I'm going a little off topic with today's post, as it's nothing to do with child photography, but I've had an increasing number of requests for information on how to take a star trail photo, so I thought an article here would be useful to anyone else who enjoys standing outside on a cold, dark night staring up at the stars and getting strange looks from passers-by....
A star trail photo is basically just a very long exposure of the night sky. The Earth's rotation means that over the course of even just an hour or so, the stars change positions in our view of the sky quite a lot. And you can capture this perceived movement on camera really quite easily.
The photo on the left was a special star trail for two of my good friends who got married recently. I hadn't done a star trail with people in before but I was pretty pleased with how it turned out. Despite their giggling, they managed to stay pretty still!
So, how to go about getting a star trail photo....
* a dslr camera, with manual controls and a fully charged battery
* a sturdy tripod
* a cable shutter release that you can then effectively lock the shutter button down with (you can get these on ebay for about £2)
* a suitable lens, but any will do really. A few considerations come into play here when selecting a lens - composition, aperture, and focal length. The wider the focal length, the slower the stars will appear to move, so to get impressive star trails at 10mm you'd need to spend a lot longer than at 50mm, for example. But 10mm might allow a much more interesting composition, so it's something you'll have to weigh up depending on individual circumstances. And the wider you open your aperture the more light you'll let in - ie the more stars your camera will be able to see (but you'll have a shallower depth of field so you'll need to think carefully about your focus).
* a clear night, preferably with little or no moon (but it can be done if there is a moon - clear nights in England are rare so we can't afford to be too picky!). If the moon is out then you won't want it in your shot though.
* a location as far away from light pollution as possible for best results, but it can be achieved even in cities.
* warm clothes!
The two methods for getting a star trail photo:
1. A single, really long exposure. This is perhaps the simplest way, however with increased length of exposure comes increased noise. You also risk blowing out any highlights that might be in your foreground etc. I tend to avoid this method but have seen some nice examples using it and it can certainly be used to good effect.
2. Using free software to stack a series of 30 second exposures and create your final photo. This is how I do my star trails as it keeps noise down. It's this method that I will detail below.
1. Find something interesting to put in your foreground. Make sure it doesn't move or it'll become a blurry mess over the course of the long exposure. The bride and groom above were under strict instructions not to move for the 30 seconds they were in shot.
2. Decide on your composition. You'll want to consider not only what you've chosen for your foreground interest, but also the direction you're facing, as this has a big impact on how the star trails will look. For example, point your camera at the North Star and the stars will appear to move in circles around it while it remains stationary (I am assuming you're in the Northern Hemisphere). Other directions will give different curves but no circles.
3. Check you're happy with your composition and settings before spending a whole evening committed to them! To do this I do two things. Firstly, I take a single 5 minute exposure at a low ISO so that I can start to see a few star trails forming but the image isn't blown out. This allows me to tell if I've managed to get the position of the north star exactly where I want in the shot (of course if you're not pointing north then you can ignore this tip, although it will still give you an idea for the type of curves your star trails will form). Next, I take a single 30 second exposure at all the settings I think I will be using, to check I can see enough stars. A good starting point to experiment from would be a fairly wide aperture and ISO 400. If you can't see enough stars, either open your aperture wider or raise your ISO. I frequently find I'm using ISO 1000 to get plenty of star trails, but I do live in a light polluted area.
4. Check your white balance from your test shots and adjust if necessary, but I find that no white balance setting is perfect for this and I frequently end up making changing during post processing to the final image so don't worry too much about which one you choose. Just make sure you don't leave it on auto or the camera may well decide to change it half way through the series of shots.
5. Camera on manual, set your shutter speed to 30 seconds, and set your camera's shutter mode to continuous burst and attach your shutter release cable. From your experiments above you should now have decided which aperture and ISO works best for your shot. I usually have my aperture as wide open as possible and work my ISO from this depending on how the stars look in my test shot). I always shoot jpeg fine for star trails.
6. Focus. I tend to use hyper focal distances to ensure my whole shot is in sharp focus despite the wide aperture. I usually focus on my foreground object by shining a torch on it and auto-focusing on that spot, then switching it to manual focus whilst it's set on this point. Leave it on manual focus for the rest of the evening so the camera doesn't decide to change it!
7. You're good to go! Lock your shutter release cable button down and have a cup of tea! Stop the shots whenever you decide you're too cold/bored/scared of the dark... I usually find an hour produces nice results, but anything from 30 minutes to 12 hours(!) will produce nice star trails.
8. If you want to, you can also use your first and last shots of the sequence to light up your foreground interest a bit. I do it for both first and last shots and then I can keep whichever I think turned out the best and bin the other one.
9. I also take a couple of dark frames to use in the software for dark frame subtraction. Just take a photo using the same settings at the start of the sequence with your lens cap on, and another at the end of your sequence with your lens cap on. It helps improve the finished result as the software uses this to isolate any hot pixels etc.
Stacking the photos:
I use startrails.exe, which is brilliant freeware designed exactly for this purpose, but it only works on PCs. There's also starstax which works on MAC and PC. And, I haven't used it, but deepskystacker also works for this. All three of these are free to download, and very straightforward to use. You just import all of your photos, including any dark frames into the correct section, and the software does the rest - it really is that easy. They provide various options for blending and automatically filling any gaps etc for you to have a play around with. The whole process takes only a couple of minutes and some even provide you with the option to create a timelapse video of the photos.
I hope this is useful to some of you! And if you discover any great sites to take star trails then please do comment or let me know on my Facebook page as I'm always on the look out for interesting locations!