How to: astrophotography

Last week we shared a list of tips for the best chance of getting your photographs noticed in the 2019 Sigma D-Photo Amateur Photographer of the Year competition. One of our recommendations was to try a new category; not only do you set yourself a challenge and expand your skills, but those new to a genre are usually more willing to approach the genre in a new way that might just get you noticed by our judges.

That said, we don’t want you jumping in totally unarmed, so here we’re sharing some Q&As on a genre that can produce some extremely impressive images that many consider too difficult to attempt themselves: astrophotography.

So, have a read of the tips, get out shooting the night sky, and enter your best images in the Astro category before Sunday, 23 June 2019.

I have used a torch for ‘light painting’ on a landscape at night, but the image ends up unrealistically bright. How do you get a moody, natural look?

Unfortunately there are quite a few different variables involved when discussing light painting. Some of the things that affect the brightness when trying to paint with light are:

• Strength of light source (flashlight/strobe)

• Distance between light source and subject. The inverse square law applies to all forms of light — if you double the distance between the light source and subject, you reduce the amount of light hitting the subject by a quarter. Or, in photographic terms, doubling the distance means losing two stops of exposure

• How long you leave the light on one particular area of the image — lingering in one area will cause it to be brighter than the rest of the area

The best way to get a good result when painting with light is to experiment with your own set-up, after all some camera/lens set-ups are better than others when dealing with long exposures. Also, experiment with how far away you are when painting with light. If you are shooting super-long exposures in near total blackness, you can actually walk through the frame with your light turned off and stand quite close to the object you want to paint, and as long as you move around, your body won’t actually appear in the image.

How do I avoid blurry stars when shooting the night sky (I don’t want star trails)?

Among serious astrophotographers there is an ongoing debate about what is the best setting to get sharp stars. But it basically boils down to what focal length lens you are using, and whether you are on a full frame or cropped sensor. The debate is whether you should follow the 500 or 600 rule — these rules come from taking the number 500 or 600 and dividing by the focal length of your lens. So if you are shooting on a full-frame sensor and are using a 16mm lens, you would divide 600 by 16, which equals 37.5, that would be the exposure time when using the 600 rule. If you used the 500 rule, your exposure would be 31.25. Some people claim that the extra bit of exposure you get when using the 600 shows a bit too much star movement. The best way is to experiment and find out what works for you.

When shooting a star-trail scene with a long exposure I get a lot of noise in the image — how can I avoid this?

The amount of noise in an image is due to individual pixels in your camera’s CCD/CMOS sensor not being exposed properly, and creating uneven gradations between pixels. This is caused either by not enough light hitting the pixels (that is why a very underexposed image will appear ‘noisy’ when you try to edit it on your computer and bring up the shadows). Or, more commonly when dealing with long exposures, you get noise due to the current running through the sensor ‘exposing’ pixels even when no light hits them, either through electrical noise or heat build-up on the sensor.

The best way to avoid noise is to use the lowest ISO setting you can, and if you are still getting a lot of noise when shooting long exposures, try image stacking instead of one long exposure. 

What is the advantage of image stacking when photographing the stars?

Image stacking is similar to creating a panorama, but instead of overlapping the edges of the image, you are building up the length of the star trails in the image through multiple exposures, instead of one long exposure.

There is another way of using image stacking involving using multiple exposures and a computerized tripod head to track stars, to build up an exposure of very faint stars, which is much more difficult to do and requires quite sophisticated equipment.

What are the best kinds of lenses for astrophotography?

Generally, most people use either really wide lenses, at least 24mm, to create wide-field astro images, or they use super-long telephoto lenses. They may even attach their cameras to telescopes to get images of deep-space objects like other planets, galaxies, and other objects outside our own solar system.

How do I use Live View to help with focusing on stars in the night sky?

Generally when shooting astrophotography you can actually set your camera to manual focus, set the focus to infinity, and leave it there. But some lenses are not very accurate with their focusing guides, and some don’t even have a focusing guide any more. If that is the case, what you can do to check focus when using Live View on your camera, is zoom in as much as possible on the LDC display on the back of your camera. Set your focus manually as close as you can to infinity, and then find the brightest star in the sky, and make minor focus adjustments until the star looks sharp.

Should I use post-processing techniques to enhance astro photos?

The first rule of thumb with astrophotography is the less post-processing you have to do, the better the image will be. The more you adjust the image, the more likely it is that the image will start to fall apart. So when shooting the stars, you really want to be sure you have your base exposure down. Also you should always shoot night images in RAW, that way you have a little more leeway when it comes to post-processing.