Imagine using a 35-mm SLR camera lens at full aperture, say f/1.8. The lens aberrations are most severe at full aperture. As the lens is stopped down, aberrations are reduced and the image quality is improved. The improvement in image quality does not last forever: at some setting of the aperture, diffraction from the aperture prevents you from gaining any further improvement in image quality.

For cases where there are many waves of aberration, the Geometric MTF calculation and GMT* optimization operands are recommended. It is based on a Fourier transform of the geometric spot data, and uses a Gaussian quadrature sampling scheme.

The primary advantage to using the geometric MTF is very high speed, compared to the diffraction calculation. The geometric MTF is very accurate for systems with large aberrations, where the diffraction MTF calculation would need enormous sampling to converge. The geometric calculation is typically factors of 100 or more faster in this regime.

The Geometric MTF calculation can even account for scattering from optical surfaces, which reduces MTF by increasing the background illumination.

The GMTF is very useful in Global Search, where one wants to efficiently search all parameter space to identify regions where good designs can be found.

The diffraction calculation should always be used when diffraction effects are important.