The layout design of optical instruments is a subject under development. In this paper some basic problems are put forward and discussed: 1. functional research of the instrument as a member of the global system; 2. analysis of performance-construction relationships; 3. optimising the general constructional parameters; 4. engineering coordination among subsystems. It is noted that there is mathematical similarity in the losses and gains of information during transferring and transforming processes via optical, mechanical or electronic parts of the instrument, and therefore they oan be treated with a common mathematical approach, The introduction of concepts of system engineering and information theory to the discussion of these general problems is found beneficial to the insight into the subject. In addition, a few remarks are made to the conditions, namely, the basiC experimental data and the mathemattical models, that should be provided for the prospective practice of computer-aided-layout design.