Optical bandpass filters are designed for a particular angle of incidence (AOI), e.g. see Fig 4 here.
However that is for the ray picture of light. Imagine I have a thin bandpass filter positioned exactly at the focal point of a lens, and a gaussian beam coming in. Right there at the filter, the beam has some beam waist and the phase fronts are parallel. Since the phase fronts are parallel, does that mean that the light will pass through the bandpass filter as if it were all normally incident? Or, will the extreme-angle rays not be filtered in the same way (the filtering function smoothed and blue-shifted, as described at the link I pasted above) as the axial rays?