Creating renders from Image Stacking using different sampling levels I read through the conversation on this question: Is there a way to render a single frame using network rendering in cycles. I tried the image stacking method that was suggested and got pretty good results. I rendered my image five times, each with 150 samples and a different sampling seed. I then rendered the same image with 750 samples to compare. (In case you're curious, the 150*5 stack had slightly more noise than the straight 750 image, so you are sacrificing some quality by splitting up the work in this way)

Then I wondered how much of a quality increase I could get if I added the 750 image to the stack, which brings me to my question. How can I reduce noise by image stacking if the images don't have the same number of samples?

So my first five layers have opacity 100%, 50%, 33%, 25%, 20%, respectively. These are the layers with 150 samples. If I wanted to add my 750 sample image to the top of the stack, what would be its opacity?
Short answer:

My guess is that it should be 50%. The combined samples of the images above it is ~750, which is the same as the number of samples in the rendered image. Since the rendered image and the combined images are more or less equivalent, They should be combined with a 50/50 mix.

Long answer:

Each image's opacity is based on how many samples it has compared to the image above it. 

E.g. if you render 5 images with 10 samples each, the second image should be mixed with the first with a factor of .5 (or an alpha of 50%). This will effectively result in a 20 sample image (as you pointed out, it's not quite the same as 20 rendered samples, but it's reasonably close), so the second image should be mixed with a factor of .33 (alpha of 33%) and so on.

Since the resulting combined image is basically a 750 sample render, it's the same number of samples as your 750 sample render. You can just tack it on top with a transparency of 50%.