Physics: Why does the camera stretch forward when it takes a close shot and retract when it takes a long shot?

When shooting close range, the object distance is relatively small, and if you want to shoot a clear image, the image distance is relatively large. The lens extends forward to widen the distance between the film and the lens, so that the film can adapt to the enlarged image distance and get a clear image. On the other hand, taking a long shot will shrink the lens.

The principle of camera imaging is that a convex lens becomes a real image. Assuming that the distance from the lens is the object distance U and the distance from the lens to the film is the image distance V, there is a relationship of1/f =1/u+1/v between them (where f is the focal length)?

When choosing the camera lens and the scene to be shot, the photographer is generally motionless, that is, the focal length and the position of the film and the scene are fixed. In order for the camera to take a clear picture, the distance and focal length between objects must satisfy the above formula.

The formula is1/f =1/u+1/v, where f is a constant. If u gets bigger, v gets smaller. Conversely, if u becomes smaller, v becomes larger. ?

Lens is an optical system for imaging, which consists of a series of optical lenses and lens barrels. Each lens has two characteristic data: focal length and relative aperture.

The viewfinder is a device for selecting scenery and composition. Anything that can fall into the frame can be photographed on film.

A rangefinder can measure the distance of a scene, and it is often combined with a viewfinder. Through the linkage mechanism, ranging can be linked with lens focusing and focus can be completed at the same time.

Extended data:

The camera uses the linear propagation characteristics of light and the law of refraction and reflection of light, takes photons as the carrier, and transmits the light information of the shooting scene to the photosensitive material in the form of energy through the photographic lens, and finally becomes a visible image.

The optical imaging system of the camera is designed according to the principle of geometric optics. Through the lens, the scene image is accurately focused on the image plane through the linear propagation, refraction or reflection of light.

Proper exposure should be controlled when taking pictures, that is, the amount of photons reaching the photosensitive material should be controlled properly. Because the range of photons received by silver salt photosensitive materials is limited, too few photons form latent image nuclei, and too many photons form overexposure, so the image cannot be distinguished.

The camera uses the aperture to change the aperture of the lens to control the amount of photons reaching the photosensitive material in a unit time, and at the same time controls the exposure time by changing the opening and closing time of the shutter.

Baidu Encyclopedia —— The Law of Convex Lens Imaging

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