The optical assembly has a small refracting objective lens and an eyepiece that is usually removable and interchangeable to give different magnifications .
A small refracting objective lens is an image erecting system that uses either image erecting relay lenses or prisms ( Porro prisms or roof prisms ) .
The optical assembly has a small refracting objective lens , an image erecting system that uses either image erecting relay lenses or prisms ( Porro prisms or roof prisms ) , and an eyepiece that is usually removable and interchangeable to give different magnifications .