In many pattern recognition applications it is required that the discrimination capability should be tunable. We propose and develop an optical set-up in which the discrimination capability could be continuously varied within a large range. Our set-up is based on the two-step JTC architecture. The pattern recognition algorithm that we realize using the set-up is the dual nonlinear correlation (DNC), which is a generalization of a number of popular filtering methods. The paper contains the results obtained numerically in a simulated optical set-up as well as the experimental results obtained in the optical correlator, which we built using two different SLMs. We analyse the correlator performance according to the different characteristics of the SLM panels.