In the case of a refractive or reflective optical element, the optical axis is the straight line that is coincident with the axis of symmetry of the surfaces. Note: The optical axis of a system is often coincident with its mechanical axis, but it need not be, e.g., in the case of an off-axis parabolic reflector used to transmit signals to, or receive signals from, a geosynchronous satellite.

 

In the case of a lens element, the optical axis is the straight line which passes through the centers of curvature of the lens surfaces.

 

In the case of an optical system, the optical axis is the line formed by the coinciding principal axes of the series of optical elements.