Sony a7Rii & Zeiss Batis 25mm f/2.0
13 seconds | f/2.5 | ISO 10,000
6 images stacked in Photoshop
A tiny section of the image (of example shown below)
What is stacking?
Stacking is the process of taking a number of consecutive images (all with the same settings) and laying them on top of each other. Then, using software to compare the data in all images, an "averaged" image can be produced.
The main purpose is to exclude anomalies in the image, i.e. noise!
As noise is random, it will appear in different patterns on the individual images. As the software is only looking for consistent elements within each frame it can exclude the noise once the images are stacked and the final image is created.
A couple of benefits arise from the reduced noise; you'll be able to bring out details in the shadows and process the stars / Milky Way without accentuating noise.
I find that 3 to 10 images is a good number to use. Any more than that means you are stacking a much larger movement in the stars from the first to the last (caused by the Earth's rotation), which will give you a little bit of trouble if working with a foreground element.
If you're a Mac user you can download "Starry Landscape Stacker" - their software is pretty much automated. For PC users there is a program called Sequator that also does a similar job. Personally I feel that Photoshop does a much better job at averaging out the noise so below is a step-by-step guide of how to do it:
Open your images as layers
Mask off the foreground in each layer
"Auto align layers" to account for the movement in the stars
Remove the masks on all layers
Convert the layers to a smart object
Use "mean" or "median" stack mode on the smart object
You will have to go through the same method for any foreground element of your image. Create a copy of the original layers - this time you do not need to align or mask them before you create the smart object to stack. Seamlessly blending the sky and foreground stacks is slightly more involved and will probably be covered through a workshop - sign up here.
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