When shooting pictures of waterfalls, here's a way to get that soft milky look to the moving water when you can't get exposures long enough to do it directly. I recently had a pocket camera and pocket tripod in my coat pocket on a nice hike along a series of waterfalls in Ithaca, NY. I didn't have a polarizer or neutral density filter to enable slower exposures and this example was shot in open shade on a bright sunny day. I used the slowest ISO I had available (80) and closed the aperture as much as I could without destroying the image with diffraction. I was still stuck with a 1/30 second exposure giving the following result:
Not bad but I wanted the falling water to look softer. With the camera fixed on the little pocket tripod, I shot 5 identical exposures of the waterfall without moving the camera. I used the 2-second timer on each exposure to make sure my shutter-button presses didn't mess up the exposures.
I brought the 5 images into Photoshop on separate layers of one file. I wanted to blend the images together so that stationary objects stayed the same and the moving parts (the water) got averaged together. It's a simple matter of changing the layer opacities to allow each layer to contribute equally to the final composite. The background layer stays at 100% opacity, the second layer is at 50% opacity, the third at 33% opacity, and so on as seen here:
If you think about it a little bit, this allows each layer to contribute equally to the averaged stacked final image. Of course I could have used fewer or more images depending on what the desired outcome is but you get the idea. The result with 5 stacked images is much closer to what I was hoping for. Ten images would have made it really milky smooth.