In light of a recent discussion I had with a photographer about the grain inherent in high ISO speeds, I thought I would take minute and write about Signal to Noise Ratios and how they affect image processing in both video in the form of gain and digital photography in the form of ISO sensitivity. I will try my best to explain this subject as I understand it however I’m no electrical engineer, and this really delves into the physics of electronics deeper than I fully comprehend soif you have more observation or clarity on the subject please let me know. Ok here we go. So what is a signal to noise ratio?

Wikipedia gives us a good concise base definition: “Signal-to-noise ratio is an engineering term for the power ratio between a signal (meaningful information) and the background noise:”

Basically for an electrical device there is an inherent noise floor in the signal, in a video camera this would be the CCD (or CMOS in some newer camcorders), and in Digital SLR cameras it would be the CMOS chip. The higher the ratio the more the signal can be “boosted” electronically without increasing noise. However as we already stated there is always a threshold where noise exist by default just because of the fact that we are dealing with electronics. For example, a S/N ration of 60db would give you a cleaner signal than say 50db ratio.

In video whenever you bump up the gain in the camera you add noise to the image, why is this? Well essentially you are adding power to the CCD to AMP or boost the signal, just like audio, however if the signal, or light, is too low it’s very close to the noise floor of the CCD. So while your signal (the image brightness) is increased you are also increasing the noise in the chips as well by electrically boosting the sensitivity of the chip. In a digital still camera the same principal applies just the terms change…now we are dealing with ISO, but for the most part its the same process. Higher ISO speed are boosting the sensitivity of the CMOS chip at low light levels to brighten the image thus also boosting the noise.

Lets examine this in terms of audio. If we record a signal to low and then have to amplify that signal we hear noise. The noise we hear is because as we are boosting the overall signal the noise is in the signal, but because the audio levels are so low as to be close to the levels of the noise we wind up boosting both of them (and the result is a “noisy” soundtrack). The noise is always there, it’s just that at the right recording levels the difference between the input and the noise floor is so great it will never be heard.

The same principal applies to image processing whether video or still. Which begs the question…is it possible to use gain or ISO and not see grain? Sometimes it is. Basically the noise increases as the signal decreases, and in imaging that signal corresponds to light. Maybe this waveform will help.

This waveform graphically represents the brightness of an image with 0 (7.5 for ntsc video) being black and 100 being pure white. The noise floor typically resides right around black and just above it. In the first waveform show above the signal is plenty strong, there ample light in the room so the noise isn’t noticed it since resides so low in the signal.

This second waveform represents an image in much lower light. You can see here how much closer the entire signal is to that lower black threshold where all that noise resides. Now to brighten the image we would have to increase the gain, which would amplify the entire signal. What used to reside in the black area of the image (around 0 or 7.5 for ntsc) suddenly gets moved up to say 20, and the noise is now a very visible part of the signal.

I hope that sheds a little light on the topic of signal-to-noise ratio. Feel free to drop us a question or comment on the subject in the form below. You can also e-mail me personally at the email address listed below with any future topics or podcast you would like to see discussed.

Kendal Miller
Kendal@LMPstudios.com