Lens and applications

A magnifying glass is actually a convex lens. When a convex lens is used as a magnifying glass, the object distance is smaller than the focal length, resulting in an upright magnified virtual image.

When a convex lens forms a real image, the size of the image changes with the object distance: "the near object becomes larger and the far image becomes larger" (that is, the object distance decreases, the image distance increases, and the image also becomes larger). The size of the photo film is fixed. When it changes from a group photo to a half-length photo, the image becomes larger, so the object should be "closer and the image far", that is, the distance between the camera and Xiao Ming should be reduced, and the distance between the lens and the film should be increased.