The performance of thermal imaging systems depends on a wide group of parameters that are integrated into the minimum resolvable temperature difference (MRTD) expression. The specific analytic dependence is determined by the system implementation and the design procedure used. This work calculates the performance dependence on detector area and optical aperture size and shape when the field of view and the detector angular subtense are held constant due to design procedure and technological constraints. Analytical models for three cold shield types (ideal, rectangular, and no cold shield situation) are described and used for the derivation of the MRTD expressions. The expression for a rectangular cold shield explicitly shows the dependence on basic parameters in contrast to the usage of a cold shield efficiency parameter that is generally accepted. In addition, an analytic model for crosstalk is described and used. It is shown for systems with negligible crosstalk that regardless of cold shield type the detector with the smallest permissible area will yield the best performance and the smallest system, while for systems with crosstalk the benefits of smaller systems, which are possible with smaller detectors, should be weighed against the MRTD degradation through crosstalk. The influ-ence on the MRTD of optical aperture size for round and square optics is calculated for diffraction-limited optics, taking into account different cold shield types. Conclusions are drawn as to the best optical subsystem, using minimal MRTD criteria, under various constraints. It is shown that when an aperture close to the diffraction limit is used, a big improvement in the high frequency range of the MRTD is achievable by using square optics.