The underlying optical mechanism of the Electronic Speckle Pattern Interferometer (ESPI) was studied. It was theoretically found that the previous design concept of conjugacy was not needed and that an alternative design gives better quality results. This was shown experimentally using different geometries and point-of-origin for the reference beam. The intrinsic problem of aliasing in the electronic apparatus of ESPI was investigated with the aid of an interferometric technique. Relevant parameters to the system were brought together to study a simplified model for the distribution of spatial frequencies at the plane of the object image. Experimentation on photographic speckle pattern interferometry revealed the need for the introduction of a variable spatial filter into the ESPI electronic system that will give way to fringes of holographic quality. Implications for future designs of ESPI are discussed.