We determined the performance of an optical correlator as the
filter SLM was reduced in resolution by a factor of M in a linear dimension. Our configuration allowed a constant reduction, by a factor of M2, in the amount of data used to describe the filter when compared to using a filter at full resolution. Reducing the resolution of the filter is shown to primarily affect its impulse response by the addition of copies of the filter image. We provide both general expressions for performance measures and guidelines for minimizing the distortion induced as the resolution of a filter is decreased. Furthermore, we show the effect of reducing the resolution of a binary phase-only filter on autocorrelation experiments and cross-correlation experiments containing competing objects. We obtained experimental results with an optical correlator with a magneto-optic SLM of 128 x 128 pixels in the input plane and a filter of 64 x 64 pixels. The results showed that our arrangement can provide comparable results to a full-resolution filter when the resolution is decreased by a factor of M and the object within the filter images is less than N/M x N/M pixels in size.