Hey guys, this post should serve as an addition to my previous post regarding noise. This article will focus on median stacking, so it is mostly for still objects, macro/product photography.
Now, lets get into it. Following my previous tips for reducing noise, we can now get more in depth on how to make it all better. Using median, we can exploit the power of averaging pixels from more shots in order to reduce noise. The more shots, the better. However, median is the last step in that process. We can practically feed that algorithm anything. The point is, the better quality it receives, the better results it produces. Note that there are many different software solutions that pretty much do the same basic job so you can use any software that does median stacking.
What do we feed the algorithm? The answer is the best stuff we can get, cleanest full resolution uncompressed .tiff files.
Before we get into file format, we need to prepare the picture.
Sample Image, 10 images stacked @ 100 ISO
Obtaining the cleanest picture for the series is crucial to the whole operation, therefore make sure you go through this checklist of key stuff when taking your pictures.
1. Overexposing. Over expose the shot as much as possible, this will make the dark areas brighter, thus avoiding noise to be generated in post production. You can go back 2-3 stops in post process without ANY damage, while if you go up 2-3 stops in post process, you increase the noise in the shadows significantly.
2. Really really tough tripod. In this case you need really tough tripod, since a slight movement can ruin everything and leave you stuck for hours trying to align the frames manually (on which you will most likely fail). So if your camera weighs 2 kilos, get a tripod that has a maximum of 5 kilos weight.
3. Light. Set up your light good, make sure it is nice and bright, because over exposing for few stops can mean great increase in shutter time and induce more noise.
4. Remote trigger/ intervalometer. Touching your camera each shot will move it by a millimeter or two, making the series of shots unusable. Make sure you have your intervalometer set to about 10 second timer before it starts the series of shots, in order to give time to any vibration from the cord (if there is any) to settle down.
5. Beat it. Yes, get away from the area your camera and photographed objects are. Doesn’t matter if you walk and breathe like a ninja or not, not all floors are perfectly solid and rigid to kill any vibration, so step away few meters while the intervalometer does its thing.
6. Keep it cool. Less heat = less noise, but turn off the a/c before you start the series, the air flow can move the object/camera slightly and that would be a problem.
When the process of obtaining the RAW files (I didn’t mention the RAW format because by now I hope all of you already use it) is complete, we can continue on the post process.
First we need to bring back the exposure to compensate for over exposing the image. It is wise to do this to all the images in the same time (as in Lightroom or similar solutions can) to save up time, and make sure that every single image looks exactly the same. It is also good to do the general editing on all the images before processing them through median, so while you have all the images selected, set your white balance as you prefer, do all the basic editing shadows, highlights, contrast, and all that stuff, but don’t do healing brushes and everything that alters the structure of the image (including sharpening) you can do all that on the output median image later. Also DO NOT use noise reduction software, while it does reduces noise it also reduces the detail which kinda beats he whole purpose of doing this whole delicate process. When you are satisfied with how the image looks, export as uncompressed .tiff files.
Why uncompressed .tiff? Because we avoid nasty .jpeg compression and artifacts it produces, while keeping most of the information that comes with the raw file. Feeding the .tiffs to the software that will do the median for you is a straight forward process, the results however are different. The more images you give it the better the results will be, however .tiff files are quite big and it can easily overload your hardware resulting in a process that lasts for hours or the software crashing. That is why it is best to stay around 10-20 pictures per shot, it reduces the noise to almost nothing and feeding it 30 or more pictures won’t make that huge impact we expect it to.
Canon EOS 1000D with Helios 44m-4 @ f8 (100% Crop)
When we take a look on this 100% cropped images ( I intentionally cropped at this area because it is darker and it is more prone to noise, while the text is just to show the detail retained), it is obvious that using the lowest ISO produces the best results. But notice the improvement in the 800 ISO shots – it is staggering. But also notice that the 10 median stacked images from properly exposed samples are just a little less noisy than the single over exposed-than-toned-down shot. The benefit from over exposing is clear and obvious and when paired with median can produce 3-4 stops better signal to noise ratio.
I hope this will help you in the process of making better photos.
Feel free to share and comment, your feedback is appreciated.
Until next time friends, Dz.