There\’s something strangely beautiful about star trail photographs. They\’re wonderfully abstract, but at the same time very, very real. I love how something as scientific as the rotation of the planet and be represented by something so captivating.
I\’ve been wanting to experiment with long exposures with my Nikon D90 for a while now, but never really had the patience. I\’ve had plenty of experience with long exposures in the past, but with a digital SLR there are other factors to take into account – such as noise.
A few months ago I made my first attempts at capturing the night sky over Ilkley in the UK – and very quickly realised that traditional long exposure techniques are not what was needed. I just pointed my camera into the night sky, and used a manual shutter release to take a series of around 25-minute exposures. I was actually surprised at the results (inset), because while they didn\’t capture as many stars as I would have liked, I was surprised by how clean and noise free the images were.
Having said that, DSLRs are not at their best when taking 30 minute long exposures, and the images that had inspired me on Flickr and other web galleries were clearly in a different league than mine. There must be something I was missing – clearly doubling my exposure time was not going to help -and was going to lead to extremely long nights of trial and error. I like to think I\’m patient, but I\’m not THAT patient!
After a bit of research I realised the trick I was missing. DSLRs performed very well at taking shorter exposures, and the results are captured digitally. So why not take a series of shorter exposures and composite them together? Well, it turns out that\’s exactly what I needed to do. Luckily, I\’d already purchased a intervalometer for my D90 (a remote shutter release that automatically takes a series of photos). I\’d bought it with timelapse photography in mind, but it would serve it\’s purpose here too.
About a month ago I was fortunate enough to be on holiday high in the hills of Umbria, Italy. The light pollution was extremely low, and it seemed a perfect opportunity to have another go.
The result is the photo you see featured at the top of this page. It was pretty much my first and only attempt on the holiday, and I\’m quite pleased with the result. It amounts to a exposure time of around 80 minutes, split into 10 separate exposures of around 8 minutes each. I used a 24mm prime lens set to f2.4, and with \’low noise\’ in mind, I set the ISO to 200.
The images were captured RAW, and imported into Photoshop as individual layers. Then, starting at the top, I simply set the layer mode to \’Lighten\’, and worked my way down – seeing the star trails extend with each step. Neat :D
Why 8 minutes? Why ISO200? No real reason, they just felt right. Unfortunately, the noise floor of the resulting image was higher than I expected. It turns out that ISO200 wasn\’t the best choice, and I see many other photographers using relatively high ISOs (1000-3200) for their photographs. I\’m keen to try again, and I\’m just waiting for the crisp, cold winter months to arrive here before I try again. I think the key next time will be a higher ISO, a much shorter exposure time (~1min), and many more photographs.