The relationship between camera focus and focal length

Introduction: With the development of digital technology, digital has been integrated into us. For an ideal lens: distant objects can be seen approximately as being located at infinity. The light emitted from any point on the infinite object and reaching the ideal lens can be regarded as parallel light. The relationship between camera focus and focal length

The so-called "optical axis" is a ray of light that passes vertically through the center of an ideal lens. When light rays parallel to the optical axis enter a convex lens, the ideal convex lens should have all the light rays converging on a point behind the lens. This point where all the light rays converge is called the focus. For example, after using a magnifying glass to focus sunlight, the smallest point is the focus. The focus must be on the optical axis.

In optical terms, the lens is used as the boundary: the space where the subject is located is called the object space; the space where the light emitted by the subject passes through the lens and forms an image behind the lens is called the object space. ?Image side space?; The focus formed in the image side space is called "image side focus" or "back focus"; On the contrary, starting from the image side, a light ray parallel to the optical axis is projected and formed in the lens object space The focus is called the object-side focus or the front focus. Note: For a concave lens, the object-side focus and image-side focus are opposite to those of a convex lens

Principal point:

The axial thickness of a lens is related to its diameter, object If the distance, image distance and focal length are small compared to each other, the lens can be considered to be a thin lens.

The focal length of a thin biconvex lens generally refers to the distance on the optical axis from the center of the lens to the focus. The center of the lens is called the "principal point".

The actual lens is composed of several convex lenses and concave lenses, and the position of the principal point cannot be directly determined. When the focus is at infinity, the distance from the principal point of the lens to the image plane = focal length

For a certain frame, the focal length of a standard lens is approximately equal to the diagonal length of the frame, and the position of its principal point Within the optical group of the lens

Focal length (focal length)

The distance from the rear principal point (h') of the photographic lens to the rear focal point is the focal length.

Although the types of lenses are different, there may be cases where the main point's front-to-back relationship is reversed, or h' falls outside the combined lens. In any case, the distance from the back principal point h' to the back focus is the focal distance.

The position of the rear principal point h' of telephoto lenses and retrofocus lenses. The principal point of the telephoto lens is further forward than that of the standard lens, and its principal point is outside the front optical group of the lens. The principal point of the wide-angle lens is further back than that of the standard lens, and its principal point is outside the rear optical group of the lens.

Back focus (back focus)

When the focus is at infinity, the distance on the optical axis from the apex of the last lens surface of the lens to the film plane is called the back focus distance.

Due to the working principle of the SLR, the reflector must be flipped up during the action. Therefore, for a wide-angle lens with a relatively short back focus distance, the reflector will touch the last set of lenses. Therefore, the wide-angle lens used for SLR cameras must use a reverse telephoto lens with a relatively long back focus distance.