Using multiple exposure to combat noise

Started Aug 2, 2007 | Discussions
jp Senior Member • Posts: 1,151
Using multiple exposure to combat noise

Inspired by the trick of image stacking used in astronomical photography, I decided to perform a few experiments with my D200 yesterday and I was absolutely amazed by the results.

I put my D200 on my tripod, chose a relatively dark static scene and shot it with 5 multiple exposures of 1/4s. Then shot the same scene with single exposure for reference. Although noise is not really a problem at iso 100 it is still readily visible in "zone 3" if you really look for it.

But by using multiple exposure to stack multiple identical images, the noise becomes absolutely invisible.
Theoretically noise should drop by sqrt(n) where n is the number of images.
This does indeed seem to be the case.

The automatic gain seems to work correctly too: the multiple exposure image and the single exposure image were equally exposed.

So noise reduction for static scenes by means of image stacking does indeend seem to work well.

It can of course be applied to any digital camera but the advantage of cameras with the multiple exposure feature like the nikon DSLRs is that you can have it done immmediately in-camera on the spot, no need for post processing, no card space wasted on multiple raws.

Is this technique really worth anything in the field?
I don't care, you decide for yourself, I just wanted to share my experiences.

(hopefully this issue hasn't already been beaten to death here in the past. In that case forgive me for my ignorance)

WyleECoyote Regular Member • Posts: 182
Excellent!!!

Great idea!

I was beginning to think that this forum had been relegated to discussing what features we'd like to see in the D6000, why full-frame is the future and why 35mm is full frame but dx/aps-c and mf are not, why canon is better than nikon and why everyone needs a 5d, and why nikon will go bust tomorrow if they don't replace the rubbish d2x... etc... etc...

I agree that uses for this may be limited, but it's a technique to be aware of.

HDR can remove noise, but it also removes shadows, which may be desireable.

Actually, there's a thought... is it possible to merge "differently-exposed" exposures into one shot - a "multi-exposure hdr"???

I may have to sit down with the manual and have a play tonight...

aclo Regular Member • Posts: 480
Re: Using multiple exposure to combat noise

jp wrote:

So noise reduction for static scenes by means of image stacking does
indeend seem to work well.
It can of course be applied to any digital camera but the advantage
of cameras with the multiple exposure feature like the nikon DSLRs is
that you can have it done immmediately in-camera on the spot, no need
for post processing, no card space wasted on multiple raws.

Is this technique really worth anything in the field?
I don't care, you decide for yourself, I just wanted to share my
experiences.

Hi,

Another interesting (and useful, in extreme circumstances) trick is to do what you describe handheld. This is an example where I exposed for ISO 6400 as a test (it's full-size, so I won't link directly) and shot 8 images handheld at 5fps, converted with positive exposure compensation, and then stacked them. I also removed colour noise before uploading it, I don't remember if I did anything else:
http://www.pbase.com/al599/image/82758775/original

Of course the frames were not all exactly aligned, so I used a stitching program (hugin and tools included with it) to align the images. I think the result isn't bad for ISO 6400 (and with some more effort it could be better, I didn't spend any time on it-although my computer did!).

A more extreme example which I did on the way home from work is (again full-size, so won't link directly)
http://www.pbase.com/al599/image/82790676

which is again 8 frames. I basically exposed to not overexpose the open window. This could have been better if I had set the camera to a lower ISO than I did, and pushed more in processing; also if I could subtract a dark frame (which I can't think of how to do with this sort of technique). It was exposed for something like ISO 50000 or so (I am estimating).

Anyway, I've used this technique a few times in actual handheld shooting, it works great (but of course it is time consuming).

Nick Wong Contributing Member • Posts: 560
Thanks JP.

Thanks JP for sharing the idea. I'll definitly try it tonite.

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rob outlaw Regular Member • Posts: 245
Re: Using multiple exposure to combat noise

In theory you are absolutely correct about this. I experimented with this method early on with my D2x and the results on "static" objects was amazingly better, with exceptionally smooth renderings on all three channels. The word static here is of utmost importance, because if you have moving objects as in a windy day, or clouds moving across the scene that sort of thing you end up with images that have ghosted elements within the image that really are quite noticeable and do not look so good IMO. And obviously a sports or wildlife photographer will not be able to utilyze this method.

Ultimately in the field I have found very little use for what could be a wonderul tool and technique, perhaps because I live in an area where something always seems to be moving. Also FWIW, this tehcnique at least with the D2x does not work tethered to the computer. From my epxeriences you can only do this when working straight to the flash card.

When running longer exposures as in several seconds the results are truly superior, but you have to have a very steady tripod as well. The slightest vibrations can cause a loss in image resolution when running multiple exposures or staking as you say. Also I got much better results on the D2x with auto gain on vs off.

Hope this helps

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jp OP Senior Member • Posts: 1,151
Re: Excellent!!!

WyleECoyote wrote:

Great idea!

I was beginning to think that this forum had been relegated to
discussing what features we'd like to see in the D6000, why
full-frame is the future and why 35mm is full frame but dx/aps-c and
mf are not, why canon is better than nikon and why everyone needs a
5d, and why nikon will go bust tomorrow if they don't replace the
rubbish d2x... etc... etc...

I agree that uses for this may be limited, but it's a technique to be
aware of.

HDR can remove noise, but it also removes shadows, which may be
desireable.

Actually, there's a thought... is it possible to merge
"differently-exposed" exposures into one shot - a "multi-exposure
hdr"???

I may have to sit down with the manual and have a play tonight...

You can not combine AE bracketing with multiple exposure, so if you want different exposures you wil have to change the exposure compensation manually between shots (at the risk of moving the camera and getting fuzzy results).

I used the 0,4 second exposure delay after mirror-up and interval shooting (1 second intervals and numer of shots equal to number of multiple exposures) with my multiple exposure experiment

That way, I did not have to touch the camera while the shots were taken so the result was absolutely razor-sharp.

It also pays to use RAW for the result: the lower bits will definitely contain useful information that can not be aquired in any other way.

On your remark about canon vs nikon.

After I switched from pentax to canon when the eos 620 came out 20 years ago, and going through several different bodies including the eos1 I have always preferred the ergonomics of canon (all softkeys) to those of nikon (all dedicated controls). When I occasionally used my friend's F4 or F80 I always cursed on the stupid ergonimics.

When giong digital I sold my most of my EF lenses and switched to the D70 and later to the D200. In my opinion, Nikon has hit the mark with their recent bodies with the pinaccle of D2 and D200 having perfectly balanced dedicated controls, softkes and menus.

The D200 is really an ergonomical dream to use. Ergonomically, it blows away all canons except their 1 series, which are closer to nikon but not still not that good.

Canon has even worsend some of the ergonomics of their new cameras, compared to earlier bodies.

Only two gripes about the D200: the Iso-auto mode has several flaws and the function of the AF button is not correclly done.

To me, as long as you do nog go to higher ISO, the quality of the D200 images is as good as any comparable Canon. It is just a question of how processed you want your images to look straigt out of the camera.

But In the high ISOs, the Canons are far superior, especially whith the D200 which has a full well size only about half of the 30D and twice as much read noise, a deadly mix for usefull high ISOs.

If I were a professional, I would bite the bullit and go back to canon for their 1Ds MKII but as an amateur photographer, wanting a good solid ergonomically sane body at a reasonable price, there is no real alternative to the D200.

Anyway, if you see that results nikon's image processing gets almost as much out of a clearly less capable sensor, I have high hopes that when better third party sensors will come available, they will give canon a run for their money. But until then, I can understand the people who are switching camp, although they may well regret their decision one day.

I am also considering an additional 400d or 30d, just for the typical low light situations where I can not use a tripod.

WyleECoyote Regular Member • Posts: 182
Re: Excellent!!!

I wasn't having a dig at canon - they appear to make excellent cameras.

i was having a dig at all the "canon is better than nikon... nikon is better than canon... i need ff... why don't nikon do... nikon is finished if the don't... i WILL jump ship d*mn it... what i want in the next camera is... etc... etc..." posts that clog this forum and make it difficult to find useful posts like this.

so you can change exposure settings between exposures... interesting... if the camera is connected to a laptop you can change the aperture/shutter between exposures without touching the camera.

it does mean hauling a laptop though which makes it even more restrictive.

but given the limited situiations in which you could use it this may not be an issue.

definitely something to think about and play with... may come in useful...

markmark Senior Member • Posts: 1,651
Re: Using multiple exposure to combat noise

a couple of nites ago i fell upon a photog (shooting canon, but that is irrelevant)

shooting a nighttime skyline.. he was taking three identical frames and was going to merge them together..he said something about 25% opacity for each of the two top layers....to smooth out the noise

i figured this was a bunch of bunk but now i see this post..i wonder if it is a viable solution for an infrequent problem

seems to me that in the realm of desirability, a bit of noise would be better than the possibility of unsharpness from stacking multiple exposures..

RBBailey2 Regular Member • Posts: 406
I've always wondered about that

I've always thought that with enough overlays the noise would cancel itself out if you used actual different exposures -- I usually use copies of the same RAW image when I do HDR type work, so I never really tested it -- do you have examples?

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Matt_Anderson
Matt_Anderson Veteran Member • Posts: 3,780
How did you stack them ?
rob outlaw Regular Member • Posts: 245
Re: Excellent!!!

WyleECoyote wrote:

so you can change exposure settings between exposures...
interesting... if the camera is connected to a laptop you can change
the aperture/shutter between exposures without touching the camera.

No, from my experience you cannot do mulitple exposures tethered or at least I have not found a way to do it with the D2x, perhaps the D200 is different though.
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drmaturin Regular Member • Posts: 215
Re: Using multiple exposure to combat noise

I have used this Mac program to stack images to reduce noise. It works well. http://www.versiontracker.com/dyn/moreinfo/macosx/25534

glasswindow Contributing Member • Posts: 758
A new feature in CS3

http://wiki.panotools.org/Image_Stacking

There are quite a few uses for this CS3 feature, actually.

WyleECoyote Regular Member • Posts: 182
Re: Excellent!!!

you said in your other post that you can only do this working straight to card.

while connected to a pc it also writes to the card if one is inserted doesn't it?

or does it write seperate images to the card regardless of your multi-exposure settings if you're connected to a pc/laptop?

rob outlaw Regular Member • Posts: 245
Re: How did you stack them ?

You don't, its done within the camera. My suspicion is that Nikon with their ME control using auto gain is really a different way of going about what Kodak did with the 14n. With that camera you had the capability of shooting at ISO 6, 12, and 25 but with some severely limited controls but the noise control was awesome with very smooth tones. Nikon seems to be doing the same more or less but by way of layering the exposures somehow. I did notice some slight deviations in exposure when using the ME setup on the D2x, thus the comparison with how Kodak did this. Its also possible that Nikon is "binning" pixels cells in different arrays much like Phase One did with their earlier scan backs to not only reduce noise, and file size while at the same time increasing ISO as opposed to amplifying the signal. I guess only Nikon engineers know for sure what is going on under the hood, but it does work well. Unfortunately I have not been able to put it to extensive use due to ghosting from movement from the different frames.

Rob

rob outlaw Regular Member • Posts: 245
Re: Excellent!!!

WyleECoyote wrote:

while connected to a pc it also writes to the card if one is inserted
doesn't it?

Well lets put it this way, using Nikon Capture Control shooting into either NC 4 or Lightroom tethered those functions are chalked out on the camera monitor when the USB cable is hooked up. Perhaps there is a work around but for the life of me I have not found it, regardless of whether a card is in the camera or not. Maybe there is something I have overlooked but so far no luck.

Maybe the D200 is different but so far this is the way it works with the D2x.

Rob

Jorgen E Senior Member • Posts: 1,730
Sony is way ahead of you... :-)

Look at page three in this pdf document, where it's described how multiple high speed exposures @60fps, combined with overlay/stacking and an advanced DSP, can be used to improve both S/N-ratio and camera shake.

http://www.sony.net/Products/SC-HP/cx_news/vol43/pdf/featuring43.pdf

Do I see a D3 feature...?

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rob outlaw Regular Member • Posts: 245
Re: A new feature in CS3

glasswindow wrote:

http://wiki.panotools.org/Image_Stacking

From what I have read this is a bit different intent of stacking with the extended version only of CS3. Its my understanding this stacking method was to assist in the cleanup of unwanted items that might be in a series of pictures similar to the iilustration in the link you provided. Which is not to say that it might not also reduce noise as well though its unclear to me how it would do that simply by layering multiple images of the same photo.
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aclo Regular Member • Posts: 480
Re: How did you stack them ?

On the computer, it can be done using layers if the exposures are from a tripod: if you have n shots, set the top layer to 100/n % opacity, the second to 100/(n-1)% etc down to 100% for the bottom layer.

for handheld shots, you can use stitching tools and export the "panorama" (of course you just use the tools to align the shots) as layers, and do the same. eg
http://www.pbase.com/al599/image/82736991/original
(it's full size), exposed for iso 6400, 8 shots handheld at 5fps

aclo Regular Member • Posts: 480
Re: A new feature in CS3

rob outlaw wrote:

provided. Which is not to say that it might not also reduce noise as
well though its unclear to me how it would do that simply by layering
multiple images of the same photo.

noise is random; so averaging lots of photos averages noise to zero.

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