What is color noise ?

[Raj]

What is color noise & how is it different from usual "noise". Is it as detrimental as noise ?
Also are there any specific scenerio's where it occurs like lets say at high iso or underexposing ?

Please enlighten me :-)

Thanks :-)

[Persian-Rice]

Get prepared to take in some info.

There are two types of noise, Colour noise which is I think is called chrominance, and black and white which is luminance or something like that.

I will explain noise first.

Noise is caused by the boosting of the photosite signal by the camera. What does this mean? Your camera has little sensors that record light and colour called photosites. These are what people call "pixels". More techy or camera knowledgeable guys call them photosites, which is the correct term. Technically each pixel/photosite is a light sensitive recorder. Now what happens is, when light hits the photosite it records a reading, and when all the readings are recorded a final compilation is created which is your image.

What happens when you have a low ISO is that the camera takes what each sensor gives it and keeps it "as is". As you increase ISO, the camera boosts this sensor reading, because the sensors are not that sensitive, if you shoot in low light, the sensor does not get a great amount of data, so the camera automatically take what was recorded and makes it stronger.

The reason you get noise is due to the lack of accuracy when a camera boosts this signal. What is happening is that the camera takes data from a photosite and adjusts it to what it thinks the proper value is. If the camera makes mistake, it might make the pixel too bright, or give it a wrong colour. When you get an incorrectly boosted colour you get that red, green, blue noise on the image, which is the colour noise. Luminance/normal noise is the cameras incorrect boosting of the brightness or darkness of the data.

This is why if you underexpose you get more noise. The camera doesn't get enough data and adjusts the data collected to what it thinks the proper value is. Now the rule of thumb is that the bigger the photosite, the better its sensitivity to light. That is why when the 20D came out, people were worried that it might produce more noise. its the same size sensor but more photosites, which also means they are smaller photosites. But what has happens is that the camera has become smarter and makes less errors when boosting that signal value.

What programs like neatimage do is adjust each pixel again. The reason colour noise is easy to remove is because all the program does is adjust colour/hue of the pixel. But what makes the loss of detail is when it tries to adjust the brightness of the pixel. Programs like neatimage or noise ninja take values from surrounding pixels and calculate a new value for the pixel at hand. in short they try to make a smooth transition from one pixel or another, which kills details. Colours are general going to generally be the same as the pixel next to it, but when it comes to shades and highlights, there are tons of variations.

Cheers.

[Raj]

Thanks for the excellent tution here Persian Rice. I have no more questions on techinical side.

So is color noise something not to worry about much (like "noise" is indeed a big concern) ?? Plus, Is it usual to get color noise at low iso's also ? From the explanation below it seems like lower iso's should be free from color noise also ...

Once again thanks for the lengthy but up to the point explanation.

Regards
- Raj

[dpp]

Sorry to but in here, I have been using a Noise reduction software on a wedding I recently shot as the light was low and I was using 800 and 1600 ISO, and the noise levels are too high for an acceptable image.

I ran the noise resuction software and the images look perfect again, but then ( and this is the worrying part) having been staring at the screen for about 5 hours you become used to the noise reduced image.

So I ran it on ISO 200 and 400 Pictures, that before I thought were fine, and when I compared the before and after I preferred the noise reduced shots.

Is this wrong, what is the limit ISO wise that noise reduction software should not go below.

[Cadwell]

Is this wrong, what is the limit ISO wise that noise reduction software should not go below.


No of course it's not wrong and there is no limit. What is important is getting an image that looks good to you. As long as you like it, it doesn't really matter what you do to get there.

[dpp]

Thanks for that I was a bit concerned that my ye was getting used to over noise reduced images.
Is it true that the effects on skin can make the person look like a mannequin?

[Cadwell]

Thanks for that I was a bit concerned that my ye was getting used to over noise reduced images.
Is it true that the effects on skin can make the person look like a mannequin?

If overdone, yes. Like any technique noise reduction can be taken too far.

[maderito]

What is color noise & how is it different from usual "noise".
A color image has two types of information: (1) brightness (or luminance) and (2) color (hue and saturation), and correspondingly two types of noise - color and luminance noise.

Brightness represents about 70% of the image data and can be displayed in one greyscale channel (L channel).

Color is typically represented in two channels (a and b) which correspond to red-greens and blue-yellows.

The net result is an image decomposed into three channels (Lab - one of several systems for representing color data).

The noise in the color channels a and b can be reduced separately from the noise in the luminance channel.

Noise is noise - bad image data arising from random events and other factors.

Color noise is commonly perceived as most objectionable and can be aggressively removed with minimal degradation to the image. Removal of luminance noise, however, if overdone, leads to featurless, plastic-like images with significant loss of detail.

Dedicated noise reduction software like Noise Ninja and Neat Image have separate controls for reducing color and luminance noise.

[robertwgross]

What does this mean: "(Unsticky Date 3/14/05)".

It was posted around 3/7/05.

---Bob Gross---

[Persian-Rice]

You pm'ed me about red dots.

Here is your answer. If you ever see any red, purple, blue, green, yellow, orange, pink dots, they are called "hot pixels". Its actually image noise, but rather due to a mistake in the proccessing, it is caused by heat and is also effected by ISO and exposure time. In short, it is caused by the overheating of a photosite because of the signal boost. What happens is that the sensor overheats and registers a bright solid colour. The longer your exposure time or the higher signal boost means the hotter the photosite becomes and the more and brighter hot pixels you will find. The reason you sometimes see them as blobs is that the heat transfers to the next photosite and causes more over heating.

Even the weather has an effect, if you shoot in the freezing cold, you will see less image noise and hot pixels, while if you shoot in warm weather, you are bound to see more noise and hot pixels.
There are also two types of hot pixels, normals ones and stuck ones. There are situations where a hot pixel will show up in the same place every time. This is due to the fact that the calibration of the photosite has gone off. Normal hot pixels are just random.

The best way to test for hot pixels is to take pictures in the dark and look at black images and see which photosites are overheating. Everyone will get them, 20D will get less, while the rebel and 10D get more.

1.make sure the camera has been off for a few hours.(this just makes the test more accurate)
2. Go into a dark room, preferably no light, and put the lens cap on. Also put a dark towel/sheet around the lens just to avoid any light.
2. Shoot in Large, fine, jpeg. Also turn off any noise reduction, sharpening, contrast.(shoot standard)
3.starting at ISO 100 take shots at 1/4, 1", 4", 8", 30", 90"(bulb). Go ahead and redo these test at ISO 400, 800, 1600.

In ISO 100, any hot pixels under 1-4 seconds should be faint, or non existent. The higher the ISO, the more you will see more and the noise will be brighter. Post some examples that seem too noisy(post exposure and ISO as well) and we can discuss things from there. I know Canon will re-calibrate a noisy sensor.

[Persian-Rice]

Robert, this thread is a sticky, and will become "unsticky" in exactly one week.

[Raj]

Guys,
Thanks a lot for all the explanation. Its really much more easy to understand than bookish stuff. & great idea to make this sticky for a while. I am sure others will benefit from this thread too. (especially when members have taken the pain to go extra mile be typing lengthy discriptions)

Thanks to everybody :-) Highly appriciated :D