FOC Instrument Science Report #75 "Format-Dependent Sensitivity Effects" by P. Greenfield Abstract excerpted from the report: The results of an investigation of format-dependent sensitivity effects are presented. In particular, it was discovered that there are as much as 30%, or greater, variations in sensitivity between formats on the same relay, as well as changes in spatial response with format. Although the effects are clearly evident, quantifying them reliably has not been easy since nonlinear effects must be corrected for to obtain meaningful results. Useful results were obtained for f/96 using two methods. One was to use ground-based LED flat fields taken for the purpose of measuring format dependent effects. The second by analyzing the stars observed in M15 using different formats. After corrections for nonlinearity, both methods give reasonable consistent results. Only the ground-based method was used for f/48. Unfortunately, the results for f/48 are less reliable because of the relatively large amounts of nonlinearity encountered. The results from this report and future work on this effect should be incorporated into the pipeline processing of FOC data so that observers do not have to specifically correct for this effect them- selves.