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In low lighting conditions I have to bring up my ISO (even when using flash) and open the aperture to a certain point to get the ambient light alongside subjects I'm photographing (which are mostly lit by bounced flash).

Just this weekend I was working with Canon 5D mk3 mostly in Aperture Priority mode (somewhere around F/7.1 with ISO5000), and I notice that some of my photographs have practically negligible amount of noise, while other had really noticeable amount of noise, which was not that big of a problem, but I was wondering:

  • What would be a factor that "reduced" noise in some shots, because the only two variables would be the shutter speed, and the amount of light in the room?
  • Does shutter speed somehow affects noise?
  • Does the amount of light affects noise?
  • Are there any other factors that would prevent or reduce noise at the moment of taking the photograph?

P.S. I'm sure you got it that I'm not talking about post-processing, but here's just another mention for those who don't.

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  • \$\begingroup\$ Of course you're talking about the same camera, isn't it? That's why in my answer I left outside sensor characteristics like size etc. \$\endgroup\$
    – John Thomas
    Commented Jun 19, 2012 at 10:37
  • \$\begingroup\$ @user952: Yes I'm talking about the same camera. I guess that it's pretty obvious that different sensors have different ISO noise behavior. \$\endgroup\$
    – Miljenko Barbir
    Commented Jun 19, 2012 at 10:51
  • \$\begingroup\$ possible duplicate of What types of noise can be present in digital photographs? \$\endgroup\$
    – mattdm
    Commented Jun 19, 2012 at 11:00

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The amount of light "affects" noise because high ISO noise is most obvious in dark areas of the photo (shadows). If you have a photo with plenty of light in it the noise will not "appear". This is one factor, but not the only one.

Longer shutter speeds usually increases a certain form of noise (called salt & pepper noise or spikes) due to photodiode leakage currents.

Also the ISO affects the amount of noise besides the well known rule of "higher ISO = higher noise". Beyond the real (base) values of ISO (100, 200, 400 etc.) the inbetween values are obtained by pushing/pulling digitally the image hence in ISO 125 (and multiples) you'll see more noise (digital push) while in 160 (and multiples) you'll see less noise than the pushed ISO (ie. 160 ISO has less noise than 125 ISO) because it is a digitall pull. Of course this quasi-'noise reduction' is obtained at the cost of dynamic range.

Another thing (related to exposure time) is sensor's temperature. At higher temperatures the image can be noisier and sometimes can appear 'hot pixels', especially in long/bulb exposures and/or in warm zones/seasons.

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    \$\begingroup\$ Is ISO 100, 200, etc. always base value ISO, or do different cameras have different base ISO values? \$\endgroup\$
    – Miljenko Barbir
    Commented Jun 19, 2012 at 10:47
  • \$\begingroup\$ Always (AFAIK). However Nikon has its main base value (the best value, let's say) at ISO 200 while Canon at 100. At least this was the case with older Nikon models. I don't know now. \$\endgroup\$
    – John Thomas
    Commented Jun 19, 2012 at 10:55
  • \$\begingroup\$ 100 and 200 are common. But I've used older cameras with base ISOs of 50 and 80. \$\endgroup\$
    – mattdm
    Commented Jun 19, 2012 at 11:10
  • \$\begingroup\$ Hey, @user952! You've been around for a while and contribute good questions and answers. Have you considered picking a non-generic name so it's easier for us to remember who you are? \$\endgroup\$
    – mattdm
    Commented Jun 19, 2012 at 12:42
  • \$\begingroup\$ Ok, Matthew! :-) ...here you are! \$\endgroup\$
    – John Thomas
    Commented Jun 20, 2012 at 6:07

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