This is because rays from the same area of the picture only diverge a little from each other, even though they may come from different distances. However small lenses mean there is a small opening or aperture for the light to enter. The f-number characterises the brightness of the image on the film. The f-number is the focal length divided by the aperture diameter. If you double the f-number you must multiply the exposure time by 4x.
Good lenses have an anti-reflecting film.
These films use the property of wave interference. The quality of the image that is formed in a camera depends on the quality of the lens system. A simple spherical lens cannot produce a flat sharp image of a scene that has colour, width and depth. A simple lens has different focal lengths for different colours. The result for an image is that it will have blurred coloured edges.