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I shoot with a Canon EOS Rebel T5 and sometimes see noise even when shooting at ISO 100. Is this just because there is not enough light in the dark portions of the picture (seen below)? Is this a function of the image sensor, lens, or both?

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This is a perfect example of "expose to the right" — that is, even though you want the final result to be low key (largely dark), take the initial exposure as bright as you can (without blowing out the brighter part of the sky, reflections, or any more subtle brighter areas). When you expose so that dark areas are really dark — either because you are underexposing or because that area of the scene really is dark — there are fewer photons to count, and so less signal, which means the signal to noise ratio is worse.

Choose a brighter exposure even for the areas you want dark eventually, and then bring down in post. If necessary, you may actually get better results by raising the ISO — see this answer for details. But in this case, since presumably you're using a tripod for your cityscape shot already, you can probably just increase exposure time. If necessarily, you might consider using HDR or exposure blending techniques to get detail in both the shadows and the highlights (although I don't think that will be necessary in this scene).

  • Needed to say, one of the big advantages of doing a HDR here is that you actually take an average or several photos during the process, which itself is an significant improvement (stops being significant after some 7-8 photos though). – yo' Mar 16 '15 at 18:02
  • "you may actually get better results by raising the ISO" - that's only true if you're planning on increasing the exposure in post, not decreasing it as you suggest here. See my question about that very post. – BlueRaja - Danny Pflughoeft Mar 16 '15 at 21:13
  • @BlueRaja-DannyPflughoeft If you have the other exposure parameters at their limit already (as wide an aperture as the needed depth of field allows and as long a shutter as practical), and there's still room to increase the ISO without overexposing highlights, I think it's still correct. It'd be better to actually let more light in, but as I say, if necessary.... – mattdm Mar 16 '15 at 21:37
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An alternative to "exposing to the right" proposed by @mattdm could be noise reduction through image stacking:

  1. align images (all images taken with "as identical as possible" intrinsic and extrinsic parameters (view point, focal length, etc.))
  2. stack aligned images as layers in a single image
  3. blend layers using median/average/...
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    One of the better tutorials IMO: blog.patdavid.net/2013/05/… and bases it all on open-source/free software which is good for those of us who don't like/want photoshop. – Holloway Mar 16 '15 at 12:29
  • Since you're using a Canon, you could install BlackMagic (the CHDK adapted firmware for higher end Canon cameras). If you take several raw photos in succession with the same camera, BlackMagic can average the files you specify. – Stephen Schrauger Mar 16 '15 at 13:33
  • Of course, if your scene changes significantly between the first and last photo (such as in a sunrise), it may not work as well, since your last image would be brighter than your first. In that case, you'll have to use multiple cameras, as described in this answer. – Stephen Schrauger Mar 16 '15 at 13:35
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Noise reduction is generally effective in dark areas because there is little detail to retain. Most noise will be chroma (color) noise so it is easy to remove in your RAW processor or with third party noise reduction.

Remember that there is no "correct" sunset exposure. I always bracket widely and choose the best result.

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