Capturing Starry Skies

Lockdowns have cleared the atmosphere a lot. Even in cities, now, the night skies can be seen dotted with stars. Earlier, I had written about creating star-trails. Here, I will write about how to photograph starry skies (and not create star-trails).

 

Starry Sky

(Starry Sky – Photographed using a 25mm lens. The photographs in this article have been processed to appear realistic. They look better on large screen displays than on mobile phones.)

 

The technicalities for the star photographs are simple. A high ISO to capture the low light, a shutter speed long enough to allow the light to reach the sensor but not so long as to let the stars create star-trails, an aperture wide enough to allow a good amount of light and let the elements be in the image be sharp! Read on for details and explanations –

 

Starry Sky above Maini's Hill Cottages

(Starry Sky above Maini’s Hill Cottages, a homestay run by us.
Shutter speed 23 seconds at f/5.6. ISO 3200. Post-processed in Affinity Photo)

 

What was that rule of 600?

This was made a long time back to avoid star trails. Dividing 600 by focal length of the lens being used gave an approximate idea for the longest time beyond which stars will start creating star-trails in night photographs. (600 ÷ the focal length of the lens = maximum exposure time). The rule was meant to prevent star-trails by keeping the exposure below a certain time limit. It still works!

With an increase in camera resolution, this rule needs tweaking and the exposure time needs to be further reduced so as to prevent star trails. I use an old camera so this rule works for me. For newer cameras 500 seems to work rather than 600.

For the photograph above, I had used a 25mm lens. As per the rules, 600 divided by 25 gives 24. So, if my exposure time for any shot increased beyond 24, I could expect stretched stars (or star trails starting to appear). My exposure time for the photograph was for 23 seconds.

 

The right ISO

For capturing the night sky with stars, the right ISO to use is a high one. I prefer mid-range ISOs in (1600-6400) in present-day cameras.

There are two reasons for this-

First is that the native ISO for most new cameras is in this range. Native ISOs give the best possible result in most night photographs.

The second and more important reason is ‘Shot-Noise’. Noise due to the irregular amount of light hitting the sensor is called ‘Shot Noise’. When the light is less, the total amount of light which forms an impression on the sensor is small and irregular. (An analogy that I read somewhere – it is like spraying a plant with water. When you spray with a little water, the water droplets don’t cover it equally and there are areas left that are dry and some areas with more than one water droplet.) This leads to irregularities in the amount of light per pixel and causes noise. With an improvement in electronics, this is being addressed and now the new cameras have reduced this type of noise. The amount of useful light increases when the image is overexposed but the noise does not increase proportionately. A high ISO is therefore useful in reducing shot noise too.

 

Aperture that works most of the time

The stars are quite dim. We are able to see them because of the wonderful capabilities of our eyes. For photographing them, apart from a high ISO and a long shutter speed (though less than what would create star-trails), a wide-open aperture is also required.

Light tends to bend when it passes near the sides of aperture blades (diffraction effect) and this causes softening of images. While using very small apertures this softening of image is quite pronounced even with regular images and here we are talking of stars that are so minute in comparison to the camera sensor. Wide-open aperture gives the sharpest image of the stars. The reason behind this is simple. The widest aperture has the most area in comparison to the perimeter (the edges of the aperture blade).

So, a wide-open aperture apart from letting in more light also helps in reducing the diffraction effects.

I prefer the widest aperture that can capture any foreground with reasonable sharpness.

 

(Stars behind an Aspen tree. The tree was lit up by a dim garden light and so it turned out to be so bright. 20.2 seconds exposure with a 25mm lens, f/5.6 aperture, ISO 3200)

 

Other things to consider –

Here are some more basics that are understood but still for the completion of this article, I am writing these down –

  • Click raw. Especially with high ISO, you’ll need a raw file to adjust the exposure properly. jpeg will not do.
  • Have the camera as steady as possible. A sturdy tripod is a must. Also, make sure that the place where you place the tripod is not too windy.
  • Use manual mode. You have to be able to set the shutter speed and aperture manually.
  • Switch of Auto-ISO. This is a common error that many people make while trying their hand at star photography. Set the ISO manually to a mid-range value as I have mentioned earlier.
  • Find a spot with the least light pollution. No street lights, no building lights, no passing vehicles with headlights! Darker the area, the better it is. The foreground structures in my images were lit by dim indirect garden lights. I could barely see them while composing my frame but as you can see even that small amount of light has lit the foreground as if I was using flood-lights!
  • There are light-pollution filters available for telescopes and cameras. You can consider these too. These improve the results even in dark places. Some of them help photograph nebulae too. (I don’t have any light-pollution filter so didn’t capture any photographs using that)
  • For ideal star photographs, a special setup is required that turns the camera in sync with the earth’s rotation. This is simply called ‘tracking’. Using tracking, clicking multiple images, and then stacking them is a the best way if you are into serious star photography.

 

Shooting Star

(A shooting star came in my frame. It was speeding past and traced this line. It looks interesting so I decided to keep it. This image itself was spoilt due to a longer than required exposure. At its full resolution, the stars appear elongated. They had moved during the exposure time of 27.9 seconds – higher than the upper limit of 24 seconds for my lens and camera)

 

Post-processing Tips

Post-processing of these photographs also requires some special precautions and efforts. Here is how I process and edit them –

The first step is to correct the exposure. Most of my star-photographs end up over-exposed (which is a good thing since I want to reduce Shot-Noise). So, I have to slide the exposure correction to the left (minus).

Next, I paint a layer on top of the brightly exposed foreground and reduce the brightness there further. Still, as you can see, my foreground elements are quite bright.

Then, I play with the curves tool. My purpose is to increase the contrast and make the sky darker still. First I drag the black-point anchor to the right, so as to cover any unused space in the histogram. Next, I adjust the curve to darken the sky while making the stars stand out. (Curves Tool at your service)

The last important step is sharpening. This is tricky. What sharpening does is brighten the adjoining pixels. Some of the stars are just a pixel or two in size, and are dull in comparison to the other stars. Sharpening brings them to the same level as many other bright stars. It is up to you to decide how you want your photograph to look. Sharpening with everything done as you would do for a regular snap-shot, will also increase the number of visible stars as compared to how you remember capturing them. Play around with the ‘Threshold’ setting in Sharpening tool to understand how many of the stars do you want to be visible in the sky. (Sharpening)

Photographing milky way is also similar but requires some tweaking in post-processing. For the milky way or nebula to appear clear, the exposure may have to be first increased in exposure compensation and then contrast raised to bring the dark shade back into the sky. For the foreground, creating composite image or combining two images is a good option.

 

Night Sky

(Some foreground elements are needed to make a good starry sky photograph. The absence of any foreground causes an unimpressive image. We are used to seeing these elements and so including them improves our subconscious association with the photographs)

 

An Afterthought

I clicked these photographs while locked up in my home. Thankfully, the village around me does not create too much light pollution. A lot of photographs still were unusable due to the light pollution, over which I had no control. On a normal day (rather a night), I would have traveled to another part where there were no such lights, but not during lock-down. The images also have been compressed since the internet is very slow nowadays. Uploading compressed images was easier for this article. Everyone seems to be using the internet during these days of lockdown!

The photographs have been processed to keep the things looking realistic. The small stars might not be visible if you are reading this article on a mobile phone! Sorry about that. Maybe I’ll write another article some day on how to click and process star photographs meant to be shared on social media and to be viewed on small screens.

The sky has been visibly clear. Maybe the pollution was affecting the hills too. Now that things are under lockdown, the air is getting cleaner.

Once the things normalize, I’ll organize a workshop on star photography and star-trails.

 

Further Reading:
Star Trails
Remove Noise (Add Grain?)

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