how to photograph the night sky.
1.0 Exposure
1.1 Shutter Speed
1.2 Aperture
1.3 ISO and Signal Noise Ratio (SNR)
2.0 Focal Length
3.0 Focusing At Night
4.0 Recommended Gear
5.0 Processing (tbc)
6.0 Cheat Sheet (tbc)
1.0 Exposure
One of the biggest challenges to capturing a photograph of the night sky, is getting a proper exposure (aka getting enough light). During the daytime this is easy, as there is so much light from the sun to capture, but at night things get much harder.
In order to do this, you need to understand the exposure triangle, covered in Sections 1.1, 1.2 and 1.3.
The importance of exposing a photography correctly is two fold, you will capture a significant amount of detail and color in the night sky, and you will also reduce noise - covered in section 1.4.
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Ensure all photos you take are in RAW format. This can be changed in your cameras settings, and will keep your photo data uncompressed.
1.1 Shutter Speed
The first aspect of the exposure triangle is your shutter speed. Quite simply, this refers to the length of time that your camera shutter remains open, recording the incoming light. For daytime photography, shutter speeds of anywhere from 1/50 seconds to 1/4000+ seconds might be used (aka 0.02 and 0.00025 seconds respectively). But at night this is not even close to sufficient.
A typical exposure at night will require anywhere from 5-10 seconds at a minimum to 30+ seconds. The number here is highly circumstantial however, and longer exposures require dedicated equipment.
This doesn't seem like a big problem though, why not just open the shutter up for more than 30 seconds? why not 5 minutes? 10? The answer here is that the stars are moving, if you keep the shutter open for too long, they will stop looking like stars and start looking like streaks. This phenomenon is referred to as star trailing. Now you can probably see the issue, shorter shutter speeds will underexpose the image and longer shutter speeds result in star trails - so how do we solve this?
Well unfortunately we need to make a compromise between the two.
A general rule of thumb is the "rule of 500". Basically divide 500 by your lens focal length (eg 20mm) to obtain the maximum shutter speed before star trailing occurs.
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eg 500/20mm = 25 seconds
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This is far from perfect but it works okay for beginners. The rule of 300 is a little more accurate but it depends heavily on factors like the declination of your target (eg. orion will trail much faster than the north star), and the size of the pixels on your camera sensor (smaller pixel reveal star trails before larger pixels). Personally I think small star trails are a worthwhile trade off for a better exposure, and as such would still recommend the rule of 500.
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1.2 Aperture
Aperture is the next piece of the exposure puzzle, and refers to the amount of light coming in through the lens. This is represented as a fraction (focal length over lens width) but you don't need to remember that - just keep in mind that the lower the number (eg f/1.8) means more light than a higher number (eg f/16).
In order to get a good exposure, you really want to have a fast lens (aka a low f number). As usual, the particular aperture needed is circumstantial, but a general recommendation is:
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>f/4 - not good
f/3 - f/4 - okay
f/2 - f/3 - good
<f/2 - excellent
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Anything above f/4 wont be able to capture sufficient light, and therefore a lens with an aperture wider than f/4 is recommended.
Now if that's all there was to aperture this would be nice and easy right? Unfortunately, photo quality often suffers at wider apertures. Therefore we need to make another compromise, as a wider aperture will give a better exposure, but will also cause a number of optical issues such as aberrations. These can be seen on my Canon EF 50mm lens (when shooting wide open at f/1.8).
The way to solve these issues is by reducing the aperture, but this will also reduce the light. The easy solution here is to buy a high quality lens that suffers minimal optical issues at wide aperture, but that's not always a realistic option. My advice would be to reduce the aperture as much as needed, but not beyond f/2.8. Any narrower and your photos will become too underexposed, a much bigger issue than aberrations.
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1.3 ISO
ISO is the last, and least well understood aspect of the exposure triangle - and the reason for this is that it doesn't really have anything to do with exposure. ISO is simply the gain applied to the sensor data, so a higher ISO will look like a brighter image, but only because the camera has digitally increased the brightness (it is of course more complicated, but this is the basic idea of it).
The problems with ISO are twofold, if you increase ISO too much, you begin to reduce your dynamic range - basically meaning bright things will locally overexpose the sensor and clip the data. And secondly, you increase the visible noise.
There is a misconception that a high ISO adds more noise but this isn't really true - it simply applies a gain to everything on the sensor (including noise) and as such, the already existing noise is amplified.
The result of noisy images when using high ISO isn't that the ISO is creating extra noise, but rather the photo has too little signal. This is what we call the signal to noise ratio (SNR). The goal of a good exposure is simply to produce enough signal to drown out the noise, if you do not do this (such as using a narrow aperture or fast shutter speed) all you will see in your image is noise as the SNR is poor.
ISO does not usually change your SNR*, only the amount of light that you capture does. (*although again it is more complicated than this due to different types of noise, and depends on the electronics of a given camera).
In general for night sky photos, I recommend an ISO of somewhere between 1600 and 6400 depending on your settings for shutter speed and aperture - but ISO 3200 is often a good starting point.
2.0 Focal Length
Focal length is what determines your field of view, and therefore how much of the sky you can squeeze into a single photo.
For beginners I would recommend a wide angle lens, something ideally between 14mm and 20mm. Not only will this allow you to capture longer exposures (rule of 500 from section 1.1), but you'll be able to see a wide section of the night sky without much difficulty.
If you want more detail in your photos, that's where longer focal lengths become useful. I personally use a 28mm, a 50mm and a 135mm lens for my detailed photos, and a 14mm for my wide angles (which I talk about here). The issue with longer focal lengths is that you need to start using a tracker to expose the photos properly - a device that moves your camera with the rotation of the earth to keep the stars still in the frame. This is the main reason I would avoid longer focal lengths at first.
3.0 Focusing at Night
Focusing your lens at night is another challenge unique to astrophotography, but one that can be mastered quickly. The key thing to know is that you will need to use manual focus.
To achieve focus at night, follow these steps:
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- switch your camera lens to manual focus
- put your camera screen onto live view mode
- point your camera toward a bright star/planet etc
- find the bright star in your camera live view (if it isn't visible try turning the focus ring until it is)
- zoom in digitally on the bright star (at least 10x zoom)
- slowly turn the focus ring on the lens until the star is pinpoint
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and you're done. Achieving perfect focus can take some practice, but you should be able to get decent photos just with these steps. Additionally you can use a Bahtinov mask if you want to make things even easier - but I personally don't bother to use one.
4.0 Recommended Gear
You don't need expensive gear to take nice photos of the night sky, but there's no denying it does help.
Any cheap DSLR with manual settings will be okay for this, but a full frame sensor camera (such as the Canon EOS 6D) will work best.
For lenses, again I'd recommend something wide angle and wide aperture. A good starter lens would be something like the Samyang/Rokinon 14mm f/2.8, but anything similar will be okay. Again, see the lenses I currently use here.