Stacking photos to reduce noise - astro photography: Anyone familiar with stacking photos to reduce noise? I'm going out to shoot the milky way and stars and wanted to use stacking for...

jayluber Loc: Phoenix, AZ

Anyone familiar with stacking photos to reduce noise? I'm going out to shoot the milky way and stars and wanted to use stacking for noise reduction. However, as I shoot the numerous photos from my tripod mounted camera, the rock formations in the foreground will remain stationary, but the stars will be slowly moving across the sky and be in slightly different places relative to the foreground as the photos progress. When I stack them, either the stars will be aligned and the foreground will not be, or vice versa.
Anyone deal with this before? Only thing I can think of is to align the stars, then use one photo to merge the single rock formation over the stacked photo.
Any suggestions?

PGHphoto Loc: Pittsburgh, PA

jayluber wrote:

Anyone familiar with stacking photos to reduce noi... (show quote)

Stacking is normally used to correct focus limitations, not reduce noise. It will actually INcrease noise because each shot will introduce noise. Usually the noise reduction process involves taking one shot normally and following it with a second shot with the shutter closed and the exposure time matching the first. The resulting image will identify the points of light caused by noise. This pattern can be used against the first shot to remove those points from the initial photo. Most cameras have this functionality in camera. Little more difficult but do-able in photoshop.

Stacking is also used to produce star trails. As you identified, the stars will move across the field of view and the multiple exposures are overlayed so the full path of the stars can be seen.

TriX Loc: Raleigh, NC

I’m wondering if the OP is referring to averaging to reduce noise as explained here: http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm

I haven’t personally tried it in photography, but averaging of repetitive signals is very effective in electronic measurements if the noise is truly random.

rook2c4 Loc: Philadelphia, PA USA

Stacking images can be a very effective way to reduce noise in color images. As noise is random, stacking will have a cancelling effect on it. I use this mostly with scanning of prints - I take multiple scans of the same print, then stack them in the editor.

Try reducing the interval between shots.

PGHphoto Loc: Pittsburgh, PA

TriX wrote:

I’m wondering if the OP is referring to averaging to reduce noise as explained here: http://www.cambridgeincolour.com/tutorials/image-averaging-noise.htm

I haven’t personally tried it in photography, but averaging of repetitive signals is very effective in electronic measurements if the noise is truly random.

Never heard of averaging - interesting concept. Seems like a rather large number of shots would be required based on the examples in the article. Thanks for the link.

Seems that using in-camera high ISO compensation would be less work and it appears to yield better or - at worst - similar results. Worth trying tough ...

jcboy3

PGHphoto wrote:

Stacking is normally used to correct focus limitat... (show quote)

It's called stacking. It averages the noise. Load the images into layers, align them, assign opacity levels based on number of layers, and merge layers. Noise is reduced by square root of number of images.

Then take one image with the ground elements you want, shift it so it covers the ground elements from the merged layers, and then use a layer mask to merge the two images.

bwana Loc: Bergen, Alberta, Canada

jayluber wrote:

Anyone familiar with stacking photos to reduce noi... (show quote)

Shooting multiple subs (images) is standard practise in astrophotography. The goal being to improve the signal and reduce noise. Since the signal-to-noise ratio (SNR) is proportional to the Sq.Rt. of the number of subs the more subs you shoot the better the result, i.e.: 4 subs === 2x SNR gain, 49 subs === 7x SNR gain.

As for the foreground, shoot it separately and merge it into the starfield image.

bwa

P.S.: And of course you can merge any foreground into the image. It's called Photoshopping for a reason.

TheDman Loc: USA

jayluber wrote:

Anyone familiar with stacking photos to reduce noi... (show quote)

Place your ground exposure on one layer, align and stack your star images in a group, then mask the ground out of the group.

TriX Loc: Raleigh, NC

PGHphoto wrote:

Never heard of averaging - interesting concept. Seems like a rather large number of shots would be required based on the examples in the article. Thanks for the link.

Seems that using in-camera high ISO compensation would be less work and it appears to yield better or - at worst - similar results. Worth trying tough ...

In the electronic world, averaging decrease in effectiveness as you average more samples - somewhere in the neighborhood of 16-32 seems to be a good compromise. If you average too many samples, the variability of the registration between samples tends to decrease the effectiveness if you continue to average large numbers of samples. I’m not sure where the “sweet spot” is in terms of imagery.

bwana Loc: Bergen, Alberta, Canada

TriX wrote:

In the electronic world, averaging decrease in effectiveness as you average more samples - somewhere in the neighborhood of 16-32 seems to be a good compromise. If you average too many samples, the variability of the registration between samples tends to decrease the effectiveness if you continue to average large numbers of samples. I’m not sure where the “sweet spot” is in terms of imagery.

Since signal-to-noise ratio (SNR) is the Sq.Rt. of the number of subs (images) there is definitely an optimum; not too sure what it is but I normally shoot 40-50 subs off a tracking mount. It depends upon the accuracy inherent in the alignment and stacking software.

It also depends upon the distortion in the subs being captured, i.e.: night sky subs shot off a tripod become harder and harder to properly align and stack as movement increases to the point where beyond about 10 subs it is often impossible to properly align and stack the session without sophisticated astro-imaging processing software such as PixInsight which can do geometric correction of the subs.

Often the best approach is to simply use the tools available in your camera for shots of the Milky Way and such. For instance, Sony mirrorless cameras will shoot four images, and align and stack them internally for a quick increase of 2x in SNR, often more than enough for "instant" astrophotography.

bwa

jerryc41 Loc: Catskill Mts of NY

jayluber wrote:

Anyone familiar with stacking photos to reduce noi... (show quote)

Yes, that's the recommended way to do star photography to prevent blurring of the stars.

http://www.diyphotography.net/guide-astrophotography-will-shooting-stars-no-time/
http://petapixel.com/2014/01/29/picking-great-lens-milky-way-photography/
https://www.davemorrowphotography.com/p/tutorial-shooting-night-sky.html
https://www.nikonusa.com/en/learn-and-explore/a/tips-and-techniques/photographing-the-night-sky.html

sb Loc: Florida's East Coast

Many Canon cameras do this automatically - "multi-shot noise reduction". My 6D takes 4 photos in rapid succession and merges them to reduce a lower-noise image. This photo was taken with just the light of a hand-held spotlight - using a 6D with the 100-400mm lens at 330mm hand-held at 1/80 sec - the auto ISO went to 25,600! But the four photos merged into an acceptable photo as long as I don't want a big print!

(Download)

koalaroo Loc: Gold Coast, Australia

Beautiful.

clemente21

Stacking images is used for :
focus: taking pictures at different depths of field
noise reduction: increasing the signal to noise ratio
eliminate unwanted objects: only the stationary objects remain.

There is a book that explains this (astrophotography included) and other uses (HDR, double exposure,etc.) then, how to use photoshop to merge them

http://www.rockynook.com/shop/photography/the-enthusiasts-guide-to-multi-shot-techniques/

TriX Loc: Raleigh, NC

sb wrote:

Many Canon cameras do this automatically - "multi-shot noise reduction". My 6D takes 4 photos in rapid succession and merges them to reduce a lower-noise image. This photo was taken with just the light of a hand-held spotlight - using a 6D with the 100-400mm lens at 330mm hand-held at 1/80 sec - the auto ISO went to 25,600! But the four photos merged into an acceptable photo as long as I don't want a big print!

Excellent capture. I’ve never looked to see if my 5D3 has this feature, but it certainly is effective! Thanks for the information.