Difference and Exclusion blending modes
The difference and exclusion blending modes in photoshop comes in very handy when doing exposure blending, a technique used a lot when photographers has to deal with a scene that has to high a dynamic range for the camera to handle.
Expose so the highlights are good and the shadows will blow, expose so the shadows are good and the highlights will blow, the solution is to take 2 exposures, 1 for the highlights and 1 for the shadows and drop them on top of eachother, the problem here is that no matter how good your tripod is and how high the shutter speed is, there will ALLWAYS be a slight difference, enough to slightly blur the image.
The solution is then to put the two images in top of eachother on different layers and put the topmost layer into difference or exclusion mode, in exclusion mode, when the two images gets closer and closer to being 100% aligned the image will turn more and more black, if they are 100% aligned the canvas will turn black and you know you have a 100% alignment, at which point you can put the blending mode back to normal and continue doing masking work to blend the 2 exposures.
Difference and exclusion blending modes are also usefull when you are not sure if a conversion is being destructive or not. For example switching color modes between LAB and RGB. Keep a copy of the original switch modes and then put the two versions on top of eachother, switch top layer to exclusion mode and if the pixels does not turn black you know that you just did a desctructive mode switch.
