1. Human Eye: This is a complex biological optical system. The cornea and lens act as refractive elements, focusing light onto the retina, which acts as the image sensor. The iris controls the amount of light entering the eye.

2. Telescope: Telescopes are designed to gather and focus light from distant objects. A simple telescope consists of two lenses: an objective lens (typically a large diameter lens) and an eyepiece. The objective lens collects light and forms an image at its focal point. The eyepiece magnifies this image, making distant objects appear closer.

These are just two examples, and there are many more optical systems used in various applications.