Night sky photography

When I started to photograph the night sky, I read that I should use a wideangle lens, high ISO, big aperture and bulb setting with remote. Unless of course I wanted to photograph the rings of Saturn or the Andromeda galaxy, in which case I needed a telescope, but I’m not an astronomer so I stick to the basics. It all sounds simple enough but it seems like there are as many settings to use as there are people giving advice. So in the end, the best way to learn is to not listen to the advice and make your own mistakes and it seems like every time I point my camera at the stars, it’s a new mistake (and a lesson)!

My interest in astronomy is pretty much limited to the stars visible with naked eye. I’ve never felt the need to invest in a telescope, but I frequently study star maps and try to learn the names of the constellations and stars. So when I photograph the night sky, I normally want to have a recognisable constellation in it. Star trail pictures can be really cool and I’ve done some of those, but in the end I found that I’m much happier with small dots of light instead of streaks and I want to top off the picture with a silhouette of something (trees are best).

ISO 200, f3.5, 15 sec, full moonSo what I’ve learned is that you have to keep the exposure at max 15 sec in order for the stars to appear as dots. There’s ever so little streaking even then, but it’s only visible in the full scale original. And if you want real star trails, then you need a minimum of 15 minutes or otherwise those streaks will have an appearance of camera shake because they are too short to come across as genuine trails. That’s my star 15 rule – you know, sunny 16, moon 11 and now star 15!

Once you’ve decided which effect you want, there are other decisions to be made. ISO 500, f2.8, 15 sec (no moon) - and yes, there are treetops in this pictureLet’s say that I want to photograph the stars as dots and I’ve set the camera to 15 seconds. Now I have to decide how many stars I want to record (or how dark I want the sky to be). A wide open aperture is almost a must, so the only thing left to adjust is the ISO speed. ISO 200 is a starting point, it will give you the bright stars but you’ll probably end up going higher to get some of the fainter stars as well and more definition in the foreground (=better silhouette). You’ll want at least a little bit of moonlight to lighten up the sky, otherwise the sky will be just as black as your foreground which makes the foreground a moot point.

ISO 100, f14, 16 min, full moonBut let’s say that I want the star trail effect. ISO 100 is doable and you can stop down, anything to get a longer exposure. However, there’s a risk that the trails will be very faint with these settings because there just simply isn’t enough light hitting the sensor. With a long enough exposure you can bring out detail in the foreground, but those trails are stubbornly faint even for the brightest stars. I’m guessing it’s because the brightness of a star is kind of an absolute? The star will keep moving along during the exposure so it will not “collect” light the same way the foreground and sky does, thus you end up with faint streaks and only for the bright stars while the foreground and sky look correctly exposed. Just compare the long exposure on the left to the first picture which is much darker overall, but the stars are much brighter in comparison to the trails.

So for the star trails, the solution is to photograph on a dark night with maybe a moon crescent to lighten the sky ever so little. Then you can bump up the ISO and stop down without a risk of your foreground and sky becoming unnaturally light, all the while gaining enough sensitivity in the sensor to catch decent trails for even the faintest stars.

What did I say about learning by mistake? The next opportunity is in early December when the first quarter moon is up in the sky in the evening. Just no clouds and no wind…

You may also like...

4 Responses

  1. Rane Olsen says:

    Nice story :)

    What I’ve kept as a rule, is to divide 500 with the used focal length in millimeters. 500/50 gives you a 10 second exposure, but with 24mm lens you can expose for 20 seconds. With 15mm the number should be 33 seconds. 24/1.4L Canon lens is quite widely used for this purpose ;)

  2. Miika says:

    Thanks, now I know what to try this evening!

  3. Minna says:

    Rane, thanks for the tip! I’m using my 15mm fish-eye which is 24mm on the 40D. :) That should give me a 20 sec exposure but I think it’s too much… but it also depends on where in the sky the stars are? The closer they are to Polaris, the less they will appear to move because the circle is smaller.

    In the above pictures, Cygnus is further from Polaris than Cassiopeia so 15 sec gave me oblong stars for Cygnus and round dots for Cassiopeia.

  4. Rane Olsen says:

    Minna, I think that those numbers are valid for 35mm film, so, go ahead and try exposing ~30 seconds with your 15mm lens. -It doesn’t cost but could pay itself ;) The thing about the angle is right, IMO. The nearer the Polaris, the better. But, as said, it’s all about experimenting and finding the answers.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>