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Night Sky and Aurora Photography Tips

Below are some notes and tips from Craig McKenzie from a Dunedin Photographic Society -

 

Night Sky and Aurora Photography Workshop

When photographing in daylight there two things that need to be achieved for a technically correct photograph, suitable focus and getting an appropriate amount of light onto the sensor.

For focus there is just one control, typically a focus ring on the lens. Although the depth of focus can be altered using the aperture setting. This can also be referred to as the f stop. It is expressed as a number in a sequence like 2.8, 4, 5.6, 8, 11, 16. The small numbers represent a large hole in the lens and let in more light. The depth of focus is also less. Stopping down, or changing to a larger number, decreases the size of the whole letting in less light and increasing the depth of focus.

For controlling the light reaching the sensor we have two controls, the aperture discussed above and the shutter speed. We can also change the sensitivity of the sensor by changing the ISO setting. Higher ISO settings mean more sensitivity.

At night we still use these same controls but a number of complications soon become apparent.

The first is that it becomes difficult to focus in the dark, both for us and the camera. There are several techniques we can use to overcome this.

The first is to use a manual focus lens that has a hard stop at infinite. Pre auto focus lenses and newer manual focus lenses have this ability. Be aware that when manually focusing auto focus lenses they do not stop at infinite and move slightly beyond.

If you know you are going to head out after dark you can focus on a distant object while it is still daylight and then use removable tape to hold the focus ring in position after setting the camera to manual focus.

If you find yourself needing to focus after dark it is worthwhile trying to see if the autofocus will work on a on a distant light or one of the bright stars. It often will. While the lens is in focus switch to manual.

Live view can also be used for focus. There is not enough light to show anything much more than a snowy image but if you zoom in and look around you will see constant sources of light. These are the stars. Zoom into maximum on a star then adjust the focus ring to get the blob of light as small as possible. Then switch to manual.

If all these methods fails it becomes a matter of trial and error. Take a photograph, examine the image and adjust the focus and repeat until a good focus is achieved.

No matter what method is used taping the lens in place can be a good idea along with periodically checking everything is still in focus by reviewing your images at a high magnification.

If you are using a zoom lens focus at the focal length you intend to photograph with. Focus is not guaranteed to remain the same as you zoom will all but a very few lenses.

With correct focus we now need to get sufficient light onto the sensor to record an image. The highest quality image will be with the ISO value set low, the aperture stopped down from maximum aperture and the shutter speed fast. Using optimum settings will result in a black image at night.

Before adjusting these settings it is time to switch to manual exposure mode.

A high shutter speed is required to stop camera and subject movement. By using a tripod we hold the camera still and can keep the shutter open a long time. It turns out that we are not keeping the camera still in relation to the stars because the earth is rotating. About 30 seconds is the maximum time we can keep the shutter open without the stars changing from points to trails and even 15-20 seconds is better. When the aurora starts to move around short exposure times help to define the structure. Longer exposure times will give more of a blur of colour.

With a maximum shutter speed determined the next best variable to change to get more light is the aperture. Because everything we are interested in photographing is at infinite we do not need depth of focus so we can open the aperture up to maximum. Use the smallest number your lens will go to.

Now it is time to start increasing the sensitivity by increasing the ISO. Increasing ISO introduces noise which degrades the image. This is why we use this last. Keep increasing to ISO until a satisfactory image appears on the monitor. The Milk Way should be easily discernible in your photograph. A typical exposure for me might be ISO 3200, 20 sec, f 2.8 for the Milky Way on a dark night.

After achieving a good base exposure you can experiment with small adjustments of one setting by compensating with another. For example you may be better off stopping your lens down a click off maximum to avoid some lens aberrations and increase the exposure time or ISO to compensate.

Now is a good time to mention a very important tip. Because we are looking at it in the dark the image on the camera monitor will look much brighter that it really is. It pays to check the histogram to ensure you are getting some values well towards the centre and right otherwise you will be disappointed when you see your images on your computer screen. I know this only too well! If the brightness of your camera display is adjustable it is worth changing it to its lowest value.

Being very familiar with you camera controls helps when it comes time to use it in the dark. If you are in a group and start turning on a torch all the time to check your camera your torch light will spill into the foregrounds of the other's shots and you will not win any friends. It will also upset your night vision.

As well as the noise introduced by high ISO noise can be introduced by long exposures. Most cameras have an option for reducing long exposure noise. They do this by taking another exposure of exactly the same length of time without opening the shutter and using this dark frame image to subtract the hot pixels from the original image. I have never found the long exposure noise reduction makes much difference so I suggest you turn it off and deal with any issues in post production. You do not be want to be waiting around while your camera takes a photo of nothing while there is good aurora action happening.

This covers the technical aspects. Next comes composition, a whole new story. Once you can photograph the night sky start to think about adding interest by including foreground elements.

Craig McKenzie