Wednesday, December 24, 2014

Northern Lights part 2 - Some practical considerations

If you tuned in for part 1 of this feature, you'll appreciate that it's important to understand how high you can push the ISO of your camera. Hopefully you'll have tested your camera to see how it performs. If you missed it, part 1 is here.

So, now you know what your camera can do. How are you going to take advantage of that? What's it actually like photographing the Northern Lights?

Well, before we talk about anything else, the first thing to remember is that it's probably going to be COLD. You can look at the temperature records for popular gateway destinations like Tromsø and Reykjavik and the Lofotens (WeatherSpark is a great source) and see that they have average overnight temperatures in the winter of around -5°C to -10°C. But on any given night it can be much colder. And if you venture inland, away from the coast, it can be an awful lot colder. As I'm writing this, it's -32°C in northern Finland, only about 100km from Tromsø. Of course, as they say in Finland there's no such thing as bad weather, just bad clothing. So make sure you look after your vulnerable extremities, and make sure you look after your camera batteries and keep them inside your clothing whenever possible. The cold can kill them very quickly.

Apart from being cold, it's also usually quite DARK. (During the winter season, hardly a week goes by here at LensesForHire without a customer conversation along the lines of "Northern Lights ... tripod ... dark ... tripped over ... smashed.") That's why you've tested the high-ISO capability of your camera - you have, haven't you? - but even so you'll want to be shooting with as wide an aperture as you can manage. And you'll want your lens to be wide too - auroral displays often take up a very large part of the sky. Most guides to photographing the Northern Lights will tell you that you need a wide and fast lens, but they don't often help you choose between wide-and-very-fast (e.g. 24mm f/1.4) or fast-and-very-wide (e.g. 16mm f/2.8). This one will. Bear with us.

We'll see some practical illustrations in part 3 of this series, but the short version for now is that in part it depends on your camera. If you have a very modern DSLR which can happily shoot at very high ISO values, then you don't have to worry so much about "fast" and you can concentrate on "wide". With an older DSLR that's more challenged at high ISO levels, "fast" is more important. So what we'll do for the purposes of this exercise is assume you have a typical consumer DSLR setup - a camera that can cope well up to ISO 1600, and an f/4 wide angle lens - and see how you get on with that.

Anyway, it's dark. How dark? Well, that depends on whether or not the moon is out. (You can check moonrise and moonset times, and a whole lot more besides, using The Photographer's Ephemeris.)

Let's look first at the situation where the moon is out. The landscape will be illuminated by moonlight, and that pretty much defines your exposure for you.  You have to set the exposure to expose correctly for the landscape, and that's that. (A photo of the aurora with no land in it wouldn't be very satisfying. Too abstract, and no sense of scale.)

You probably know the Sunny 16 rule: in bright sunlight, at f/16, the correct exposure is the reciprocal of the ISO setting. So 1/100th of a second at ISO 100, 1/400th at ISO 400, and so on. Obviously you can use this to calculate the correct exposure time for any combination of ISO and aperture. For example let's look at your assumed limiting conditions of ISO 1600 and f/4. ISO 1600 is 4 stops more exposure than ISO 100, and f/4 is 4 stops more exposure than f/16, so you'd need your shutter speed to be 8 stops faster than 1/100th, which would be ... something which no regular camera can achieve. Obviously. Shooting at ISO 1600 and f/4 in bright sunlight would be madness.

But moonlight from the full moon is about 17-18 stops fainter than sunlight. (Reference.) Plus, I think a moonlight photo would want to be underexposed by a stop or two - you want it to look like moonlight, nor sunlight. So at ISO 1600 and f/4, you're looking at shutter speeds which are 7-9 stops more exposure than 1/100th, which is around 1½-5 seconds. That's quite workable, I would suggest.  With an ultra-wide lens, your stars will still be points of light (i.e. no visible trailing) as long as your exposure is less than 20-30 seconds. (Reference.) And if the aurora is moving, it shouldn't move too much in that sort of time.

So that's an important result.  If the moon is out, you should be fine with consumer-grade DSLR equipment like we've assumed. What you'll see by way of aurora in your photos depends, of course, on how bright it is relative to the landscape. If it's bright enough, you'll get good images.  If it's less bright, your best bet is probably to take composite images, one for the landscape and one for the sky with different exposures, and merge them later.

But around new moon the illumination is much harder to quantify - starlight, aurora, street lights or village lights perhaps, clouds, light from the moon or sun below the horizon scattered by the atmosphere, and so on. Determining the correct exposure is much more difficult and tends to be much more trial-and-error. We'll look at that, and look at some specific equipment recommendations, in part 3.

1 comment:

  1. Oh, I'm so glad it's not just me that has knocked over a tripod that way. One of my several mistakes was not taking my own tripod and learning how it worked properly first. And I gave up trying to argue with the insurers (have you had the filter inspected at a camera shop? no, there are no camera shops where I live...) but Nikon fixed the lens.
    It was worth the shock of seeing lens in pieces and the financial loss to have seen the aurora and got any pictures at all. We'll go back and I'll know better.