Quantum sensitivity is an important emulsion property for photographic image formation and is usually calculated from a fraction of grains developable vs. log exposure curve (F-log E curve). A new method, residual grain size analysis using the electrolytic grain-size analyzer (EGSA) to detect undeveloped grains, has been developed that will allow one to readily obtain Flog E curves for each grain size class in polydisperse emulsions. The correctness of the F-log E curve derived by this new method has been examined by comparing the F-log E curve with that calculated from the normalized D-log E curve in the case of monodisperse emulsions. The problems in EGSA measurement, such as noise and emulsion removal from film base, which affect the accuracy of F-log E curves derived by such a method, have been solved and are discussed in detail. The method has been applied to the case of an emulsion with a bimodal grain size distribution.