At short wavelengths, optical systems can be designed such that a single aperture defines the beam that is used (system light gathering power), and another (the system field stop) defines the field-of-view (FOV). These components define the beam envelope and all other components are oversized so that they do not 'clip' or vignette this envelope. At longer wavelengths the diffraction caused by such clipping can seriously degrade the FOV response function and cause an increase in stray-light background. It is thus even more desirable to avoid clipping the beam as it passes through an instrument by oversizing all the optical elements. In space borne instruments, however, accommodation constraints can turn such oversizing into an unaffordable luxury. Instrument design must therefore consider the impact of multiple beam clipping and in particular any degradation in the FOV function. In this paper we describe such an analysis, based on advanced ray- tracing software, and give results for its application to two instruments: (1) The infra-red space observatory Long Wavelength Spectrometer (ISO-LWS, wavelength range 46 - 198 micrometer), where the FOV response is modeled for use with on-board calibration and data retrieval. (2) The imaging photometer in the Far Infra-Red Space Telescope SPIRE instrument (Spectral & Photometric Imaging Receiver, wavelength range 200 - 650 micrometer), where the analysis is needed for (a) Trade-off studies between instrument sensitivity (aperture size) and FOV degradation by clipping (b) Predicting the FOV performance of the final proposed design.