What is Focal Length?

Focal length is a parameter for all optic devices. Speaking by optics theory, focal length means the distance from focus point to lens centre. Focal length is significant for camera lens. Focal length and image size is in direct-ratio, the longer the focal length, the larger the image size; the shorter the focal length, the smaller the image size. Focal length and visual angle is in reverse ratio, the longer the focal length, the smaller the visual angle; the shorter the focal length, the larger the visual angle. Focal length and field depth is in reverse ratio, the longer the focal length, the shallower the field depth; the shorter the focal length, the deeper the field depth. Focal length and sensitivity of perspective is in reverse ratio, the longer the focal length, the weaker the sensitivity of perspective; the shorter the focal length, the stronger the sensitivity of perspective. Focal length and contrast is in reverse ratio, the longer the focal length, the clearer the contrast; the shorter the focal length, the clearer the contrast. The longer the focal length, the shallower the field depth is. The shorter the focal length, the deeper the field depth is. Therefore, when taking remote photos, we should choose long focus lens, but near photos choose short focus lens. Focal length of lens is measured by “cm” which is very noticeable.