The Earth's rotation makes the stars appear to move from east to west in the sky. For a stationary, unguided camera of focal length F, a star having declination d, will photograph as a trail of linear length, L, in an exposure time of t seconds:
t * F * cos ( d ) L = -------------------- 13750
Note that the units of length, L, will be the same as the units of F.
Shorter exposures will result in shorter trail lengths. Assuming an 8x enlargement of a print and "normal" viewing distances, a trail of 0.1 mm or less will be virtually unnoticeable. The following table indicates maximum unguided exposures which will meet this criterion with several common 35 mm camera lenses:
| Lens | Maximum exposure in seconds at declination |
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| Focal Length | 35 mm field | (degrees + or -) | |||
| 0° | 30° | 45° | 60° | ||
| 28 mm | 49 x 73 | 49 | 56 | 70* | 98* |
| 35 mm | 39 x 59 | 39 | 45 | 55 | 78* |
| 50 mm | 25 x 37 | 26 | 31 | 37 | 52 |
| 100 mm | 14 x 21 | 14 | 16 | 20 | 28 |
| 135 mm | 10 x 15 | 10 | 12 | 14 | 20 |
| 200 mm | 7 x 10 | 7 | 8 | 10 | 14 |
| 300 mm | 4.6 x 7 | 4.6 | 5.3 | 6.5 | 9 |
| 400 mm | 3.5 x 5 | 3.5 | 4 | 5 | 7 |
*Note: exposures over 60 seconds with "fast" lenses (f/2 or better) will show sky fog.
The angle of sky coverage, B, of any film having linear dimension s used with a lens of focal length F is given by (s and F must have the same units):
( S ) B = 2 * arctan |-------| ( 2F )
Instrument Requirements for Various Objects
| Object Type | Lens | Focal Length | F/ ratio | Comments |
| Star Trails |
as available | 50mm - 400mm | f/2.8 - f/8 | f/4 lens having 40° coverage best. |
| Moon | telescope | 50" or greater; use amplification | f/8 - f/80 | 35mm SLR on telescope with projection. |
| Sun | telescope | same as Moon | same as Moon | Use proper filters to avoid eye and equipment damage |
| Planets | telescope | 100" or greater | f/16 - f/80 | Projection necessary to enlarge image. |
| Stars (general) | 25mm diameter minimum |
any, but 100mm - 300mm best | f/4 - f/8 | Use medium speed f/ ratio for best definition. |
| Star Clusters | 50mm diameter minimum for open clusters | 400mm or longer | f/4 - f/16 | Use longer focal length for smaller clusters |
| Nebulae | 50mm diameter minimum | 150mm lens or short focus telescopes | f/2 - f/8 | Need greatest possible light collection. |
| Comets | 25mm diameter minimum | 100mm lens or short focus telescopes | f/2 - f/5.6 | Wide angle for tail coverage; long focal length for coma. |
| Aurora | 25mm diameter minimum, fast | 28mm - 100mm | f/2 - f/4 wide open | Wide angle; short exposures to minimize motion. |
| Meteors and Satellites | as available | 50mm - 100mm | f/2 - f/4 | 40° - 60° sky coverage |
