How to focus the camera?

The focusing of the camera is actually what we often call focusing. Focusing is divided into automatic focus and manual focus.

The focusing of the camera lens is usually carried out in the following three ways:

(1) The focusing method of changing the image distance

The camera lens is focused on infinity. When a distant object is focused, it is imaged on the focal plane of the lens, that is, l'=f'. When the photographic distance is shortened to a limited distance, such as 7m, 3m, ? (referring to the distance between the subject and the camera film plane), the image distance l' will be lengthened. In fact, the film position of the 135 camera is relatively unchanged, so the entire lens can only be extended forward a limited distance x'. This increase is the only way to ensure that the image points are correctly placed on the film plane to maintain the image plane. Clarity. This focusing method that keeps the focal length of the lens constant while changing the image distance is also called whole group focusing. This increase x' is called the focusing amount. When using this focusing method, you only need to turn the focusing ring on the lens. The distance scale between the film and the subject corresponding to the focusing amount is engraved on the focusing ring. The focusing ring drives the lens barrel. The multi-thread thread allows the lens to move axially so that the focus of the lens is on the film plane. Because it is a whole set of moving lenses, the relative positions between the lenses are fixed, so the imaging quality of the lens can always be maintained at its best.

(2) Focusing method that changes the focal length. This focusing method slightly changes the focal length of the lens by moving the axial position of a certain group of lenses in the lens, so that it can be maintained when the object distance changes. The image distance remains unchanged. This is a schematic diagram of the front group focusing, which is one of the most commonly used focusing methods. The front group can be focused individually, or the front groups can be moved together to adjust the focus. In addition, there are focusing modes that use the middle group or the rear group. The advantage of this focusing method is that the entire lens can remain motionless during focusing, the focusing amount is small, and the focusing mechanism is relatively simple. Since zoom lenses have many lenses and are large in size, it is difficult to move the entire set, so this method is often used to adjust the focus.

(3) Fixed focus method Most of the simple cameras currently available on the market have a fixed lens position. That is, no matter how far the object is, the distance between the camera lens and the film is always fixed. This focusing method is called the fixed focus method. Despite this, due to the limitation of the size of the circle of confusion, the camera's shooting quality is still guaranteed. In fact, this type of camera uses "depth of field" to adjust focus, also known as the hyperfocal distance method.