different focal length, aperture and distance will produce different depth of field (picture from the network):

Different aperture values can pass different amounts of light, and the best exposure speed can be obtained by adjusting aperture and ISO parameters. You can also control the depth of field through the depth of field characteristics of large aperture scenes and small aperture scenes (picture from the network):