Saturday, November 7, 2009

What is noise?

What is noise? Thought you would never ask.

Here are a few facts about noise you won't find in a camera's hype sheet. Or on the review sites either. While things have gotten better--well regarded review sites like dpreview aren't pontificating absurdities about camera noise like they were a few years ago--but there is still much confusion.

All the facts I'm listing are out on the web somewhere-either in plain language or more often hiding in mathematical formulas. But I think it would be interesting to pull them together in one place. In more or less plain language,

Fact one.

Unlike film which works in a totally different way, digital cameras count photons. Photons are little hunks of light that work like bullets and knock photoelectrons out of the silicon that camera sensors are made out of and into the pixels that sit on the top of the silicon.

(Einstein won his Nobel Prize for working out how this works. He received the honor. His first wife collected the money. It was written in their divorce settlement.)

Fact two

Pixels work like penny banks. If you take a picture of a uniformly illuminated source--a white box by the window lit by a clear sky for instance--all the pixels will collect their photoelectrons, the pennies, during the exposure. Then if you add up all the photoelectrons and divide by the number of pixels in the camera you have your signal which tells you how bright it is outside. To make the math easy, today the signal is 1000 photoelectons. ("pe-" in engineering talk.)

If you empty a penny bank and count the number of pennies that are short or over 1000, that is the noise. For this example lets say you have 33 extra pe- in a pixel--a magic number I will explain in the next post.

Of all the ways to explain and quantify noise, the number of noise photoelctions is the easiest to work with. So we won't get into decibels, the noise numbers loved by electrical engineers. Today your noise is 33 pe- and your signal to noise (S/N) is 33. That's nothing to brag about but it's still a useable S\N

Fact three

The vast majority of the noise you counted is not from your camera. It from the light. I repeat. THE NOISE IS FROM THE LIGHT!

The noise is caused by the random emission of photons from anything that is hot enough to give off light--that means everything in our universe. Sun, flash lamp, candle, your big toe, puddle of liquid air, everything. The amount of light and spectra of the light will vary of course. With a medical tomographic camera your big toe becomes a bright source, but taking off your shoes won't help any if you are shooting a wedding. Still, regardless of where the light comes from, it carries its noise along with it.

What does this mean. Nothing a camera maker has done or ever will do can get rid of this noise--photon shot noise in engineering talk. Short of a trip to a sf alternate universe, the noise is not going to go away.

And if you noticed that I said "vast majority", what are the real numbers. By my calculations, if you own a Canon 5D up to 98 % of the noise comes from the light in a low ISO and bright exposure. And if you don't, with my carry-it-everywhere Oly SP350 up to 95% of the noise come from the light. Something I measured.

If you now think Old Scrib is sprouting total nonsense--his cheapo old tiny sensor SP350 doesn't take clean pictures like a Canon 5D--we'll explore the differences between noise and signal to noise in more detail in the next blog. If you want to understand what's going on inside your camera confusing the two terms can causes much confusion.

We will also get into how you can, with free software and not that much effort, measure how noisy or clean your camera is. A buddy of mine just bought a Canon 7D--their latest that's been on the market for only a few weeks. Next week he's bring over his older 5D and 7D and we will measure and compare them with my D60.

Be new info. Before the review sites post their noise figures. So keep watching the blog.

*edit* Not new info now--but first time bloggers have high hopes of scoping the big sites.

If you see this edit you are the fifth visitor ever, all today, to read this post. Put a comment in the comment box--so I know there is one-- and I'll think up a suitable prize.

To be continued:

No comments:

Post a Comment