Arnoud Quanjer Wildlife Photography

I'm primarily a wildlife photographer, but besides wildlife photography a think that I like to do is star trail photography. Star trail photography is basically taking long exposure pictures of the night sky. As the world turns in relation to the stars a long exposure picture will result in the stars making 'trails' in the picture. The longer the exposure, the longer the trails will be. How long you expose for is up to you, I usually make one to one and a half hour exposures. The technique used for photographing star trails is very easy. You setup your camera on a tripod, set the camera to bulb mode and use a cable release to make a long exposure picture. Although this sounds easy enough there are a couple of things to keep in mind though. These things are exposure, composition and focus, camera settings and battery power.

Exposing your star trail

When you shoot your first star trail picture it's easy to get too enthusiastic and immediately go for very long exposures. If you use long exposures you get long star trails and that's exactly what you want right? Under perfect circumstances that is correct, unfortunately the circumstances are hardly ever perfect. In most parts of the world there will be cities nearby and this ambient light in the sky can ruin your picture if you expose for too long. The light that your eye sees can pick up is nothing compared to the sensitivity of a digital sensor. The ambient light that the sensor picks up during a long exposure can be so overwhelming that you don't see any stars at all in the resulting picture. Another thing that can ruin your star trail is the light that the moon reflects. When the moon is full you can forget about taking any star trail pictures, the reflected light from the moon will be much too bright. Another thing that can ruin star trail pictures are clouds. Ideally you want to make star trail pictures on a cloudless night as clouds reflect lots of light. If the clouds are not too dense then they might give an interesting effect in the resulting star trail, but in general they just ruin the shot.

To sum up: it's best to shoot star trails on a cloudless night, far away from cities and either when the moon hasn't risen yet or during a new moon. From my experience I can tell you that these three conditions unfortunately seldomly coincide :(

Composition

One of the most important things in star trail photography is composition. A picture of just star trails doesn't make for very interesting picture. For a star trail picture to be aesthetically pleasing you need to find an interesting foreground object like a tree, a formation of rocks, a house or whatever else you can find. If you're shooting under perfect circumstances (no moon, cloudless night and no ambient light) then getting the foreground object correctly exposed can be a problem. Under these circumstance the foreground object will not reflect enough light and will thus look a bit dull in the resulting picture. If this is the case then you can experiment a bit during the exposure. You can for instance use a torch or a flash gun to extra expose the foreground. It takes some practice to get the exposure right, but once you et a feel for how long to expose the forground object you can get amazing shots. You can also experiment with different colours of light by using filters.

Setting the focal distance (focussing)

When setting up your camera you need to make sure that the foreground object and the trails are in focus. Depending on how far the foreground object is away from you this may mean that you need to select a small aperture to get the whole scene in focus. I generally select a foreground object that is at least 5-10 meters away from me and I shoot using a wide angle 10-22mm lens on my DSLR (16-35mm equivalant for normal 35mm cameras). Using wide angle lenses getting the whole scene in focus is usually not an issue as the infinity mark is usually close to the lens. Ideally you'd have to set your camera to focus near the hyper focal distance of your lens. The hyperfocal distance is defined as "the point of focus where everything from half that distance to infinity falls within the depth of field.". What this means in laymans terms is that at every aperture every lens has a distance setting that produces the greatest depth of field. Landscape photographs are often taken with the lens focused at the hyperfocal distance; near and distant objects are sharp in the photos. This also works for star trails as you're usually also working with wide angle lenses when taking star trail pictures. Normal to wide-angle lenses are good lenses to use for star trail photographs as these lenses all have a relatively short hyperfocal distances even when set to small f-numbers (large apertures). A 16mm lens set to f/2.8 for example has a hyperfocal distance of around about 4,80m. This means that everything from 2,40m to infinity will be sharp in a photograph taken with this lens focused at the hyperfocal distance. This also means that focussing further away, for instance at the horizon or a distant tree, is useless. Determine the hyperfocal distance of your lens before you take the shot and focus at half that length. It's probably a good idea to calculate the hyperfocal distance at the various apertures for your lens of choice and write them down before you go on a trip.

When you determine the hyperfocal distance of your lens it's important to take into effect the extension factor of your camera if you're using a DSLR. If you're for instance using a 24mm lens on a Canon 30D then the 35mm equivalent of the lens will be a 38,4mm lens as the 30D has an extension factor of 1.6x. You can use one of the many available online hyperfocal distance calculators to calculate the hyperfocal distance for your lenses on the various aperture settings. A good calculator which takes into account the extension factor of your camera can be found here.

Circular trails or stripes?

When you set up your tripod for a star trails picture you should consider the kind of trails that you want to show. If you're on the Northern hemisphere and you point your camera North then the stars will make circular trails. In the Northern hemisphere you can find North by locating Polaris. If you want to make circular star trails in the Southern hemisphere then you need to point your camera to the South. In the Southern hemisphere you can find South by using the "Southern Cross" constellation. When you have found the Cross and the Pointers, take an imaginary bisector of the pointers and extend this line. Now draw another imaginary line through the long axis of the cross and extend it. Where these two lines meet is Celestial South Pole. By dropping a line from the celestial South pole to the horizon will give south. It appears difficult, but once you have tried it, you will see how easy it really is and with a little practice you can simply look up at the Pointers and Cross and find South immediately. If you don't want circular trails then point your camera away from the Southern Cross or Polaris and you'll get stripes instead of circles. You can find a number of examples in the photo gallery at the end of this page.

Getting the horizon straight

Obtaining focus at night can be extremely difficult. In your viewfinder you will struggle to see any stars, and if you see a star then obtaining focus on it will be near impossible. As I use a wide angle lens my infinity focus point is pretty close to the lens which gives me the opportunity to use a torch to light illuminate an object close to me so I can focus on that . As the foreground object as far as my lens is concerned is as far away as the stars (eg beyond the infinity focus of the lens) getting the foreground object in focus will make sure the stars are in focus. Besides getting the focus right I've found that it is extremely difficult to get the horizon straight in nighttime pictures as you can't really see the horizon. I've solved this problem by using a Hama Double-Bubble Spirit Level which fits the flash hot-shoe of my camera. Using this spirit level it's extremely easy to see if the camera is level.

Noise reduction

If you are shooting digital then there are a couple of settings that you need to check. If your camera has got an option to reduce noise by performing a 'black-frame subtraction' then you need to turn this function on. This will dramatically reduce noise in the picture. You also need to select your lowest ISO number to reduce noise. Most camera's offer a 100ISO mode and some professional DSLR's even offer a 50ISO mode.

Battery power

Most camera's use battery power to keep the camera's mirror up during a long exposure. This means that your battery will drain during the shot. You need to make sure that the battery doesn't run out while making the shot because that would mean that the image is not saved to your memory card. If you've enabled 'black frame subtraction' then you also need to take into account that after you take the picture the camera will take another picture for performing the black frame subtraction. The exposure time of this black frame subtraction picture is identical to the initial exposure. This means that if you make a one hour exposure that the camera battery will need to last for two hours! Always use a fully charged battery when making star trail pictures!

The most important thing in star trail photography is practice, practice, practice! I've only recently started making star trail pictures and I learn from every picture that I take. I hope that I'll get it right in a couple of years.

Stacking images

A while ago I read about another technique to make star trails on the Internet. In this technique not a single long exposure image was made, but a series of short exposures which were then stacked and turned into one star strails image. In this technique you make exposures of around 30 seconds using the Tv mode on your camera and a remote release. You set the camera to continuous shooting and shoot as many pictures as you want. When the camera has exposed for the set amount of time the shutter will close, but because the camera is in continuous shooting mode and the remote release is still engaged a new image will be made instantly. I tried out this technique in Africa in January 2008 and found out that it has some distinct advantages, but also some nasty disadvantages.

On of the main advantages of shooting a series of relatively short exposures is that the noise is reduced by a drastic margin. If you use long exposures you'll need to use the in-camera noise reduction because otherwise you'll end up with too much noise. Using short 30 second exposures the noise problem is not that bad. Another advantage of shooting a series of images is that you can let the camera take pictures until the battery dies. When it dies you'll lose only the last picture, but the rest will already be stored on your compact flash card. When you're taking long exposure star trails you'll lose the complete star trail picture if the camera runs out of juice. There are also some disadvantages of using this method though. Most DLSR's can perform automatic noise reduction when using long(ish) shutter speeds. What the camera does is it takes the image and after the shutter is closed is takes another picture using the same exposure time. This image will be substracted from the original image thereby removing non-image artifacts like hot and cold pixels and static interference. The drawback of noise reduction is that with cameras you can't take a picture while the camera is performing the noise reduction. This means that if you take 30 second exposures for your startrail that you will miss 30 seconds each shot. This leads to visible gaps in the trails. This means that you need to turn off noise reduction if you want to use the multiple images stacking technique.

The biggest drawback of this method for me is that if you shoot in a situation where you don't have a lot of light illuminating the foreground that you can't get a properly exposed foreground. I made a one hour star trail using 30 second exposures and was not impressed with the result. I probably have the ultimate camera for using this technique as I use a Canon 1D Mark III which can perform noise reduction while taking pictures, but I was not impressed. The results I've gotten using long exposures are infinately better. I think that this technique of stacking multiple images will work like a charm with objects that are brightly lit, for instance the sky line of a city, but in near total darkness it doesn't work for me. If you have another opinion and have succesfully used this technique please drop me an e-mail to let me know.

For turning the stack of images into one star trail image I used Startrails.exe from startrails.de. This is a briljant little freeware program which does exactly was it says.

Star trail photo gallery

These star trail images were all made during my trips to Southern Africa.

• Mpila camp, Imfolozi Mpila camp, Imfolozi
• Mlilwane Wildlife Sanctuary, Swaziland Mlilwane Wildlife Sanctuary, Swaziland
• Twee Rivieren, South Afica Twee Rivieren, South Afica
• Twee Rivieren, South Afica Twee Rivieren, South Afica
• Kalahari Tented Camp, South Afica Kalahari Tented Camp, South Afica
• Augrabies National Park, South Afica Augrabies National Park, South Afica
• Dune Camp, South Afica Dune Camp, South Afica
• Quivertree Forest, Keetmanshoop Quivertree Forest, Keetmanshoop
• Quivertree Forest, Keetmanshoop Quivertree Forest, Keetmanshoop
• Aba-Huab campsite, Namibia Aba-Huab campsite, Namibia
• Urikaruus wilderness camp, South Afica Urikaruus wilderness camp, South Afica
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