As RMS wavefront error goes to zero, the MTF approaches its diffraction limited performance. The default RMS wavefront merit function is therefore an excellent place to start when optimizing a system to have a desired spatial frequency response. You can then switch to explicit MTF optimization operands as needed for a final adjustment of system performance.

MTF calculations take longer than wavefront calculations, and so initial wavefront optimization is strongly preferred. In addition, when system performance is not already good, direct MTF optimization can stagnate. For example, if the lens used to form the images on the previous page is simply defocused, the MTF can drop to zero and then rise again, as shown below.



This is referred to as spurious resolution, and can cause the local optimizer to stagnate, as the MTF must get worse before it can get better. Although Hammer and Global Search can handle this, it is almost always best to optimize on wavefront error until all spatial frequencies of interest are within the first minimum of the MTF plot. Only then should you optimize for specific MTF targets.