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Low light Photography and Digital Noise

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

How Do Digital Cameras Work?

Digital cameras have sensors that receive photons (the thingies that comprise 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.

Amplifying weak digital signals (low light)

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 its ‘sensitivity’; instead, what your camera does is, it turns up the amplification of the photons that are received. Making them seem brighter. In audio, this is called ‘gain’.

So why are higher ISO photographs so grainy, and noisy? If you guessed “because of the amplification,” you’d be right… It’s 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.

What is Digital Noise?

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’ or ‘static’. 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!).

A Simple Work-around to Reducing Noise

Give the photograph more light. Simple, right? Well, not quite… If you’re photographing the stars, then you’re out of luck. Instead, you could use a tripod, and allow more light to accumulate. This then has the issue of causing blurring. But you can’t always have things go your way. There’s usually a price to pay for every perk.

Low Noise, High ISO, Fast Shutter Speed – Possible?

Ok, now don’t ask me how to keep the S/N ratio high even with low light and a high shutter speed!

That’s something that the engineers who built the cameras will have to figure out. What I can tell you, is to use lower ISO numbers, and only rarely use higher ISO numbers … only you absolutely have to. This way, you’ll have significantly fewer noisy pictures (always keep in mind that a good picture is meant to be seen and not heard, LOL).

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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Published: June 15, 2007 | Last Updated: July 23, 2021

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