# Afocal Photography Basics

Some math is required to get the best results, but don't worry; it is fairly easy to grasp. The formulas for afocal photography are listed in Table 2.1. The projection magnification is the camera lens focal length divided by the eyepiece focal length.

Figure 2.3. In afocal photography, the camera with its lens is aimed directly into the eyepiece.
 Table 2.1. Afocal Photography Formulas Ft Telescope focal length D Telescope aperture diameter ft Telescope focal ratio = Ft/D Fep Eyepiece focal length Telescope exit pupil = Fep/ft Fc Camera focal length fc Camera focal ratio Camera entrance pupil = Fc/fc Mp Projection magnification = Fc/Fep Feffective Effective focal length = Ft*Mp feffective Effective focal ratio = ft*Mp

The projection magnification can act like a Barlow when it is greater than one (shaded cells in Table 2.2) or as a focal reducer when it is less than one (clear cells in Table 2.2). Your telescope effective focal length is the telescope focal length multiplied by the projection magnification (Table 2.3). The telescope focal length and focal ratio are increased in the shaded Table 2.3 cells and decreased in the clear cells.

 Table 2.2. Projection Magnification Factor Nikon 990 Camera Focal Length Eyepiece Focal Length Minimum Zoom 6 .2mm Maximum Zoom 23.4mm 4mm 2.05 5.85 10mm 0.82 2.34 14mm 0.59 1.67 18mm 0.46 1.30 20mm 0.41 1.17 24mm 0.34 0.98 35mm 0.23 0.67 40mm 0.21 0.59 55mm 0.15 0.43
 Table 2.3. Effective Focal Length and Ratio for a 1800mm Focal Length, f/12 Telescope Nikon 990 Camera Focal Length Minimum Zoom 8.2 mm Maximum Zoom 23.4mm Eyepiece Focal Length Effective Effective Effective Effective Focal Length Focal Ratio Focal Length Focal Ratio 4mm 3690mm 24.6 10530mm 70.2 10mm 1476mm 9.8 4212mm 28.1 14mm 1054mm 7.0 3009mm 20.1 18mm 820mm 5.5 2340mm 15.6 20mm 738mm 4.9 2106mm 14.0 24mm 615mm 4.1 1755mm 11.7 35mm 422mm 2.8 1203mm 8.0 40mm 369mm 2.5 1053mm 7.0 55mm 268mm 1.8 766mm 5.1