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noise and average-stacking

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Sat, 08/21/2010 - 04:43
Art_Wannabe
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noise and average-stacking
Submitted by Art_Wannabe on April 15, 2009 - 9:30am

I feel like I've had some success in reducing noise by average-stacking. I'm pretty happy, and want to share this and see if anyone else finds it useful or has any comments.

The idea of average-stacking is to take several identical shots and then average them into one. Noise being random, the tiny random color spots should be reduced in proportion to the number of pictures you are stacking. Just stacking 2 shots should cut your noise in half!

What this means to me: I've been trying to take shots where the tones go gradually from light to dark, so I inevitably had noisy darkish areas. Even at my lowest ISO, noise was showing up where there was less light. I had resigned myself to having noise in the darker areas. The only ways to reduce the noise seemed to be to
•add blur to the dark areas during editing,
•take brighter pictures that you then darken during editing, or to
•get a more expensive camera (ouch).
Selective blurring and taking brighter shots are useful techniques but they don't always work. I'm happy to have found another approach that won't cost me any money.

Other kinds of stacking, exposure "HDR" stacking and focus stacking, seem to increase noise. I'm happy to find something that can make these techniques work better.

Software: PS elements and photomatix (free version) can average-stack. Maybe other software too.

When it could help: tripod shots when the subject doesn't move.

Boringness alert! The following content may be interesting only to the author. Continue at your own risk!

Example: For the "oyster Mushroom (4)" shot, I took pictures at 7 different focal lengths (each had a different part in focus). For each of these 7 focal lengths, I took 3 different exposures (to have some choice of exposure later and to allow exposure-stacking if I wanted to). For each of these 21 focus and exposure combinations, I took two identical sets of pictures, totaling 42 pictures.

I decided that the medium exposures were adequate, so I discarded the brighter and darker shots. Now I was working with 2 identical shots at each of 7 focal lengths.

I average-stacked each identical pair using photomatix, creating 7 averaged shots (one at each focal length). The average-stacking process didn't take long (unlike focus stacking). I noticed that the process seemed to reduce the contrast. I'm not sure what to make of that. Anyway, the results were pleasing.

I then focus-stacked the 7 averaged shots using combineZM, which I've described elsewhere in this forum. I finally edited the shot in PS Elements.

By the way, I took the pictures with my Rebel xsi, a 100mm 2.8 macro lens, set at f/11.

Here's the result, so you can judge for yourself.

http://www.triangle.com/your_photos/?id=15466291

Enjoy!

-Art

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Wed, 09/01/2010 - 13:44
#1
Art_Wannabe
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Hey Java! Just to add my comments... My experience is that the noise just lurks in those dark areas, and I suspect that is true even if the shutter is open longer. It makes sense to me to think of each little sensor element on the sensor is a little photon counter. The gradations in counts will by necessity be rougher when there are fewer photons. So if one sensor element gets 2 photons and the next one over gets 4, that's sort of what you would expect from random variation, but it means big differences in the relative brightness of the pixels. Thsee differences really are visible if you brighten up these dark areas during processing. The nice thing about averaging shots together is that the editing programs allow subtle gradations in the darker areas, and the noise, being different for each shot, tends to cancel itself out. I've read that some photogs also purposefully over-expose a bit, planning to darken the image during editing, so the dark areas will have smoother gradations in brightness. It occurs to me that combining over-exposure and averaging might be even better at creating smooth gradations in dark areas. The other problem with leaving the shutter open longer is noise from hot pixels, those randomly over-charged sensor elements that get brighter and brighter the longer the shutter is open. Unfortunately, stacking won't remove hot pixels, because they stay in the same spot for each shot. Fortunately, Elements seems to remove most of them automatically, at least when importing from raw. To find your hot pixels, leave the lens cap on and leave the shutter open for 30 seconds. I think lots of little colore specs will show up on your image, and will even be visible on the LCD if you zoom while viewing the image. Here are 2 shots that show the benefit of averaging. http://www.triangle.com/your_photos/?id=15466336
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Mon, 08/30/2010 - 21:54
#2
Java55
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Just curious, I wonder if using a ND filter and a longer exposure would produce the same results, or if it would be just the same as no ND filter and a shorter exposure? By the way, I have never tried this before but I am thinking that if there is even the most remote possibilty, it can happen.
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Mon, 08/30/2010 - 21:54
#3
Java55
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Just curious, I wonder if using a ND filter and a longer exposure would produce the same results, or if it would be just the same as no ND filter and a shorter exposure? By the way, I have never tried this before but I am thinking that if there is even the most remote possibilty, it can happen.
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Sat, 08/21/2010 - 04:43
#4
Art_Wannabe
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noise and average-stacking Submitted by Art_Wannabe on April 15, 2009 - 9:30am I feel like I've had some success in reducing noise by average-stacking. I'm pretty happy, and want to share this and see if anyone else finds it useful or has any comments. The idea of average-stacking is to take several identical shots and then average them into one. Noise being random, the tiny random color spots should be reduced in proportion to the number of pictures you are stacking. Just stacking 2 shots should cut your noise in half! What this means to me: I've been trying to take shots where the tones go gradually from light to dark, so I inevitably had noisy darkish areas. Even at my lowest ISO, noise was showing up where there was less light. I had resigned myself to having noise in the darker areas. The only ways to reduce the noise seemed to be to •add blur to the dark areas during editing, •take brighter pictures that you then darken during editing, or to •get a more expensive camera (ouch). Selective blurring and taking brighter shots are useful techniques but they don't always work. I'm happy to have found another approach that won't cost me any money. Other kinds of stacking, exposure "HDR" stacking and focus stacking, seem to increase noise. I'm happy to find something that can make these techniques work better. Software: PS elements and photomatix (free version) can average-stack. Maybe other software too. When it could help: tripod shots when the subject doesn't move. Boringness alert! The following content may be interesting only to the author. Continue at your own risk! Example: For the "oyster Mushroom (4)" shot, I took pictures at 7 different focal lengths (each had a different part in focus). For each of these 7 focal lengths, I took 3 different exposures (to have some choice of exposure later and to allow exposure-stacking if I wanted to). For each of these 21 focus and exposure combinations, I took two identical sets of pictures, totaling 42 pictures. I decided that the medium exposures were adequate, so I discarded the brighter and darker shots. Now I was working with 2 identical shots at each of 7 focal lengths. I average-stacked each identical pair using photomatix, creating 7 averaged shots (one at each focal length). The average-stacking process didn't take long (unlike focus stacking). I noticed that the process seemed to reduce the contrast. I'm not sure what to make of that. Anyway, the results were pleasing. I then focus-stacked the 7 averaged shots using combineZM, which I've described elsewhere in this forum. I finally edited the shot in PS Elements. By the way, I took the pictures with my Rebel xsi, a 100mm 2.8 macro lens, set at f/11. Here's the result, so you can judge for yourself. http://www.triangle.com/your_photos/?id=15466291 Enjoy! -Art
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