Now, if you use eyepiece projection, you are changing the image scale by increasing the projection distance, and must therefore calculate Effective Focal Length (EFL), but the system focal length remains unchanged. On 5/7/10, Chuck Hards <chuck.hards@gmail.com> wrote:
Tele-extenders and focal reducers have a multiplying factor printed on them (most do) For example, .85x. Assuming no eyepiece in the system, simply multiply the objective's focal length by that factor. Example, 8" f/10 = 80" = 2032mm focal length. 2032 x .85 = 1727mm = ~ f/7. You've changed your original system from f/10 to f/7.
Filter wheels and other accessories like extension tubes with essentially no magnifying power add or subtract nothing to focal length. They just take up "IN" travel.
On 5/7/10, Patrick Wiggins <paw@wirelessbeehive.com> wrote:
Looks nice. But I've always wondered how one calculates the exact focal length of a given setup. I suppose one could just use the manufacture's stated focal length of the OTA. However, once one starts adding accessories to the back like focal reducers, tele-extenders, extension tubes and filter wheels the advertised focal length becomes useless.