what is a "noise" in a photograph and how can it be removed?

Question

I'm surprise to know that pictures have "noise" and I'm curious what kind of noise is it.

Answer 1

Noise typically is seen in black areas of a photograph where there are specs and dots of different colors. Sometimes you'll notice them in solid colors like red, blue and white, where there are tiny pixels of dots and colors appearing.

It's basically the lack of clarity with speckling in a photo, the way white noise appears in an audio clip or old record and it takes away from the actual music or sound.

You would usually notice noise when you look at them closely on the computer or when you blow them up at a larger size.

The best way to get rid of it is use the Noise reducing filter in Photoshop.

The common causes of noise in photographs are usually bad lighting or low res cameras.

Answer 2

There are some good answers above, but here are some better visual examples. Krayzie's sample looks more like a typical "grain" plug in than noise.

Here are a couple of good examples of noise. Look at the blobs of color that don't belong. You know that the rose petals in the first example are fairly smooth, but this image makes it look like it has acne or something. There are little "bumpy" looking things and spots of dark and light that just don't belong. This was taken at ISO 3200 for 1/4 second - a sure formula for creating noise - high ISO with a long exposure time.

http://www.flickr.com/photos/samfeinstein/842992586/

Look the image over on the page linked for a second and read the caption. Then click on "All sizes" and look again. It's not too bad yet... Click on "Original" and then you will easily see the obvious noise.

Here's another one. Scroll down to the shelf with the colorful notebooks on it. You will easily see the noise in the areas that you know are supposed to be a uniform color.

http://www.flickr.com/photos/samfeinstein/842827784/

Here's an example of a low light long exposure using a small sensor point and shoot camera. If you look at the original size, the noise situation isn't too bad. That's because, even though this was a full one second exposure, I kept the ISO at 100 to try and tame the noise that I would have expected in the shadows if I had used a higher ISO.
http://www.flickr.com/photos/samfeinstein/441244806/

Noise is related to sensor size. Generally speaking, the bigger the sensor, the less problem it will have with noise. This is not "the best bad example," but I don't have any on-line right now. (There's a new challenge...) Although this shot was made after sunset and the shutter was set at 1/8 second, you will not really see any noise. Enlarge the image and look in the darkest areas of the frame. For a good bad example, I will have to try ISO 1600 next time... You might notice some blips just above the horizon to the left of center. That's not noise - it's some geese.
http://www.flickr.com/photos/samfeinstein/449115543/

Film does not have "noise" per se, but rather it will show grain, which is no more than the crystals on the emulsion of the film. Generally speaking, higher ISO films show more grain, but it is nothing at all like digital noise. View this as large as you want and you will not see those alterations of color the way you do in a noisy digital photo.
http://www.flickr.com/photos/samfeinstein/856121089/

Here's perhaps a little more than you wanted to know, but read on for an explanation of where noise comes from.

ISO is not really an adjustment to the sensitivity of the sensor. A sensor can either detect photons or not. It can not vary the sensitivity to those photons.

ISO in a digital camera refers to the gain applied to the signal sent from the sensor to the processor. Think of the ISO adjustment as an analogy to a volume knob on an amplifier. Adujsting the ISO upwards boosts the signal sent from the sensor to the processor. More signal ("louder" signal, if you will) leads to digital noise as some of the signal from one individual diode jumps over to an neighbor, creating noise. Think of it as being analogous to distortion as an amplifer runs out of headroom and eventually starts to clip the signal because it can't handle it. There's your "hot" or blown pixel that creates noise in high ISO settings.

This is one reason why a larger sensor has less digital noise. The pixels are bigger and have more of a "buffer zone" around them so that it is less likely that an amplified signal from one photodiode will jump over into the territory of a neighboring photodiode. They have more "breathing room," if you will.

Most cameras have noise reduction software that kicks in at higher ISO's unless you intentionally turn it off. This software smooths things over, but it is at the cost of losing detail. It's a question of whether you want a loss of detail or presenceof noise and it's a balancing act sometimes. Sometimes it's better to use it just to get the shot that you wouldn't otherwise get and sometimes it's more important to preserve that detail. Sometimes you might even WANT to introduce noise as an effect.

Photoshop and many other kinds of image processing software offer one click "noise reduction," but this is also at the expense of some detail. You'd have to try it and see which is your preference. I guess I have to make a sample for that, too.

Answer 3

Most have answered you very well. Those that compare film grain to digital noise need to learn the difference. Grain can be used as a creative part of making images. Noise is something no one really wants. They have come along way since my first 10d.

Answer 4

"Noise" is a result of using a very high ISO and/or long shutter speeds. Most cameras have a "Noise Reduction" setting which helps but at the expense of detail. There are after-market programs like "Noise Ninja" which also reduce the effects.

Answer 5

In the old days noise meant an image was "busy" or "noisy" - with alot going on.

Nowadays its a term used to describe imperfections in digital images. Its nothing like film grain, film has no noise.

Enlarge most digital images and you will notice noise - or discoloured pixels.

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Answer 6

Edwin's answer is spot on... the problem occurs with long (over 1 second) exposures and high ISO speeds and it leads to speckling of information in your image, rather like film grain. Noise Ninja or other products can deal with it well.

Answer 7

Not sure what you mean by noise...do you mean the white circles that sometimes end up in pictures. if so, those are orbs and they cannot be removed. They are balls of light energy.