Stretching in Photoshop
Hello and welcome back to my blog. This post will be one for the absolute beginner and will cover stretching your astro images in Photoshop.
One of the main questions I hear from people when the first get into astrophotography is:
"why are my images so dark after stacking?!"
The answer is, you need to "stretch" your image.
I'm not really sure on where the term stretching came from when used in regards to images, but I assume its because essentially you are widening the histogram of your image.
In Photoshop we can use the levels and curves tools to accomplish this.
Step 1: Make sure that the image you save from your stacking program is unstretched and saved as a 16bit tiff. This means Photoshop will open the file without any issues. If you accidentally save your image as a 32 bit tiff file, don't worry...we can easily change it. Simply select "Image > Mode > 16 Bits/ Channel". You will then be faced with a HDR toning box, scroll to the method box and select "exposure and gamma". Click ok.
Once you click ok, you should have your 16 bit tiff image ready to stretch!
Now we can move to the levels tool. To access this tool you can either press "Ctrl+L" or go to "Image>Adjustments>Levels".
You will be faced with the following levels box.
Can you see that 'spike' to the left of the image? That has all your data squished into there!
All we will be using at this point to stretch the image is the black and mid point sliders.
Step 2: Adjust the mid point slider. This slider needs to be moved to the left, just so that it is sitting just next to the right edge of the spike. As you do this you will notice some faint detail start to appear in your image.
This is your first stretch. Click ok to save and then we go again.
Step 3: Stretch again and repeat as necessary. You will notice on the second stretch that the spike has moved to the right. This time move the black point slider to the right, just so that it sits to the left of the histogram spike and the mid point slider to the left, just so it sits to the right of the spike again. Press ok.
The key is to do little stretches and always being conservative.
We don't want to move the sliders into our 'spike' of data as this means we are 'clipping' some of the data and therefore losing detail.
Repeat the stretches as necessary, but be aware you can stretch too far...
Eventually you will get to a point where the histogram is nice and wide and you're quite happy with the level of detail that you have managed to drag out of the data. If the background is looking too bright, you can adjust the black point slider a touch more, however also being aware of how it affects your image. You do not want to clip the data. At this point you can use the curves tool to make your image 'pop' a bit.
Step 3: Open the curves tool. You can either do this by pressing "Ctrl + m" or by going to "Image>Adjustments>Curves". Upon opening the curves tool you will be greeted with a box that contains your histogram spike and a diagonal line.
The key here is to be conservative also, we don't want anything too drastic. If you click just below the diagonal line about one third away from the bottom and just above the diagonal line one third from the top, you will then give the line a slight "S" shape. See image below for example.
If you do this you will notice the background become slightly darker and the colours and brighter parts of the image pop out a bit more. Press ok.
You have just successfully stretched your stacked astro picture in Photoshop!
Astrophotography and the associated processing can be very daunting, so don't worry if you don't quite get it the first time. It just takes practice!
Here is my final image after a bit of extra tweaking (mainly the saturation and reducing the stars a touch).
Here's the youtube video of how to stretch this image also.