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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?

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    Would it not be easier to manually adjust the images to "normalise" their exposure, then stack as normal? – Digital Lightcraft Nov 6 '13 at 16:58
  • What are you trying to gain by using different settings? Why not simply use all images that have 4 second exposures if 4 seconds is ok? If it isn't ok, altering the blending is going to result in more negative sides of the 4 second exposure showing through. – AJ Henderson Nov 6 '13 at 17:19
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    @DarkcatStudios the image exposures are already normalised at the time was increased to account for the ISO decrease. The question is how to bias the opacities so the less noisy files contribute more to the final image. – Matt Grum Nov 7 '13 at 12:51
  • @MattGrum - Ah right ok, i missed that! carry on... – Digital Lightcraft Nov 7 '13 at 14:09
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The better route is to covert the images into a Smart Object / Image Stack, then set the stack mode to 'mean' or to 'median'. I do that all the time for night images.

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The suggestion by Cambridge in Colour is a bit of a hack to get round the fact that earlier versions of Photoshop had no "average" function for image stacks. It falls down when you reach a certain number of images as the opacity values have to be rounded to the nearest integer.

Anyway, the effect you want is that the images are combined in a certain ratio, ideally each of the 4-second exposures should have 4 times the influence of the noisier exposures. One simple way to enforce this would be duplicating each of the 4 second exposures 4 times, then applying the CiC method for setting the opacities (100%, 50%, 33%, 25% etc.) or using the smart stack and average functions as mentioned by b shaw in the other answer.

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