October 22, 2021
Winter is the ideal time to try your hand at astrophotography. (After all, there’s more night than day this time of year!) So, here’s an interesting fact: there are roughly 6,000 stars in the sky that can be seen with the naked eye (younger folk – with their better eyesight – tend to do better than old ‘uns). Unfortunately you can’t see all 6,000 in one go as only one small section of the sky is visible at any one time. That, and the fact that there’s often a planet in the way. The earth is divided into to two hemispheres: north and south. There are stars that are only ever visible to observers in the northern hemisphere and some that can only be seen from the southern. Take Polaris (or the North Star), this is an important - if rather dim - star. Polaris is a northern hemisphere star. Once you’re south of the equator you won’t be able to see it. (We’ll come back to Polaris in a moment.)
Light pollution from street lighting is typically seen as an orange glow. Modern LED street lights produce less light pollution and have a much colder light. (18 minutes at f/4, ISO 800)
Ambient lighting has an effect on the number of stars you can see too. Light pollution – from streetlights typically – will significantly reduce the number of stars you can see. The moon has an effect too; if you want to see – and ultimately shoot! – lots of stars then it’s best to wait until a moonless night. Ideally you want as little ambient light as possible and this usually means the countryside. If you’re lucky you’ll live close to a dark sky park. This an area where efforts have been made by local authorities to reduce the effects of light pollution, making the night sky as dark as possible.
Cameras can – with the right settings – record far more stars than can be seen with the naked eye. Starlight is relatively faint however and it’s not wholly straightforward to produce a pleasing photo. There are two solutions to overcome this problem. The first is to leave the shutter open for as long as necessary until a satisfying image has formed. The second is to increase the ISO so that the sensor is effectively more sensitive to light. This means that the camera can then record faint stars with even relatively short shutter speeds. Both create more problems, however. (Life is never simple, is it?) Let’s start with the long shutter speed approach first.
Star trails and long shutter speeds
The earth constantly rotates and it’s this movement that cause the stars (including the sun) to move across the sky. With a sufficiently long shutter speed stars won’t appear as a point of light at all but will create a visible trail - the longer the shutter is open, the longer the resulting trail. However, this is quite a neat effect and shouldn’t be dismissed. It’s not how we naturally see stars, but it does create an interesting sense of movement. There’s no right or wrong length of time to shoot star trails. However, 10 minutes is the minimum to see a reasonable trail, with an hour or more better still.
If you’re including a landscape feature pick something that will make a good silhouette against the sky, such as a tree. (21 minutes at f/8, ISO 800)
Stars appear to rotate around an imaginary point in the sky known as the celestial pole. In the northern hemisphere, it’s Polaris that sits almost – but not quite! – on this point. Sigma Octantis is the equivalent in the southern hemisphere. Unfortunately, neither star is particularly bright, though Sigma Octantis is by far the dimmer of the two. If you point your camera towards Polaris/Sigma Octantis and shoot a long exposure then the star trails will arc around these stars. If you were able to shoot an exposure of 24 hours then the arc would join up to form a circle. (You’d obviously need 24 hours of darkness, but this is possible above the Arctic and below the Antarctic Circles during their respective winters)
Unfortunately, there are two more problems to solve when shooting star trails: battery life and long exposure noise. Digital cameras are completely dependent on battery power. Once it’s gone you have to stop shooting. Shoot a long exposure and there’s a danger that the battery will run out before the exposure is finished. (Which means you’ve lost the shot) So, before shooting it’s worth checking that your batteries are charged to 100%.
Long exposure noise builds up during… well, long exposures. Cameras have a neat trick to get round the problem called long exposure noise reduction. Unfortunately, it takes the same length of time to remove the noise as it did to shoot the original image. (With the camera usually locked up until the process is complete.) I tend to switch long exposure noise reduction off before shooting. In fact I now don’t shoot long exposures star trails. Instead I shoot lots of 30-second exposures, one straight after the other, and then blend the results together in post-production. That said, here’s the procedure for shooting a star trail in-camera:
Turn Long exposure noise reduction off before shooting if you intend to shoot several long exposures over the course of an evening. If you don’t you’ll only be able to shoot half as many photos during the session.
What you’ll need: tripod, freshly-charged batteries, remote release, and (optional) stopwatch.
Using higher ISOs will reveal more stars, but this can make the final shot look ‘busy’. 124 separate shots blended (each shot 30 seconds at f/4, ISO 800)
It is possible to shoot with a long exposure and avoid trails with a piece of equipment known as an equatorial tracking mount. This is a mount with a built-in motor that moves the camera so that it precisely follows the movement of the stars. This keeps the stars in the same position within the image frame. The result are pin sharp images, without the penalty of noise from using a high ISO. The downside is cost: equatorial tracking mounts aren’t cheap, but they do help to capture images that would be impossible otherwise.
As mentioned, increasing the ISO does allow you to shoot with a relatively short shutter speed. (Though still long compared to ‘normal’ photography!) Increase the ISO high enough and you would be able to use shutter speeds that are mere fractions of a second rather than whole seconds. The problem is image noise. A high ISO adds noise to an image – noise that could well obliterate the stars you’re trying to shoot. That said, modern sensors are incredible and produce great images even at higher ISOs, so there’s less reason now not to use a high ISO. Full-frame cameras come out best in this regard, but APS-C/cropped cameras aren’t too far behind.
Off-camera flash is great if you need to add a bit of light and detail to a foreground during an exposure. (30 seconds at f/8, ISO 400)
There is one factor that you still need to consider to shoot pin-sharp stars: the focal length of your lens. The longer the lens, the more quickly star movement is apparent. You’ll therefore need to use a shorter shutter speed when using a long lens than when using a wide-angle. Fortunately, there’s an easy way to calculate the shutter speed to use.
All (!) you have to do is divide 500 by the focal length of the lens multiplied by the crop factor of your camera (or as an equation: 500/crop factor x focal length*). The result is the time in seconds that should produce pin-sharp stars (providing you’ve focused correctly) with the lens you’re using. So, for example, with a 50mm lens on a full-frame camera you would set a shutter speed of 10 seconds or faster (500/1 x 50). The same lens on an APS-C camera would require a shutter speed of 6.25 seconds (500/1.6 x 50). The difference is simply due to the fact that a 50mm lens is more ‘telephoto’ on a crop-sensor camera and so requires a faster shutter speed. Now, this calculation is a bit rough and ready so it’s a good idea to experiment with shutter speed until you find a value that works for you.
That all said, here are my checklists when shooting stars:
What you’ll need: tripod, freshly-charged batteries and remote release.
1) Set up before it gets dark if possible, when there is no moon and the sky is clear with a low chance of cloud for a few hours.
2) Select your lens and calculate the shutter speed required.
3) Choose your composition.
4) Switch the lens manual focus and focus on ∞.
5) Set the ISO to 400 initially.
6) Switch the camera to manual exposure and select the lens’ maximum aperture. Set the shutter speed calculated above.
7) Make an exposure using the remote release to fire the trigger.
8) Experiment with varying the exposure, particularly the ISO.
Have fun shooting stars, and don’t forget to wrap up well. It can get very, very cold standing around while your camera exposes…
Polaris is the star at the centre of this circle of stars. The tree was lit by a torch during the exposures. And the straight lines of lights are aircraft passing through the shot over the 55 minutes it took to shoot the sequence. (109 separate shots blended (each shot 30 seconds at f/4, ISO 200))
This guest post was wrote by David Taylor.
November 19, 2021
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