What points are called side focal points: a) collecting lens; b) a diffusing lens?

Side foci are points in the focal plane in which parallel beams of light are collected, falling on the lens not along its main optical axis.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.