noise

[ricnak]

Please excuse me if this is a really dumb question but I have not quite got my head around noise..... Does digital noise increase with decreased shutter speed in low light situations? Is there a relationship like there is with increasing noise and increasing ISO? If there is, does one produce less noise or is it directly proportional?

Thanks in advance.

Helen

[ricktas]

Noise is the digital equivalent of grain in film. Remember the grainy black and white photos that used to appear in your newspaper?

There are several ways noise can be introduced to a photo, the most common being increasing your ISO. Give it a try, take your camera out and set it to Av (aperture mode) and ISO100 and take a photo, now dial the ISO up to the highest setting available, leave your camera on Av and take another photo. Go inside and load them up on the PC and see what the results are.

There are different sorts of digital noise too, high ISO is only one form. Long exposure noise is another, with very long exposures (shutter speeds of 30 seconds or more) long exposure noise can appear. The longer the exposure the higher the possibility that long exposure noise will appear. Most camera's have a Long Exposure Noise Reduction (Long Exp NR) built in, have a read of your manual on how it works.

Now why would we increase the ISO when it makes our photos noisy (grainy). It is a balancing act. Sometimes we need a fast shutter speed, but the light is not sufficient to capture a sharp, in focus shot. Increasing your ISO means you can achieve a faster shutter speed. But you need to increase your ISO only to the level needed to get the shot you need. So you may need to increase your ISO to 400 or 800 to get the shot, but not go to 3200 or 6400. By only going to the ISO you need, you are balancing the quality of the result (noise level) against the need for a shutter speed needed to capture the photo you want.

It is all part of the exposure triangle. Have a read of the NTP learning guides on shutter speed, Aperture and ISO. All three are inter-related and work in unison to achieve the results you want. Sometimes you do not need to change your ISO and a change of aperture can get you what you want from your shoot. These three camera features work together, each reliant on the other when taking photos.

Sometimes noise/grain in a photo can add to the end result, so noise is not a bad thing, it is just another option available to you. However, there is a range of noise reduction software available to help you rid a photo of noise if needed.

[jjphoto]

Noise can be a problem with very long exposures. Here's an extreme example that illustrates the point.

This is a one hour exposure on Canon 20D with lens cap on and at ISO100. This is a straight RAW conversion, no tweaking at all.
http://www.johnjovic.com/temp/Noise/1_hour_IMG_1142_1000px.jpg
The full image (all 10Mb) can be downloaded from http://www.johnjovic.com/temp/Noise/1_hour_IMG_1142_1.jpg if any one wants to see it.

100% crop of the above image, no tweaking at all.
http://www.johnjovic.com/temp/Noise/1_hour_100percent.jpg

JJ

[ricnak]

Thanks for that explaination Rick and the photo example jjphoto.

I had a similar shot the other night and it got me wondering if the noise from the long exposure was the same as the noise from high ISO. The mind wanders when your sitting on a roof at 2am freezing your bits off!

Cheers

Helen