Wednesday 4 April 2012

How To Photograph The Moon

There are some questions that come up time and again, and one of them is "how do you photograph the moon?"

The result many people get when they first try is something like this:

Pentax K-5, Pentax DA*300mm f/4 lens @ f/8, 4s, ISO100, handheld.

Now the problem here is that cameras have to make assumptions about what they're being pointed at.  One assumption they make is that over all, the brightness of the image will add up to an average 'mid-tone', so the dark sky and relatively small moon mean that the exposure becomes much brighter than we want it.  In the above example the camera chose an exposure time of 4 seconds, which resulted in lots of camera shake, and a grossly overexposed moon.

This is one of the times when manual exposure is a good mode to use.  The surprising thing about photographing the moon is the short exposure time required, but when you think about it, it does make sense.  Rather than being dark, like our surroundings at the time, the moon is being lit by full sunlight, so the exposure we need to give it is pretty much the same as we'd use (or the camera would pick) for a daylit landscape shot.

There's a rule of thumb called the 'Sunny 16' rule, which says for bright sunlight, an aperture of f/16, and a shutter speed that's the reciprocal of your ISO value, will give you a correct exposure.  So if your ISO is 100, a shutter speed of 1/100s will give you a good exposure when used with an aperture of f/16.

We rarely have to use this rule anymore, but there was a time before lightmeters were built into cameras, and being able to estimate the exposure was very useful.  But it's still useful as a starting point for getting a good exposure with the moon, although a slightly wider aperture generally gives you better results, especially when the moon is lower in the sky.

So switching to 'M' (Manual) mode on the camera, set an aperture of f/11, set your ISO to 200, and pick a shutter speed around 1/200s, and see if that doesn't give you a pretty good exposure on the moon.  For a more hand-holdable shutter speed, drop your aperture to f/8, and you can raise the shutter speed to around 1/400s.  If it's too dark, drop your shutter speed a bit, or pop the ISO up to 400, or both.  Have a play.

Pentax K-5, Pentax DA*300mm f/4 lens @ f/8, 1/350s, ISO200, handheld.

The above was shot last full moon, and was then heavily cropped to get the above composition from the relatively short focal length of 300mm.  Try shooting the moon when it's not full too; you'll need a slightly longer shutter speed, or a higher ISO, but the sidelighting on the moon will help highlight the craters and texture.

The next full moon is just two days away, give it a go!

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