To demonstrate the process, I’ve whipped up this 75-second video demo using Martin’s images (hoping he won’t mind). Check out his story on Photoshop News for details and images. Having said that, photographer and author Martin Evening has come up with a great example of how combining multiple images into a stack, then aligning them and running the Median filter, can make moving objects (tourists, pigeons, bits of noise) disappear. It was to enable technical applications that image stack processing was added, and it’s the reason that one finds the feature in Photoshop Extended. If this doesn’t yet sound scintillating, it’s probably because (I’m guessing) you’re not doing technical image processing work. Other algorithms include Entropy, Skewness, Summation, and Kurtosis*. So, for example, you could take a range of frames, then have Photoshop show you the average value of each pixel. Yesterday I mentioned that Photoshop CS3 Extended features 'image stack analytical filters.' Er, yes, so that’s useful and relevant… how, exactly? In a nutshell, you can now treat multiple images as a single entity, running an algorithm across them non-destructively.