The performance of starting PtSi infrared cameras is characterized based on estimating their spatial frequency response. Applying a modified knife-edge technique, we arrive at an estimate of the edge spread function (ESF), which is used to obtain a profile through the center of the 2-D modulation transfer function (MTF). Using this technique, the complete system MTF in the horizontal and vertical direction is measured for various imaging systems. The influence of charge transfer efficiency (CTE) on the knife-edge measurement and resulting MTF is also modeled and discussed. An estimate of the OlE can actually be obtained from the shape of the ESF in the horizontal direction. In addition, we demonstrate that this technique can be used as a field measurement. By applying the technique at long range, the MTF of the atmosphere can be measured.