Tutorial - Astrophotography - Part 2

In Part 1 of this Astrophotography tutorial, we looked at how to pick your lens, make sure you have a tripod as well as consider the light pollution and weather conditions. Now in this section we will look at shutter speed, stabilization, focus and actually taking the photo.

The 500 rule of Astrophotography

While keeping your camera perfectly still is crucial for astrophotography, because your light source is in fact tiny pin-pricks of light. Ideally, you want them to show up as dots of light rather than short lines due to movement in the camera. When your subject is something that isn’t on the earth, you realize that you and your camera are never actually perfectly still. The fact is, we’re all actually moving thousands of miles an hour around the sun, and the stars that we’re photographing aren’t. So, enter the 500 rule. The 500 rule is very simple and very useful. Divide 500 by the focal length of the lens you’re using, and the number you get will be the number of seconds you can leave your shutter open before the stars start to move in your frame. This is another reason why the 24mm is such an appealing option for astrophotography. 500 divided by 24 gives you a maximum length shutter speed of 20 seconds.

500 / 24mm = 20.8 sec

If you have a 15mm lens, you could leave your shutter open for 33 seconds before the stars would start to move in your frame and begin to look like your camera moved during the photo. As you can now imagine, telephoto lenses are not what you want for photographing the night sky. Even a 50mm lens would only allow you to use a 10 second exposure. Even if you’re shooting with a lens like the 50mm f/1.2 from canon, that 10 second shutter speed is going to force you to bump up your ISO higher than you’ll want to in order to get a properly exposed photo of the stars. This all changes if you’re shooting star trails and want the stars to move through your frame; but that’s a subject for another tutorial.

Turn off your image stabilization

Not all lenses have images stabilization, but if the one you’re using to shoot the nice sky does, make sure you turn it off. Image stabilization is designed to compensate for your hands minor shaking when hand holding your camera. Optics inside your camera or lens (depending on the brand) actually move to compensate for your movement to keep the frame more stationary. This is a great feature, but when you’re camera is on a tripod and not moving, that motion the lens creates ends up destabilizing your frame instead of stabilizing it. If you leave your image stabilization on during a long exposure, you will end up with a blurry image.

Setting focus for Astrophotography

Just like you need a camera with full manual control to photograph the stars, you also need a lens with manual focus. For the time being, I’m unaware of any camera that can autofocus on a star filled sky; so you’re going to need to focus your lens manually. This can be a challenge. Most lenses do not have a manual focus that locks off or stops at infinity; which is where you’re going to want to have your focus set. Depending on the lens, it may have the ability to focus beyond infinity, which isn’t as cool as it sounds, it just means everything is blurry. If your lens has this extraordinary ability, there two things you can do.

1. If you’re in an area where there is a light in the distance, you can turn your camera into live view mode, digitally zoom in on the light with your camera, and manually focus your lens. Most digital SLRs allow you to digitally zoom in while in live view much like you would when zooming on after you took the picture. The view doesn’t change, it just enlarges a portion of the frame. In order to see the light well enough to set focus in live view, you may have to adjust your settings. Once you have your focus locked in on that light in the distance, you can go back to using your viewfinder and aim the camera at the sky again. Just make sure you don’t accidentally move your lenses focus ring.

2. This way involves a little bit more planning. You can set your camera up in the middle of the day, pick a subject at your lenses infinity focusing point, and then put a strip of tape on your lens’ focus ring to hold it in place for your night shoot. This is a technique I’ve never used, but apparently it’s fairly popular. Just make sure you use a tape that will come off without leaving anything on your lens.

Don’t forget your shutter

This ties in with the concept of keeping your camera very still. When your cameras mirror opens and closes, it causes your camera to vibrate. It’s a very minor vibration, but it can still be enough to soften an image when taking night shots. There are two ways to deal with this. One is to go into your cameras settings and select “mirror lockup.” When this is enabled, your camera will lock the shutter up when you push the shutter button, and then take a photo the second time you push the shutter button. This eliminates the mirror movement during the actual photo taking process. However, not every camera has this option, so the second thing you can do is shoot in live view. Live view locks the mirror up so that your sensor can see out of the lens and allow your camera to display your frame on its LCD screen. This will effectively get you the same result as shooting with the mirror lockup feature, but restricts you from being able to use your viewfinder.

Use your cameras timer

Again, this ties in with keeping your camera very still. With a solid tripod and soft hands, you can get away with ignoring this step, but I always do it anyways. When you push your cameras shutter button, you pretty much always move the camera. Again, any movement can ruin an astrophotography shot because the stars show up as lines instead of dots in the sky. Luckily, every camera I’m aware of has a timer mode. Most cameras have two timer options; one being ten seconds, and the second being two seconds. Most people use the ten second timer for group shots and probably wonder what the two second option is even there for. Well, it’s for long exposure shots. The two second timer allows your camera to settle from any movement that might be caused by the shutter button being pressed, before it takes a photo. Any movement from you touching the camera should be completely gone by the time two seconds has passed, which is why I recommend using this feature.

After you’ve completed all those steps, you’re ready to set your frame and start photographing the night sky! You may have to take a few photos before you get your settings where you want them to be for a properly exposed photo, but that’s all part of shooting in the dark.

- Guest Author

Jared Heynen - www.venrayimages.com

Jared is a professional wedding and fashion photographer from Calgary AB Canada. When he is not out photographing people, he relaxes by shooting landscapes and the night sky.