What is the principle of focusing in photography? Why do you feel that the distance and perspective of colleagues who are focusing are changing?

1. focusing principle: adjust the distance between the convex lens and the film or CCD, so that the light of the object is imaged on the film or CCD. The object distance is different, so this distance is between one focal length and two focal lengths of the convex lens.

2. For a fixed-focus lens, focusing will certainly not affect the focal length, because the focal length is a parameter of the convex lens itself, and this number is fixed.

Of course, most of the current cameras are equipped with zoom lenses, and the focal length will change when zooming, which mainly affects the imaging range, that is, to achieve the effect of pulling the lens.

3. Why is the distance and angle of view changing when focusing?

Let's take a look at this light path diagram.

I drew the positions of three negatives, and only B was the position where the focus was successful. At this time, the candle head just touched the edge of the film. Obviously, A and C failed to focus, and the range of objects included was different. A will accommodate more, and C will accommodate less, which means different perspectives. The perspective is different. Of course, I feel the distance is changing. It's like a telescope looking at something closer, but it just increases the angle of view.

Furthermore, if we don't change the distance (object distance) between the camera and the subject, but want to get clear photos from three different perspectives (abc) (that is, focus is successful), then we can only change the focal length of the convex lens, that is, use a zoom lens. Because the fixed-focus lens can only focus successfully at the B position, although the ac position changes the viewing angle, it is impossible to focus successfully.