2.5 Afocal Attachments: Power and Field Changers

As shown in Fig. 2.8, an afocal device may be placed before another optical system to change its effective focal length. The resulting focal length for the combination F _c is simply the product of the afocal magnification MP and the focal length of the original system F _p . Thus

Figure 2.8: An afocal attachment can be placed in front of another system to change its focal length, power, or field. Shown here is a galilean afocal which, in the upper sketch, increases the focal length by a factor equal to its magnification and, when reversed, shortens the focal length by the same factor.

(2.12)

From the figure, it can be seen that, depending on its orientation, the afocal attachment can produce a combined focal length F _c which is longer or shorter than the focal length of the prime lens F _p . When used as an attachment to a camera lens, the systems of Fig. 2.8 are usually referred to as telephoto or wide-angle attachments. Note that, with a galilean afocal camera attachment, the iris of the camera lens is the aperture stop of the system. Occasionally a binocular (which is a keplerian telescope) is used as a telephoto attachment. The real exit pupil of the binocular may cause problems because, for this arrangement to work well, the exit pupil of the binocular and the entrance pupil of the camera lens must be coincident. If...