How do I choose the right ISO for astrophotography on a Sony A6000?
Asked 2/13/2018
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I photographed the night sky with a Sony A6000 and a 135mm f/2.8 lens (about 202.5mm full-frame equivalent). My settings were ISO 25600, 3 seconds, and a stack of about 10 images in lxnstack. At lower ISO settings, such as around ISO 5000 with a 2-second exposure, I was able to record fainter stars, but at ISO 25600 the brighter stars looked washed out and dim stars seemed to disappear.
I often see advice to use very high ISO for astrophotography, but in my results that seems counterproductive. How should I balance ISO, shutter speed, and dynamic range for this kind of setup?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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As you increase the ISO, your dynamic range decreases. While your camera has a base dynamic range around 13 stops, when you set the ISO to 25600 you only have a dynamic range of a little under 7. This means that bright stars will wash out much easier. This graph shows the effect of ISO on dynamic range for your camera. 
Source: https://www.dxomark.com/Cameras/Sony/A6000---Measurements
To properly balance your ISO value you want to use the "500 rule". In a nut shell:
500 / [focal length] = maximum exposure time (to avoid star trails)
For your lens: 500/202 ≃ 2.5 seconds
Now that you know the shutter speed, open the aperture all the way and drop the ISO as low as you can.
Shooting at such a long focal length means that you might want to make a "barn door tracker" or buy a telescope tripod that adjust in live time. This will keep your camera pointing at the same place in relation to the sky, allowing you to put the ISO all the way down to 100 and shoot a much longer exposure without the introduction of star trails.
Originally by user71865. Source · Licensed CC BY-SA 4.0
user71865
8y ago
0
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Very high ISO is not always better for astrophotography. On your A6000, raising ISO reduces dynamic range, so bright stars clip sooner and look washed out. That can also make faint detail harder to recover.
For an untracked shot, start by setting exposure time from the 500 rule: 500 / focal length. At about 202mm equivalent, that’s roughly 2.5 seconds max to limit star trailing, so 3 seconds is already a bit long.
Then open the lens as wide as possible and use the lowest ISO that still gives a usable exposure. In other words: don’t push ISO higher than necessary. A lower ISO can preserve more highlight detail and star color.
Also, some cameras apply stronger noise reduction at very high ISO, which can suppress dim stars (“star eater” behavior). That may be part of why ISO 25600 looked worse than around ISO 5000.
At 135mm, tracking becomes very helpful. A simple barn-door tracker or other star tracker will let you use longer exposures at lower ISO for much better results.
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