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

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