Say you have a dichroic filter with a bandwidth of 10nm centered at 500nm, so it will pass 495nm - 505nm light through. In an optical system the light is generally focused as a cone through the filter, which means that most of the light, if not all, is entering the filter at an oblique angle. What effect does this have on the light passing through the filter? I assume that there is a small change in the bandpass for light that enters at an oblique angle. Is that correct? And the same thing happens if you tilt it too? Does the bandwidth stay relatively the same if you tilt the entire filter, with only the upper and lower bandpass frequencies going up or down, or does the bandwidth change too?