I'd like to be able to stack multiple images to reduce noise. In the past, I've followed the approach found at Cambridge in Colour: averaging images. Simply put, I bring all of the images into Photoshop, one on each layer. Then I change the opacity of each layer such that image 1 has opacity 1/1 (100%), image 2 has opacity 1/2 (50%), image 3 has opacity 1/3 (33%) and so on. This process operates under the assumption that each of the images were taken with the same settings, therefore they have roughly the same amount of noise.
What I'd like to know is how I could average images that I've taken such that the exposure level ends up the same, but the amount of noise is different due to using different ISO values.
For example, I have 4 exposures at ISO 1600, each with an aperture of f/4 and a shutter speed of 1 second. Theoretically, if I had another exposure at ISO 400, f/4 and 4 seconds, it should have the same exposure level as any of the first four exposures but have 1/4th the amount of noise. (Right?)
If I wanted to stack all five of these images to reduce noise (4 @ ISO1600 plus 1 @ ISO400), how would I set the opacity levels of these layers? 100%, 50%, 33%, 20% for the first four, but what would the fifth image have for its opacity?