To an astronomer, astrophotography is the photography of all kinds of celestial bodies, from star fields to constellations, clusters to galaxies. It also includes planetary bodies such as our neighbours in the solar system, and our own moon. It’s a pretty broad field encompassing a huge range of magnifications and specialized equipment and techniques. It’s a very scientific undertaking. Astrophotography for photographers is rather different.
With ‘astronomical’ astrophotography you could say that the objects being photographed are the principal interest, and the photography is simply to capture a visual record, or to record parts of the spectrum invisible to the naked eye, or capture objects so dim that they would be invisible without special equipment and post-capture stacking, averaging and alignment processes.
For photographers, astrophotography is subtly different. It’s the capture of the full spectacle of the night sky, usually across a wide angle of view, often with foreground interest and ground detail to add a sense of scale and grandeur. It’s the capture of a spectacular image, where the identification and recording of individual celestial objects is secondary to the overall effect.
This is not an argument about which is right, or which is ‘better’! They are two distinctly different things. This is just to explain why the gear and techniques for ‘photographic’ astrophotography are different to those that an astronomer might use.
Astrophotography for photographers
There is one principal challenge for photographers in capturing the night sky. It moves. The earth rotates slowly in space, though we are on the earth so to us it looks as if the sky is rotating. This means you need to keep exposure comparatively short to keep stars as pinpoints of light not streaks.
How long should this exposure be? Astrophotographers use the ‘500’ rule (some use ‘600’ not ‘500’). All you do is divide 500 by the effective focal length of your lens (in full frame terms) to get the longest exposure time you can use before stars turn into streaks. So let’s say you are using a 20mm equivalent wide-angle lens – this would give a maximum exposure time of 500/20 = 25 seconds.
Now an astronomer would use a motorised equatorial mount to keep the sky stationary in the field of view, but this won’t work for astrophotography where the landscape is part of the shot. That’s because the sky is moving relative to the ground. If you use a motorised mount to keep the sky still, the ground will blur.
Image stabilization won’t help at all, of course, because all astrophotography demands a tripod anyway because of the extended exposure times. The problem is not camera movement, but the constant rotational movement of the sky.
The alternative is to do the opposite and photograph star trails, which demands exposures of many hours. Here’s an example:
This idea of maximum exposure times is central to photographers’ astrophotography gear. It means that a wide-angle lens will be more useful because (a) it lengthens the maximum exposure time and (b) it gets a wider area of sky into the frame.
It also has a bearing on the best cameras and lenses for astrophotography. To keep within that maximum exposure time you need a fast (wide-aperture) lens and higher ISO settings. Generally, it’s easier to find fast wide-angle lenses for full frame cameras, and these cameras also deliver cleaner results at higher ISO settings.
You can get very good ‘astro’ shots with smaller-format APS-C or MFT cameras, but the reduced lens choice and more restricted ISO range will work against you. You can still get great astro shots with smaller format systems, but the ‘window’ of acceptable ISO settings and exposure times will be narrower.
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