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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
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
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.
The Stars - If you don't want
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