What is photo stacking and why should you use it?
I’ve been pondering what to write about on this blog for a little while now. I started my photography journey by picking up a DSLR to go and take some shots of the milkyway, having always had a fascination for the cosmos and our place within in. Having never studied photography and having no real idea what I was doing, I did some quick googling, pointed my camera up to the night sky with some basic setting adjustments and had a go. Needless to say, my first few attempts were pretty horrible. Since then, I have stumbled my way through camera settings, planning for shots and post processing. Time for me has always been limited with a very busy medical career and family life, and I didn’t want to spend too much time on courses or books, preferring to go out in the field and get my hands dirty so to speak. So I read some blogs, watched some YouTube videos, and spoke to (and tagged along with) some photographer friends and slowly picked things up.
So having said all of that, I’ve decided that I’d like to use this blog to mostly try to share some of what I’ve learnt, and hopefully help others who were in a similar situation to where I was 3 years ago. I may not have the most technical explanations, but I hope that I can at least share some of the techniques that I use (whether right or wrong) to achieve the results that I do.
One thing I get asked about quite frequently is about star stacking for my nightscapes. More specifically, what it is and how I go about it. In this week’s blog post, I want to dissect what stacking is and then in a future post I will go into how I go about it and what tricks I’ve picked up to make life a bit easier.
Photo stacking is a process used quite often in low light photography to counter the effects of high ISO settings on the camera and reduce the noise that it creates.
Here are some examples of a shot that is zoomed in 100% - the first shot is a single exposure that is unstacked, and the second shot is composed of 8 images stacked together.
A single exposure shot at ISO 5000 at 15s, 14mm. Zoomed in 100% you can see a lot of noise
The same “image” but this one is a stacked version that contains 8 successive milkyway shots. Same camera settings as above, zoomed at 100%. Notice how there is less noise in this version
You can see the effect that stacking has on the final image – much less grainy noise distracting from the image with sharper stars. Interestingly (and this is something I didn’t appreciate until recently), stacking is useful not just for astrophotography, but can be used in basically any photography of a static image to clean it up and make it sharper. The process of taking the images is very easy these days – simply set your camera up on a solid tripod and take multiple successive shots without moving the camera or changing the settings/focus. Even shots taken at low ISO (such as those below taken at ISO 100) can benefit from stacking.
A single image at ISO 100, 5s exposure at 14mm, zoomed at 100%. Whilst less noisy than a low light capture, due to a lower ISO, there is still noise present when enlarged/zoomed in
The same camera settings, however this image contains 4 successive shots, stacked together. Notice how when you enlarge the image/zoom in, it makes a marked difference to the amount of noise, even with low ISO. This effect can be improved by stacking more images together
My basic understanding of how stacking works is as follows; each pixel within a photo will have a value assigned to it depending on what makes up its final colour. You may have heard of Red/Green/Blue or RGB – each colour has a range from 0 to 255 and the final pixel created will depend on the combination of each of the values of these three colours. White for example has an RGB value of 255,255,255 and black a value of 0,0,0.
The concept of “Noise” is also useful to understand. Noise in essence looks like grain all over your image. It can decrease the sharpness that you can achieve and makes the image quality significantly worse. Noise is imperfections in the way in which your sensor captures what it sees. Even if you take a photo with the lens cap on, the resulting image won’t be completely black and there will be some pixels that have discolouration. The easiest way to see this is to take a photo with the lens cap on and then start increasing your exposure (a lot). The resulting image as you do so will show all of the noise you have captured.
There are two main types of noise; shot noise and digital noise. Shot noise is that which occurs from the scene you are photographing and is a result of differences in light emission and refraction. This type of noise is dependent on the number of photons sensed by your camera in each shot at a given exposure and can lead to “hot pixels” along with grain.
Digital noise is the noise produced by your camera (the sensor and internal electronics). This noise is dependent on many things, including your sensor size, sensor fill factor and the temperature; for example, as your camera warms up, it will introduce more digital noise as the sensor heats up.
Since static pixels within a shot (“signal”) remain relatively consistent, and the noise is random, your computer can analyse each pixel between the “stacks” (successive shots) and keep the signal and reduce the noise – also known as boosting the Signal-Noise ratio.
To increase the ability of your computer to figure out what is signal and what is noise in nightscapes, you can also add to the process “dark frames”. These are shots taken with the exact same settings you used to take your image, but with the lens cap on. Dark frames will contain no signal, but will contain digital noise as discussed above. This noise can then be subtracted from the light frames to improve your signal-noise ratio.
It sounds like a lot of effort, and if you do it manually it certainly is, however the process of actually taking all of the required shots is trivial, and a lot of programs exist these days to help automate the stacking procedure. The benefit of stacking to reduce noise is the large increase in detail that you can accomplish, the improvement in the colours within your photo, and improved sharpness which all really shine when you want to display your image large (especially in prints) or if you need to zoom/crop.
In my next blog post, I’ll share with you quickly the program I use to automate stacking, and the method I use to manually stack, including some nifty tricks, when automatic stacking fails.
