Some Trick For Noise Rejection

posted on 16th of february, 2011

Lately I took a set of pictures with night urban scenes. Such a subject requires relatively long exposures and ISO should be in most cases high enough. All this stimulates noise and so, normally night photos contain a good deal of noise artefacts. To supress noise I applied some techinque that I took from my astrophoto experience and now I would like to speak on this matter.

1.

So, for all my night shots I used tripod and took several shots from the same point and with all the same sets (typically from 5 to 10). Surely, I saved pictures in RAW format since (a) it does not use any compression and so, it does not rape any information; (b) it uses 13 bits to code brightness (for my Canon 350d) in contrast to JPEG which does only 8. 8 bits are quite enough for resulting image but it maight be not enough at post-process stage, e.g. if one needs to stretch histogram significantly. For cathedral image at the left I took 5 shots (ISO400, f/16, t=2 sec).

2.

I start preprocessing with converting my 5 RAW images to 16-bit TIFFs correcting color temperature and exposure if needed (it goes without saying that corrections must be the same for all 5 pictures). TIFF is not crucial, any 16-bit format would be OK.

3.

In PhotoShop I open new 16-bit canvas with the same sizes that my 5 images are of (in my case it is 3456x2304px). I put these 5 images into new canvas as 5 layers. Now the main trick. I set transparecies for each layer by some specific way (using 'normal' mode): 100% for lowest layer, then 50% for layer that is just above the lowest one, 25% for next one etc... For my case with 5 images transparency values will be as following (from top to bottom): 6.25%, 12.5%, 25%, 50%, 100%. Then I join all layers into the only one.

With such set of transparencies when each previous layer is with factor 2x more transparent comparing to the next one, the brightness value for each pixel on resulting ('stacked') image represents a mean value for 5 images that participated in stacking. With such stacking real objects which are presented on all 5 shots and which are the same on all 5 shots (sky, clouds, buildings) do not change, while noise (which is result of spontaneous dark current and is not the same on each of 5 shots) becomes lower.

It is illustrated by the picture below on which part of single shot with sky (that typically is the most noisly place) is shown at the left with 200% magnification to show more details and result of stacking procedure (as described above) is shown art the right with the same magfnification. Here is link to this picture

Essential improvement for SIGNAL/NOISE ratio is clearly visible. Note, that with 10 shots effect would be even more remarkable. From my experience I may conclude that sometimes even 2 images (instead of 1) are enough for successfull noise supression. In most cases I use 4 or 5 and it works properly. 10 images were enough in all cases I faced with.

I would be glad if my post will be of any use for somebody.

Tags: night noise processing rejection supress

Comments (68)

	Posted by Igordabari on March 13, 2011 to Maatzey: Hi Maciej. The technique I described here does not work for stars if you want to keep also landscape. I am going to write several blogs on shooting stars but, unfortunately, have noy fount time for this yet. Anyway, thanks for the link - your image is nice.
	Posted by Maatzey on March 13, 2011 Well I have done some test using Your method and some other. unfortunately with 18mm and 20sec. exposure I was not able to keep the stars in one point :( As this creates also nice effect I was more heading for "one point stars". Please have a look at the outcome of my tries Noise reduction works
	Posted by Keki on March 10, 2011 fantastic work! thanks for sharing :)
	Posted by Maatzey on March 09, 2011 Thanx for answer.... I am waiting for this long, long story :) I want ALL MY STARS :)
	Posted by Maddrew on March 09, 2011 Thanks for sharing :) very comprehensive tutorial and the results are fantastic :)
	Posted by Igordabari on March 09, 2011 If focal length is 50, then period up to 2-3 seconds are allowed to keep a star within the same pixel (given that optics are ideal, which is not a case :-) ). Actually one can use exposure even 4-6 seconds and stars will look still points. If focal length is 18 mm, 'allowed period' could be 3 times longer. So, quite enough time to take 3-5 shots to be stacked. But with such short exposures one will get only the brightest stars. To get all stars visible by eyes one needs to use VERY sophysyicated technique. It is another story (and very long one :-) ).
	Posted by Maatzey on March 09, 2011 very nice technique, but I really love stars on my pictures... how will it look with stars, how cen we keep stars and not loose them as noise (stars will significantly move during 5 shots...)
	Posted by Igordabari on March 08, 2011 to Rzs: This I do not know exactly but think that result will be the same. But it is much more simple and faster to stac 10 photos and then appy PSh noise filter for the only image. Otherwise one would have to apply noise filter 10 times and then stack which would be an obvious mess.
	Posted by Rzs on March 07, 2011 Sophisticated tutorial indeed! Thanks for new technique (for me) you share with us. What do you think? Is it better to reduce noise first by using noise reduction for instance in Photoshop and then use your technique, or conversely?
	Posted by Ziprashantzi on February 27, 2011 very nice article
	Posted by Igordabari on February 25, 2011 to Fra73:It MUST work :) Glad for you, hope to see results in your PF. Good luck!
	Posted by Fra73 on February 25, 2011 I tried and it works!!!! Thanks again :)
	Posted by Igordabari on February 22, 2011 to Uptall: Thanks for reading and commenting. Good luck. to Fra73: I would be happy to know that you try it getting wonderful results. Best of luck to you!
	Posted by Fra73 on February 22, 2011 I have always wondered how to efficiently reduce noise in night pictures. Thanks a lot, great tip!!!
	Posted by Uptall on February 21, 2011 thanks for share.nice pics
	Posted by Igordabari on February 21, 2011 to Kelpfish: Because there are no miracles in this world :) Even 1.000.000 copies do not contain more information comparing to original. So, stacking photo with itself would be of no use. Technique is based on the fact that on each of N photos object (sky, cloud, building, ...) is THE SAME while noise ACIDENTALLY CHANGES from shoot to shoot. So, with stacking one makes noise to 'compensate itself' while real objects do not change. So, noise decreases and signal is the same and so, SIGNAL/NOISE ratio becomes better. If one will use copies for stacking nothing will change, at all.
	Posted by Kelpfish on February 21, 2011 If all exposures are the same why not just duplicate the layer five times?
	Posted by Igordabari on February 21, 2011 to Anhong, Liubaohua & Davulcu:Glad to share ideas that you find to be useful. Good luck!
	Posted by Davulcu on February 21, 2011 Thanks for sharing Igor. I will try this and share my ideas asap
	Posted by Liubaohua on February 21, 2011 Thank you for sharing your technique.Very interesting...I will also try it out next time I shoot at night.
	Posted by Anhong on February 19, 2011 Wonderful pictures. Very good skills. Thanks for sharing!
	Posted by Igordabari on February 19, 2011 Any special considerations in determining your exposure or white balance, when you initially took the images? No, this is irrelevant. Good luck!
	Posted by Adeliepenguin on February 19, 2011 Very interesting...I will also try it out next time I shoot at night. Any special considerations in determining your exposure or white balance, when you initially took the images? Thanks for sharing.
	Posted by Igordabari on February 18, 2011 to Sigurdurwilliam & Jjumawan:It would be interesting to know your opinion AFTER you get some results using the technique.
	Posted by Igordabari on February 18, 2011 to Sigurdurwilliam & Jjumawan:It would be interesting to know your opinion AFTER you get some results using the technique.
	Posted by Jjumawan on February 18, 2011 Thank you for sharing your technique. I'll try it this weekend.
	Posted by Sigurdurwilliam on February 17, 2011 Neat trick... Can´t wait to try it out... If only I had read this before I went out to shoot last night... XD
	Posted by Igordabari on February 17, 2011 to Kelpfish:No, I was not going to make HDR (which I do not like) and so, exposure was exactly the same for all 5 shots. So, layer order was irrelevant.
	Posted by Kelpfish on February 17, 2011 Very good info. Thank you! Are you shooting all 5 images at F16 @ 2 sec? I'd think you would have to bracket to get dark to light before applying the tiered transparencies. Then what exposure order do you use? Darkest at top? And I assume that bracketing is done through EV adjustments or shutterspeed? Joe
	Posted by Igordabari on February 17, 2011 to Ospictures: For your wonderful and charming shots with models this technique seems to be of no use. Nevertheless, thank you for reading and good luck!