A collimator is an optical device used for producing a beam of parallel rays. It consists of an objective lens fitted in a tube and an illuminated reticle placed at its focal plane. In this setting the image of the reticle is projeted at infinity (Diagram 1).

A focusable collimator allows for slight adjustments between the position of the objective lens and that of the reticle along the optical axis – thus providing the option to create setups where the image is projected at finte distances:

By moving the reticle away from the objective lens the rays leaving the reticle are focused by the optics on the opposite side forming a real image (Diagram 2).

Conversely, moving the reticle towards the objective lens results in a diverging beam and a virtual image at the apparent origin of the rays (Diagram 3).

When using the collimator for evaluation of parallax error in riflescopes it is operated in this third setting – that is the virtual image of the reticle is observed through the eyepiece and serve as a reference for parallax correction.