Mult-exposure averaging on 5DM3 to reduce noise
/forum/topic/1120557/0



snapsy
Registered: Feb 24, 2008
Total Posts: 5204
Country: United States

Here's a demonstration of the 5DM3's multiple exposure feature used to reduce noise through averaging. All images shot raw on tripod, MLUP, timer-release, ISO 6400.

http://horshack.smugmug.com/photos/i-XmKnDjZ/0/O/i-XmKnDjZ-O.jpg
http://horshack.smugmug.com/photos/i-L32hBcj/0/O/i-L32hBcj-O.jpg

Full-sized images:
Averaged in-camera
Averaged in CS5
Individual exposure



AGeoJO
Registered: Jul 08, 2003
Total Posts: 12145
Country: United States

Are those images of the exact same exposure then? If they are, I guessed I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value... .



snapsy
Registered: Feb 24, 2008
Total Posts: 5204
Country: United States

AGeoJO wrote:
Are those images of the exact same exposure then? If they are, I guessed I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value... .


The predominate noise in an image is photon shot noise, which is random across multiple exposures, so if you average multiple exposures together you average out the noise.

http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm



Photon
Registered: Jan 19, 2003
Total Posts: 10159
Country: United States

If the scene is static and you're using a tripod to enable this technique, why not a low ISO exposure?
Astro shooting without a clock drive?
Edit: After reading the article linked, I see various possible reasons listed at the end. The fact that the 5D3 can do it in camera with jpegs probably won't be of use to me, but multi-exposure averaging in Photoshop is a useful tool to keep in mind. For some purposes, I think HDR and low ISO make more sense, but there are always exceptions. Thanks.



RobDickinson
Registered: Sep 25, 2009
Total Posts: 3576
Country: New Zealand

I didnt realise the 5d3 can do this in camera?

Longer exposure low ISO shots are subject to noise too...



Photon
Registered: Jan 19, 2003
Total Posts: 10159
Country: United States

RobDickinson wrote:
I didnt realise the 5d3 can do this in camera?

Yes, with jpg output, though it can also save all the raw files.
It can even align hand held shots pretty well!
RobDickinson wrote:
Longer exposure low ISO shots are subject to noise too...

Yes, and not generally helped by raising ISO to shorten exposure. Multiple exposure averaging, though, is routinely used in astrophotography, so I suppose there would also be occasional uses for it in other types of low light level shooting.



TeamSpeed
Registered: Dec 17, 2005
Total Posts: 1991
Country: United States

I used to do this with my 1D3, but using bracketing. I would take bracketed shots within a 1-2 stop window, and average them, the results were quite nice. Of course that is more along the lines of HDR methodology, but it helped with the noise as well.



Andrew Welsh
Registered: Jan 20, 2007
Total Posts: 4973
Country: United States

This is how astrophotographers get shadow details, reduce noise, increase signal to noise ratio. It's called stacking.

http://astro.welsh-house.net/img/m45-5min-web.jpg
Stack of 27 images, 5 min exposure, ISO1600, Canon 400mm f/5.6L and Rebel XT body.



RobDickinson
Registered: Sep 25, 2009
Total Posts: 3576
Country: New Zealand

I assume that takes some tracking device though?



Andrew Welsh
Registered: Jan 20, 2007
Total Posts: 4973
Country: United States

RobDickinson wrote:
I assume that takes some tracking device though?

Yes, at that exposure length and focal length, tracking is necessary
An 8mm fisheye is limited to about 20 seconds before starts start to become elongated.



Monito
Registered: Jan 28, 2005
Total Posts: 10110
Country: Canada

Interesting thread, snapsy. Thanks.



jchin
Registered: Jan 02, 2005
Total Posts: 2717
Country: United States

How do you do this? Do you need Photoshop?



Andrew Welsh
Registered: Jan 20, 2007
Total Posts: 4973
Country: United States

jchin wrote:
How do you do this? Do you need Photoshop?

It's a menu option on the 5D3.

For other cameras, you would have to do it in any image editing program that has layers, or an astrophoto image processing program (Iris, ImagesPlus, Registax, MaxDSLR, Astrostack, DeepSkyStacker aka DSS, MaxIm, Nebulosity, K3CCDTools, Pixinsight etc etc)



Pixel Perfect
Registered: Aug 16, 2004
Total Posts: 19928
Country: Australia

snapsy wrote:
AGeoJO wrote:
Are those images of the exact same exposure then? If they are, I guessed I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value... .


The predominate noise in an image is photon shot noise, which is random across multiple exposures, so if you average multiple exposures together you average out the noise.

http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm


Yes and this is why it won't work for fixed pattern noise as that's not random. PS uses it's median filter to blend several images to reduce noise

Also the use of PS's median filter can also be used in a different way. Say you have a nice scene in front of you but there are people in the shot that spoil the image. Take several shots (the more the better) with the camera locked down on a tripod of the scene and as long as no one is stationary for all the shots you can eliminate them from the scene.



BrianO
Registered: Aug 21, 2008
Total Posts: 8552
Country: United States

AGeoJO wrote: ...I didn't quite get how the noise reduction works by averaging several images exposed at the same exposure value.

As others mentioned, a lot of noise is random in nature, generated by the amplifier in the sensor, stray photons, etc., and so it appears in different places in the image, whereas actual image elements will be in the same position.

By stacking multiple images, the fixed data is increased in "density" by the number of images, but the random noise isn't.

If we were to shoot four exposures of a sixteen-block color array it might look like this, with the red blocks representing introduced random noise and the bottom image representing the combined "noise-reduced" image.



Monito
Registered: Jan 28, 2005
Total Posts: 10110
Country: Canada

Pixel Perfect wrote: Also the use of PS's median filter can also be used in a different way. Say you have a nice scene in front of you but there are people in the shot that spoil the image. Take several shots (the more the better) with the camera locked down on a tripod of the scene and as long as no one is stationary for all the shots you can eliminate them from the scene.

That can be done manually as easy as pie with any software that has layers and layer masks, like GIMP which is free and Free.



RobertLynn
Registered: Jan 05, 2008
Total Posts: 11710
Country: United States

I move a slider in Lightroom lol.

Sorry I had to.