When only certain filter pixels are allowed to transmit light, those `on' pixels constitute the region of support (ROS) for the filter. At those frequencies where noise power overwhelms signal power, or where the available values of the filter are not well aligned with the ideal spectral filter, the filter can actually work against the intent of the metric, and zero transmittance is preferable to what the SLM can do. In contrast to prior theories, which applied to only phase-only filters, we present algorithms for optimizing and studying ROS for an arbitrary SLM. We also present laboratory demonstrations of improvement in signal to noise ratio when a blocking mask is inserted into an optical correlator. A mask has detailed functional dependence on signal, noise power spectral density, the selected metric, and the realizable filter values. Rules are given for designing a mask that will be more broadly useful.