This section introduces the fundamental components and parameters of an imaging system, including focal length, the system stop, entrance and exit pupils, and the diffraction-limited resolution. The pinhole camera is considered as an illustrative example, demonstrating that unlimited resolution is theoretically achievable with a simple pinhole, but the solution is impractical due to extreme size requirements and very low light-gathering ability of a pinhole. For lens-based systems, key parameters such as the field of view, numerical aperture, and F-number are defined in terms of system geometry. The paraxial (parabolic) approximation is introduced through a Taylor series expansion of a spherical wavefront. This approximation is then applied to derive the thin-lens equation. Parameters of an imaging system, such as transverse and longitudinal magnification are introduced. The section concludes with an estimate of the information capacity of an optical system as a function of its parameters.