Low light Photography and Digital Noise

Last Updated on June 17, 2010 by Susheel Chandradhas

You’ve all had digital pictures with “noise” in them… want to know why?

Digital cameras have sensors that receive photons (the thingies that make up light). These sensors are analogue devices that turn analogue signals into digital data. These charges are converted to digital data in cells that are sensitive to three different colours of light: Red, Green and Blue (RGB). The cells that receive more photons have a higher charge and are designated ‘lighter’ while the ones with a lower number of photons are darker.

Now, what happens when you take a photograph in a really dark place? Once you’ve opened up your aperture to the max, and used the lowest shutter speed possible, you turn the ISO up, right? And voila! Your sensor is more sensitive, right? Naah! The sensor can’t really adjust it ‘sensitivity’; instead, what your camera does is, it turns up the amplification of the photons that are received. Making them seem brighter. Simple!

So why are higher ISO photographs so grainy, and noisy? If you guessed “because of the amplification,” you’d be right… Its similar to amplification on your stereo. If the signal level is low, the more you amplify it, the more likely it is that you’re going to amplify the background static too. And this will give you lower quality sound.

Your camera’s sensor has a basic charge, so if we turn up the amplification because the signal level (number of photons) is too low, this charge, along with the charge from neighbouring cells interferes, creating ‘noise’. This ratio of ‘signal’ to ‘background noise’ (or static) is called, quite simply, Signal to Noise ratio. If the signal to noise ratio is high, you’ve most likely got a clear picture, if it is low, you’ll have a noisy one (how about that, next thing you know, I’ll be talking about loud-mouthed pictures!).

Simple Work-around

Ok, now don’t ask me how to keep the S/N ratio down, that’s something that the engineers who built the camera will have to figure out. What I can tell you, is that if you stick to lower ISO numbers, and use higher ISO numbers only you absolutely have to, you’ll have significantly less-noisy pictures (always keep in mind that a good picture is meant to be seen and not heard).

I’ve tried hard to keep this post jargon-free. Do let me know if you would like a more techie explanation, and I’ll give you the whole story with diagrams and tech-talk.

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