At autocollimators with fixed focus setting the distance between reticles and objective is fixed. Usually the adjustment is made for infinity at a wavelength of 546 nm. Adjustment to other fixed distances or wavelength is also possible on demand.

Measuring tasks requiring different wavelengths of light, or measurements requiring variable focus (finite and/or infinite) are best accomplished using focusable autocollimators. With the aid of the tube extension the position of the reticles in respect to the focal plane of the objective is changeable. As a result of that the former parallel beam converts into a convergent or divergent beam and yields a real or virtual image of the reticle.