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Started Jan 13, 2012 | Discussions thread
MOD Dale Buhanan Veteran Member • Posts: 4,271
Re: High ISO noise, colour temperature, and crying 'Fake'

Really excellent explanation, Andy. I learned something here myself. Thanks!

Andy Westlake wrote:

What you seem to be getting wrong here is a very simple error, and very common indeed. It's the result of shooting under artificial light at high ISOs, seeing lots of noise, and deciding that this means all 'real world' high ISO shots will have lots of noise. It's incorrect because it fails to understand how colour temperature and white balance has a huge effect on noise, which is most visible when shooting under artificial light.

If you shoot under domestic artificial light, you're generally looking at light sources with colour temperatures below 3000K. This means they're very orange, with little blue light. If you set the camera to daylight WB and shoot under artificial light you'll see this very clearly indeed - the picture will have a horrible orange colour cast.

To produce a visually-acceptable image that looks correctly colour-balanced, the camera has to amplify the blue channel massively compared to red, and the green channel by a couple of stops too. This is why the noise is so bad - in essence you're shooting at a much higher effective ISO than you think.

The point here is that excessive high ISO noise under artificial light is not due to the light level but the colour temperature. If you shoot at the same ISO under conditions closer to daylight white balance (ca 5500K), you'll get much better results. If you must shoot under artificial light, then you'll generally get better results using an 80-series blue colour-correcting filter over the lens and shooting at slower shutter speeds, if you can get away with it. This allows better exposure of the blue channel without overexposing the red.

Now, in the real world there are many high ISO shooting situations where the colour temperature is within the camera's comfort zone, and in these cases you'll get much the same performance as our tests show. The most obvious examples are daytime indoor shots using window light, or simply shooting outdoors early or late in the day. Or perhaps stopping down for maximum depth of field in a close-up while shooting hand-held. High ISO isn't only for shooting under artificial light.

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kind regards
Dale

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