This techique describes a method to stack several high ISO source images in Photoshop CS3 or CS4 Extended, in order to minimize random noise – in effect creating a synthesized long duration low-ISO exposure.
It is the first in a series of techniques relating to ‘computational photography’ – the process of utilizing several source images and raw computer power to enhance your final image output.
So you are at an art museum. You’ve got your shiny new DSLR with you, but the strict rules of the museum excludes both tripod and flash use. Unfortunately the art pieces are not all that well lit, maybe just the equivalent of a single incandescent lightbulb from the distance of a couple of meters.
– But you really want to get that depiction of a museum piece… What do you do?
My Canon 40D features a maximum of 6.5 frames per second. Instead of taking one exposure at high ISO, I can take a sequence of exposures at high ISO, and then combine all of them in a Photoshop Stack to create an image devoid of the rampant random noise. Photoshop aligns, then averages the pixels from each shot to create noise free pixels, and extract details otherwise obscured. The reason this works is because the noise is random – it is rarely in the same spot in two seperate images.
Astronomers has stacked images like this for a long time, indeed they have some very sophisticated software available to them, but now everyone with a copy of Photoshop Extended can utilize its powerful align and stack algorithms to do this trick.
Here’s the workflow:
1) Set an exposure suitable for the lighting and for handholding. For a painting, you don’t need much DOF, so an open aperture is suitable, just choose the first aperture that is sharp for your lens. In this example, my camera settings were: RAW, 1/160s f 2.2 ISO 3200. I want a fast shutterspeed for my 50mm 1.4 to ensure sharpness across the handheld sequence I’m about to make. I choose the highest ISO setting on my camera, ignoring any noise issues for now.
2) Angle the shot, focus, brace the camera as best you can, and hold down the shutter for as long as necessary. The more shots you have, the better a result you can expect, but you should also expect a longer processing time. My Canon 40D filled its buffer at 15 photos at 10MP RAW format, and it took it just a bit over 2 seconds to do so.
3) So, now you’ve got 15 noisy, virtually useless, slightly underexposed ISO 3200 shots of the same washed out painting. Great.
At this point, back at my computer, I open up Bridge, and examine details with the Loupe on all of the frames. If one or two are out of focus, I delete them or exclude them. Then I open up all of the remaining sharp shots in Camera Raw. I make sure to select them all, dial in a +1 exposure, because I had to underexpose the shots a bit to get a faster shutter speed. This will push my source images to ISO 6400(!). I carefully adjust the WB and other settings. I like to keep a linear tone curve for this technique, but your mileage may vary. You may even add a bit of capture sharpening, disregarding the hit you take in noise. – But do turn off all noise reduction, as we will deal with this in the next step.
I usually keep to a 16-bit Prophoto workflow, but I find that for a 15 image stack in Photoshop, it might just be to much data crunching to wait for, so I made my source images 8-bit Adobe RGB in this case. See screenshot:
Camera RAW - 15 photos selected, note positive exposure compensation.
Note that these are not photos of an actual piece of art, but of a 1 euro reproduction print I bought in a tourist trap in Spain. It’s hanging on a wall in my room, and it came in handy for illustrating this article.
Next, I click Done.
4) In Photoshop, go File>Scripts>Load Files into Stack. Select your images, and make sure to check ‘Attempt to automatically Align Source Images’ and ‘Create Smart Object after loading layers’.
If you have a lot of source images, this process can take a little while. Go make a cup of coffee, pet your cat or sumthin’.
5) When done, go Layer>Smart Objects>Stack Mode>Mean.
The pixels of all the layers will be mean blended, and the noise will be seriously diminished. Taking into account my photos were ISO 6400 originally, they now look great. I now flatten my image (Layer>Flatten Image), clean up any residual noise with Neat Image, apply a transform to straighten up the image, then apply a healthy dose of Filter>Sharpen>Smart Sharpen. The final step is a slight crop, because when handholding for a sequence like this, framing will differ somewhat, and around the edges of the photo there’ll be a margin where there’s more noise, because there are fewer overlapping frames.
Here’s some before / after crops at 100%:
Comparison of 1 original ISO 6400 frame vs stacked, Neat Image + Smart Sharpen result.
And here is the final result:
Result of 15 image mean stack
It sounds like a lot of trouble, but once you establish this workflow, it is quite easy. And it can get you those shots that would otherwise be impossible, such as in museums or other low light situations with static subjects, where you prefer not to use, or cant use, a flash or a tripod.
If you think this sounds like to much work, here’s an idea for a $1 camera stabilizer.