The paper deals with a type of optical-electronic instruments based on the optical equisignal zone. For the most
applications the difference signal in the equisignal zone is required to be independent on the distance. The purpose of the
study is to find interrelation of parameters of an optical system, of sources and of the receiving part providing constant
difference signal. It is known that if the source has a uniform radiance and an ideal objective (without aberrations) is
used, in order the width of the transitional zone be independent on the distance, the objective must be focused on infinity.
In practice homocentricity of the pencil of rays is influenced by the spherical aberration. Study of impact of the 3th order
abberrations on the width of the transitional zone, and as a result, on radiance distribution, allowed to find equations
linking the width of the transitional zone, exit pupil diameter, and irradiance of the sources. It is shown that the relative
difference signal depends on relation of the product of spherical aberration value and distance to the exit pupil diameter,
thus relative difference signal can be reduced with refocusing on a required distance. Based on obtained relations and
experimental results we conclude that using an appropriate processing of the difference signal, correction of spherical
aberration of the objective and focusing the projector into infinity, constant static characteristics may be achieved.