Tuesday, June 21, 2011

How to... top tips for urban star trail success

For years I'd heard people say urban star trails couldn't be done: the ambient light's too great; you need to get far away from any light pollution; everything turns orange. Stand in the middle of a city on a clear night and you'd be forgiven for believing it. From down amongst the garish glow of streetlamps and neon signs promoting food you'd never eat when sober it's near-impossible for our eyes to pick out pinpricks of starlight high above.

For our cameras, though, it's a different story altogether. f/2.8 is f/2.8, wherever you happen to be pointing your lens- as far as environmental awareness is concerned, the camera is pretty dumb. So I figured it was worth searching out a composition to disprove the theory and see what happens when you shoot urban long exposures at wide apertures.

There's one thing in particular that continues to fascinate me about capturing urban star trails and that's how the result can appear to flip reality. You might expect to see a human element in some frames but at ground level there's just stillness. Instead, what you don't expect is how the sky's brought to life by the earth's perpetual motion.

So that's my take. Whatever your motivation, the complexity of shooting urban star trails means the results can be very rewarding. Before you get out there and try it for yourself here's a handful of tips to point you in the right direction.

Ditch the wide angle and look up.
You need to get above the level of the streetlamps. Most of the time that's not physically possible so let a longer lens do the job for you - my 35mm and 50mm primes have seen lots of use for this reason. Avoid areas in direct street lighting, which usually means compose your camera upwards, but try to find an angle that captures an ambient glow to shed a little light on your foreground.

Shoot wide open.
Stars don't disappear from view just 'cos you're at an urban vantage point, they simply become harder to see with the naked eye due to the contrast between dark sky and ambient streetlighting. If you can find a composition that lets you shoot long exposures at f/2.8, the stars will be there. Use exposure duration to control the ambient light - how bright the scene looks is down to personal preference but I tend to use 15-30 second exposures and run consecutive frames for 20-60 minutes.

Learn your hyperfocal distances.
You might think shooting a scene at f/2.8 would give you depth of field headaches but there's a factor of DoF that often gets forgotten. Sure, it's about focal length and aperture but the final piece of the triangle is focal distance - how far away you are from your subject. Hyperfocal distance gives you the data you need to find a winning composition with your lens wide open. Get the data you need at DoFmaster

Get orientated.
If you want circular trails, find Polaris (the pole star) by running an imaginary line from the end of the scoop of The Plough constellation. A good starting point is to face north and look up. For straighter trails, shoot south or a few degrees either side. In the same way the rim of a bike wheel covers more distance than the hub in the same time, pointing your camera at Polaris means a long wait for extended star trails, whilst shooting away from the pole star makes your trails seem to grow at a quicker pace.

Set up the camera right
If you're shooting RAW you can skim this bit as you'll make your adjustments in PP. I tend to shoot JPG - stacking using StarStaX freeware is so much quicker than using PhotoShop so it matters how the camera's set up.

> White balance: neutralise the orange colour cast from streetlamps by manually setting your WB all the way down to 2500K. This adds blue back into the image.

> Contrast: max it right up to cut through the city haze that often appears around well lit areas.

> Brightness: your pic's going to be pretty bright anyway, you can probably keep this at zero.

> Saturation: adding saturation also adds contrast so a little can be a good thing. If you can't shift the urban colour cast with manual WB, though, the option's there to turn down the saturation- or you could convert to mono in post. Shooting star trails using mono in-camera is an unreliable method as often the toning can eliminate subtle star trails that would be captured in colour and could be retained by subsequent careful conversion to mono.

Know when to give up
Not every excursion ends with a successful image. If your efforts are coming to nothing because of location, weather, streetlighting or some idiot trying to jump in front of your camera (I've packed up on occasions because of each of these), it might just be a sign to take an early bath.

There's always the chance you'll reflect on what you did manage to capture and appreciate it for other reasons. The shot above got clouded out after just a few minutes but I've reached the conclusion that trails any longer would have over-complicated the image.

If reading this has left you inspired to get out and try it for yourself I'd love to see what you come up with. Feel free to outlink to your work in the comments below.