Reducing noise with multiple exposures

[giuliogiuseppecarlo@xxxxxxxxxxxx]

I'm trying to reduce noise using multiple photos of the very same subject, using both a tripod or simply repeating a scan a number of times ( i have a flatbed scanner with a lot of noise especially in dark areas... )

So far i have tried with my old broken camera that had powerful noise at 400 iso  and first results are not bad at all.

Samples:
http://img90.imageshack.us/img90/6967/prova1wc2.jpg
http://img131.imageshack.us/img131/448/prova2xh7.jpg
http://img116.imageshack.us/img116/7589/prova3on7.jpg
( heavy purple fringing is probably due to the fact that i choosed 5/7 of the max resolution, 5 mp instead of 7 )

But i have still some questions:

I have used the first level, as normal, with 100% opacity, then i have added other 9 levels ( 9 photos ), with opacity 100/9=11,1, and mode set to normal.

If i use 10 levels, at 10% opacity each one, the brightness of the result is different from the original ( lighter ).

An example: i would like to use a grayscale image, shot 3 times.

For a given pixel, i have that first image is 200, second 190, third 192.

So i would like to have as a result (200+190+192)/3=194

So what do you think i should use?

Thank you in advance.

Giulio.


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