In previous version of OptiFDTD, there existed 2 options to obtain spectral response – FFT and DFT. The frequency domain sampling step was 1/(2N*Dt) for FFT. For DFT, it gave spectral response for a series of wavelengths which was identified by users. the more selected wavelengths, the slower DFT (FDTD) runs. so a max. number on sampling points (e.g. 200) was set. We removed FFT some time ago and only keep DFT.