The focal length is the distance required for an optical device (such as a camera lens) to focus a parallel light on the imaging plane. It is an important concept in photography.
The focal length determines the perspective and magnification of the lens. A shorter focal length means a wide angle, able to shoot a wider scene, but less effective in object amplification. A longer focal length means a narrower view and a higher magnification for shooting distant objects or scenes requiring better detail capture.
The focal length is also related to the depth of field. Typically, long focal length has a smaller depth of field, while short focal length has a greater depth of field. Therefore, when the shooting needs to highlight the theme and blur the background, the telephoto lens is often selected to achieve the shallow depth of field effect.
In the field of photography, common focal lengths include wide-angle lens (generally less than 35mm), standard lens (about 50mm), and telephoto lens (more than 70mm). In addition, there is a zoom lens that can change the actual focal length by adjusting the corresponding optical components.
In summary, the focal length is the distance required for the optical device to focus a parallel light on the imaging plane. It affects perspective, magnification, and depth of field, and has an important impact on photography.