We are testing an infrared camera, which produces images fine across a reasonable range of wavelengths. Now we want to see what images we get at certain specific wavelengths. So we (naively) acquired a bunch of narrow band-pass filters from Edmund Optics and then adapted them between the camera and lens. We are at the point of giving up trying to get any images this way because the focus is completely destroyed by doing this. It is as if the back focus distance of the camera has changed with the introduction of the filter, despite all its elements being flat glass.
Can anyone explain this? Is it a futile exercise, or are there some kinds of filter where this would work? Clearly most visible cameras use an infrared cut filter, and that works fine, but it is a single-element filter, and we need a multi-element filter to get the narrow wavelength bands we are interested in.