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Photography of the Sun, Moon and Stars

by Bob Atkins, 1998

This short article deals with photography of the sun, moon and stars using normal photographic lenses mounted on non-tracking mounts (tripods!). It's not an astrophotography course!

1 - Image Size

Both the sun and moon subtend an angle of just over 1/2 degree. They may look big to you, but they don't look very big to a lens. Their image size is given (approx) by the following formula:

Image Size (of the sun or moon, in mm) = Focal Length / 110

That means to get a 5mm diameter image you need a 550mm focal length lens. To "fill" a 35mm frame, you need a lens around 2500mm in focal length. So you will need a telescope, or a long telephoto (maybe with a 2x teleconverter) to get a decent sized image.

2 - Exposure

The Moon - The full moon is, in fact, an object in direct sunlight, and requires the same exposure as any object in direct sunlight (i.e. the "sunny f16 rule applies). This means 1/ISO at f16 for a "mid tone" object. The moon is in fact darker than a mid-tone, but we don't normally want it to look dark, so 1/ISO at f11 is a better starting point. So with ISO 100 film and an f8 lens, you would shoot at about 1/250. Bracketing exposure is a good idea too. When the moon isn't full, you will need a longer exposure, perhaps a couple of stops more for 1st and last quarter. Note though that there is a great range of brightness across the moon, so you may not find a single exposure which will give you optimum results for both the terminator (dark/light or day/night boundary) and the limb.

For images taken by moonlight, exposure starts around 4 minutes at f4 for ISO 100 film under a clear sky with a full moon. You almost certainly want to bracket around this value.

The Sun - The sun, when in a clear sky and anywhere but a few degrees above the horizon can easily damage your camera/lens, and more importantly your eyes, if you look at it for long enough. You need a filter, and if you intend to look through it as well as photograph through it, it must be a filter designed for visual use. Some filters block visible light but pass IR. Exposed film is an example of one such filter. Unless you are an idiot, intent on becoming a one-eyed idiot, don't use anything but a properly designed visual filter to look at the sun! Check out any issue of "Astronomy" or "Sky and Telescope" magazine for suppliers of solar filters for visual use. You can also check this link to a NASA site for a list of solar filter suppliers. With any lens or telescope used to observe the sun, the filter goes on the FRONT of the optics. This is very important. Stick a 5ND or 6ND filter in the slot at the back of your 600/4 and point it at the sun and you will soon be very sorry indeed that you did. Al the light and heat will be concentrated on the filter and it will crack, possibly damaging your lens, camera and you. Even if it doesn't crack there is still a good chance you will damage some of the smaller internal lens elements due to heating. Exposure depends on the exact filter you use, but you should aim for something in the 1/250 and faster range to avoid problems with camera/lens shake whe using a really long lens.

For solar eclipse photography, check out:

The Stars - If you don't want star trails, you have to keep the exposure fairly short. For 25 micron star trails (i.e. stars that look like points), you need to keep the exposure under about 2 seconds with a 500mm lens. This scales with focal length, so you get only 1 second at 1000mm, or 20 seconds at 50mm. The faintest star you will record doesn't depend on the speed of your lens. This may come as a suprise, but it's true. For point sources and images (and stars are as close as you will ever come to a point source), the physical aperture of the lens is what counts. So a 50mm f1.0 and a 500mm f10 lens, both of which have a 50mm diameter objective, will both record the same stars at the same brightness. If you allow the stars to trail, longer focal length lenses will produce longer (but dimmer) trails than shorter focal length lenses for any given exposure time.. Faster lenses do, however record the diffuse sky background illumination faster (it's not a point source), so "sky fog" builds up faster with faster lenses.

You can find references to making simple tracking mounts for cameras at the following URLs:

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Article created 1998