Filters and Focus Shift

When we install a filter - any filter - into an eyepiece, we always have to refocus slightly. But why does a flat piece of glass cause the point of best focus to change? The reason is that the glass in the filter has a refractive index different from air, so it slightly changes the focal point. But because the surfaces of the glass are parallel, it doesn't change the focal length. The filter acts as a relay lens to move the focal point outward, but without introducing any magnification or reduction. The image scale stays the same, so the effective focal length stays the same, even though the physical distance from the objective to the focal plane has changed.

Another reason for this effect in some refractors is that the constituent colors of white light focus at different points, so if you isolate one of those colors (as in a red filter), you then have to refocus for where that color is now sharpest. However, this does not apply in a telescope where all colors focus at the same point.

Finally, most run-of-the-mill filters are not optically flat - the thickness of the glass substrate may not be exactly the same at every point. In other words, depending on the quality of the surface finish, a filter could cause some light rays to divert in odd directions, resulting in a slight loss of detail in the objects observed. You'll note that ads for filters may say they're made of optical glass, but generally say nothing about the surface accuracy of the glass used. This has led some finicky observers to avoid any filter that is not certified as optically flat. However, the closer a filter is to your eye, the less effect there will be from optical imperfections. As a result, several experts have advised not screwing filters into your eyepiece barrels, but instead placing them directly in front of your eye.

Published in the April 2003 issue of the NightTimes