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17 February 2012

Star-Trail Photography

written by: Don

Images of star trails are maybe some of the most impressive pictures. Getting them right requires just a the basic camera equipments, some careful planning and a little bit of luck with the weather. This is how I do it.


Figure 1: Star trails over Kerala, India
Canon EOS 550D, Canon EF 17-40mm f/4.0 L USM
f/13, 2740s, ISO100

Intro

My first attempt on star-trail photography was out in Andalusia, Spain, when we were staying in a B&B out in the country side just south of Antequera. Far away from any light pollutions of the city, the night sky look absolutely amazing with a lot more stars visible, and that instantly prompted me to give star-trail photography a go. Not knowing much about it, I simply started to experiment with my camera and luckily, after a couple of failed attempts, I was starting to get decent pictures even during the first evening, and that showed me just how 'easily' this can be done. So, to share this with you, this is how I do it.

Equipment

  • a DSLR camera with a fully charged battery (we will really need a fully charged battery because we are working with very very long exposures - my battery died on me during my first attempt...)
  • a remote shutter release (cable or infrared)
  • a tripod
  • a torch (optional)
  • a clear night sky
  • quite a lot of time on your hand

Composition

First off, a tripod is indispensable for star-trail photography, since we are exposing for a long time to capture the movement of the stars in the night sky (it's kind of more like the movement of the earth, but never mind...).

Then, obviously, since the star trails are the main 'eye-catcher' of the image, we want to reserve a large portion of the frame for the sky. However, just photographing the sky without any other object in the frame would not be a good idea, because your image will look pretty lost. What we really want instead is to include an easily understood foreground subject into bottom 1/3 of the frame. This could either be a house (as seen in Figure 1), a plant or something similar. By doing so, what you achieve is to show something relatively commonplace for the viewer against a very unusual background. The combination of the two should ideally intrigue the viewer. With that in mind, star trail photography often means that we have to set up the tripod pretty low close to the ground, with the camera pointing diagonally upwards.

A couple of things to be borne in mind:

  • You will have to set your camera up in a dark place. Even your object should be in pitch darkness (It will eventually become visible on the photograph through the long exposure). However, this means that you will need to frame your picture in darkness as well, so be prepared to use your torch, if needed.
  • On the same note: pay attention to the moon. Do not include the moon into your frame and make sure it will not move into the frame throughout the exposure either. The moon is much brighter than the stars and will cause less star trails to be visible. In fact, just having the moon anywhere near the frame will give you less satisfactory results.
  • if there is no good foreground object available or if the framing of such an object is difficult for whatever reason, you can resort to using something in the middle ground instead, like a forest or a not so distant mountain. However, this is really not ideal and should be avoided whenever possible. The results (as seen in the Figure 2) is never quite satisfactory, because there is nothing close enough to the viewer to 'lead' him into the image.
  • When using a plant / trees as your foreground object, make sure that the night is windless. The last thing you want is to have a blurred out object in the foreground. If you are unsure about the wind conditions, try to stick to solid objects.

Figure 2: Star trails over Andalusia, Spain
Canon EOS 550D, Canon EF-S 18-200mm f/3.5-5.6 IS
f/4.0, 510s, ISO100

Camera Settings

Now that we have got our composition sorted, lets sort out our camera. We will be shooting in fully manual mode, because the conditions for star trail photography is so unusual that you cannot expect the computer to be able to have anything preprogrammed for you. I will try to keep it simple and make a check list containing all the crucial settings that you should check before firing. These are:

  • Aperture: The aperture should be set to as wide open as possible. f/4.0 to f/5.6 is probably a good place to start. This is because you want to capture as much light per milli-seconds as possible before the stars move on to another place in your frame. That is to say: the smaller your aperture, the fainter your star trails will become / the fewer star-trails you will capture. You can obviously use this as a tool of composition. In Figure 1, I have used f/13, which is rather small. The result is that not many star trails are visible.
  • Shutter Speed: The shutter speed can be varied to anything between a few minutes to several hours. The choice is only guided by how long you want the star trails to be - and how much battery life you have got left.
  • ISO speed: turn the ISO speed to the lowest value available. This is most likely either 80 or 100, depending on the camera. Any higher ISO number will increase the colour noise in the dark sky, and this will be very difficult to remove afterwards.
  • Focus: since you are shooting in the dark, your camera will find it very tough to automatically focus on anything due to the lack of contrast, so switch over the manual focus. Ideally, you should have set up your camera and framed your picture and all before sunset when there is still enough light available, but then hey, life"s too short! So, presuming you are trying to manually focus in total darkness like me, this is the trick I use: as long as the foreground subject is not too close to the lens, I simply focus on infinity and twist the focus ring back just a tiny little bit. I found that this trick works most of the time. If in doubt, make a test shot with the highest ISO speed (and hence fast shutter speed) and check the focus results before moving on. Remember to turn the ISO speed back to 80 or 100 afterwards.
  • White Balance: Given the low light, it is impossible for the camera to produce a correct prediction using auto white balance (AWB). That is not a big deal, however, as long as you shoot in RAW format. You can adjust the white balance later.
  • Other settings: Two useful settings are mirror lock-up prior to releasing the shutter (eliminates potential vibrations in the camera during releasing) as well as enabling the built-in long-exposure noise reduction tool (far better results than reducing the noise later). Both settings can be found deeply buried in the custom functions menu on Canon DSLRs. N.B.: especially the noise reduction will process the image as long as you have exposed, i.e. if you are shooting a 1 hour star-trail photo, the camera will be occupied for two hours. So what this means is that (a) you need to make sure your battery is going to hold up and (b) you effectively only get one shot per night if you are looking at the longer exposures, unless you are able to stay up all night and manage to somehow kill the time during each of the shots...

Execution

Now that we have sorted it all out, all you need to do is to release your shutter with the remote / cable release, then walk off and do something fun. I tend to go indoors and read a book then but obviously you can also just sit there and wait for 45 minutes or so. Just make sure you do not ever touch the camera until you have finished exposing (via the remote, of course).

And that's it! Once you have got the shot in the camera, you can transfer it to your computer and do the usual post-processing routine: white balance, exposure, sharpness etc.

So go ahead, give this a try. If you have any comments on this guide, any suggestions or if you have discovered any flaws, please do not hesitate and leave me a comment below. Have fun and happy shooting!

 

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