Astrophotography is where you really test out your camera capabilities. Unlike photographing sunrise/sunset, you need to bump up your ISO to 8000, 12800 or even 25600 to get an ideal exposure in a low light environment.
There are two challenges that you would probably face if you are into Astrophotography.
One, you found out that there are too many noises on your photo due to taking photos using the high ISO setting.
Two, no matter how you crank up your ISO setting, you are still getting a photo with an underexposed foreground.
Then, you may think of upgrading your camera gears. Well, if you have the budget, then I would say go ahead. I believe you pay for what you get, a better camera or lenses would always give you a better advantage in low light performance.
However, these gears are pretty expensive.
If you can’t upgrade your hardware, you can use software to go beyond the hardware limitation to overcome the above challenges. That’s also what smartphone companies are currently doing, using technologies like built-in HDR, Deep Fushion on Apple iPhone, Night Sight on Google Pixel.
Let’s start with the first one.
1. Separate exposure for the foreground and Milky Way
For you to photograph Milky Way, there’s a max Shutter Speed you can use before the stars start turning into star trails.
For example, the max Shutter Speed you can use at 24mm Focal Length is 20 seconds, based on the 500 Rule. (If you never hear of 500 rule before, you can check out my previous blog post HERE)
However, this 500 rule may restrict you in getting an ideal exposure for your foreground. You may have the sky proper exposed, but the foreground underexposed.
The solution for that, take separate exposure for both foreground and the Milky Way.
You can stick to the 500 rule for the sky, and then you change your camera setting and use a slower Shutter Speed for the foreground. Here’s an example.
The left photo was using ISO 25,600, f/2.8, 21mm and 15s Shutter Speed. The right photo was using ISO 25,600, f/2.8, 21mm and 30s Shutter Speed.
If zoom in on the stars, you can see that the stars on the right photo are showing star trails effect.
You can also go for a lower ISO and a much slower Shutter Speed for lesser noise.
After that, using Brush and Layer mask in Photoshop, you blend both the photos together into one. 😀
2. Noise Reduction using Image Stacking technique
There are many ways to reduce the noise, but the most common and effective method would be using the Image Stacking technique.
Using the same camera settings and composition, you take multiple photos of a scene. If possible, I suggest taking at least ten photos.
Then, you import them as Layers into Photoshop.
With all the Layers selected, right-click on any one of them and choose Convert to Smart Object from the menu.
Once all the Layers merged into a single Smart Object, go to Layer > Smart Objects > Stack Mode > Median.
Wait until the process complete, and you will see noise on the foreground has been greatly reduced.
Right on the Smart Object layer again, and choose Rasterized the layer to convert it back to a normal layer.
However, note that the Milky Way and the stars have also become blurry. This is due to the stacking process tend to remove anything that’s not static across the photos.
You can resolve that easily by blending back the sky from one of the photos that you took.
Alternatively, you can check out Sequator (Windows) or Starry Landscape stacker (Mac), which is easier to use and able to auto-align the photos to perform a separate Noise Reduction for the sky.
It is also possible to apply Noise Reduction for the sky in Photoshop, but that’s required additional steps, and sometimes it is not that straightforward.
3.Understanding your camera capabilities
This may sound contradicting to the objective of this post, but for you to be able to utilize the above two techniques, you need to understand the limitation of your camera first.
You need to decide that up to what ISO setting that the output photo is still considered usable without overwhelming noise. How many photos will it take to reduce the noise down to an acceptable amount?
For example, when I use Nikon D750 or Z7 for Astrophotography, I have no problem go all the way to the max ISO, which is 25,600, because I’m confident that the photo will look “clean” enough after the Noise Reduction.
If you are not sure about this, the best way is to try out different ISO settings during the shooting.
You can start with the ISO setting that you are more comfortable with, and take as many photos until you are satisfied.
Then, you increase the ISO setting and take another set of photos. You can repeat this process until you reach the max ISO setting.
After that, you compare those photos and perform Noise Reduction on your computer. From there, only you can tell which ISO setting provides you with the best result after Noise Reduction.
I did these testing with my cameras, tested ISO 8000, 12800 and 25600.
P.s. sometimes, higher ISO may produce lesser noise. There’s a technique called ETTR. Perhaps I will write a blog post to explain this in detail in the future.
That’s all for this blog post.
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