Zoom and focus
May 11, 2023
It usually refers to lengthening or shortening the focal length by moving the position of the lens in the lens, also called ZOOM.
Zoom can be divided into two types: optical zoom and digital zoom:
Optical zoom is to change the position of the focal point by moving the relative position of the lens inside the lens, change the length of the focal length of the lens, and change the viewing angle of the lens, so as to realize the enlargement and reduction of the image. if filmed
The position of the object remains unchanged, and the focal length of the lens will be proportional to the magnification of the object. If there is one word to describe optical zoom, it is "telescope". (horizontal movement of the imaging surface)
Digital zoom is to increase the area of each pixel in the picture through the processor of the camera phone, so as to achieve the purpose of zooming in, just like we forcefully enlarge the pixels of the image in image processing software such as ACDSEE,
It's just that this process is carried out in the mobile phone, and a part of the pixels on the original sensor are enlarged by means of interpolation. Different from the optical zoom, the digital zoom is a change in the vertical direction of the sensor, which gives people a sense of
zoom effect. Since the focal length does not change during the whole process, the image quality will gradually decrease as the magnification ratio increases. (Scaling in the vertical direction of the imaging plane)
Digital zoom can also be divided into interpolation algorithm zoom and pseudo digital zoom:
Interpolation algorithm zoom: Interpolation operation is performed on the image to expand the size of the image to the required specification. This algorithm is not ideal in terms of its effect, especially when used on a mobile phone, the camera on the mobile phone
The data obtained by itself has a lot of noise, and if it is interpolated again, the obtained image is almost unusable.
Pseudo-digital zoom: When the camera is not in the maximum resolution format, for example, when a 1.3-megapixel sensor uses a 640*480 specification to take pictures, still set the sersor to work at a resolution of 1280*960, and then capture the central part The image is used to obtain a 640*480 photo, which doubles the size of the captured object on the mobile phone. This method requires almost no additional algorithm support and has no effect on image quality. The disadvantage is that it can only be used in small size cases.
Optical zoom without sacrificing clarity
Digital zoom sacrifices sharpness significantly
1. Focus concept
When a convex lens is used as a camera lens, the clearest image it forms generally does not fall on the focal point, or in other words, the distance (image distance) from the clearest image to the optical center is generally not equal to the focal length, but slightly greater than the focal length.
The specific distance is related to the distance between the object being photographed and the lens (object distance). The larger the object distance, the smaller the image distance, (but in fact it is always greater than the focal length).
Because when we take pictures, the distance between the object to be photographed and the camera (lens) is not always the same. For example, when taking pictures of people, sometimes, if we want to take pictures of the whole body, we should stay far away; That is to say, the image distance is not always fixed. In this way, in order to get a clear image, the distance from the photosensitive surface to the optical center of the lens must be changed according to the different object distances. This changing process is what we usually call " focusing". Therefore, what we call "focus" adjustment is not the focal length in the true sense, but just a consensus term for the process of adjusting the image projected on the film or photosensitive element to the clearest in photography. That's all.