Focal length of basic knowledge of photography

Focal length refers to the distance from the center of the lens to the imaging plane such as film or image sensor to form a clear image.

The focal length is expressed in millimeters. For example, a 50mm lens is the distance between the lens center point of this lens and the imaging plane of the film or image sensor that forms a clear image, that is, the focal length is 50mm, which is not adjustable, and it is also called a fixed-focus lens, while a 18-50mm lens means that the focal length of this lens is adjustable between 18-50mm, which is also called a zoom lens.

1. A lens with a focal length of 40-60mm is called a standard lens, and the photos taken with this focal length are basically the same as those seen by human eyes, about 45.

2. A lens with a focal length less than 40mm is called a wide-angle lens, and the viewing angle of photos taken at this focal length is greater than that of human eyes, which is greater than 60.

3. A lens with a focal length greater than 60mm is called a telephoto lens, and the viewing angle of photos taken at this focal length is less than that of human eyes and less than 30.

4. Fisheye lens refers to a lens with a very short focal length and a viewing angle close to or equal to180. A lens with a focal length of 16mm or less can usually be regarded as a fisheye lens.

5. Macro lens refers to a lens with a copy ratio of about 1: 1. It can be a lens with any focal length. Compared with the general lens, it can focus the subject more closely and form an image with the size of a real object on the imaging plane.

1. The focal length of the lens determines the size of the image formed by the object on the imaging plane. Assuming that the same subject is photographed at the same distance, the longer the focal length of the lens, the greater the magnification of the image formed by the subject on the film or image sensor (the smaller the viewing angle).

2. The longer the focal length, the less light reaches the image sensor and the darker the image.

We know that for a camera, the larger the aperture, the greater the amount of light entering and the brighter the picture. But for the focal length, the longer the focal length, the less light reaches the image sensor. So the brightness of a picture is related to the aperture and focal length, so how to consider it when copying? This is a function of f value, which refers to the relative value of lens focal length/lens diameter. No matter what lens, as long as the f value is the same, the amount of light reaching the image sensor is the same.

4. The longer the focal length, the smaller the depth of field. As can be seen from the following figure, the depth of field (red part) is the longest when the subject is away from the camera with a focal length of 28mm 10 meter, followed by the focal length of 70mm and the shortest when the focal length is 200 mm ..