What is causing increased noise in my recent photos?

Question

In the last two shoots I've taken I have noticed much more noise in the resulting photos. In the first instance, I put this down to my fault, as I was shooting at high ISO levels, but in the second case I overcompensated and went to a very low ISO level.

Now, this could be a software thing — I've only just started using Adobe Lightroom and Photoshop, having been using Gimp for several years — but that doesn't sound likely. I've tried the noise reduction tool in LR but could not get a satisfactory result.

Equally unlikely, to me at least, is that the CF card has changed. I was running low on space during my days out with the camera and so changed my 1GB CF to a 4GB number. Could this be the culprit?

My camera is an EOS 30D and the lens in both cases was a Sigma 18-125mm 3.5-5.6

What could be causing this to suddenly start happening? Is there anything I can do to rescue the photos?

It might not be noise I'm seeing but something else. The background is certainly speckled which I've always assumed was noise but more worryingly is the loss of detail I'm seeing in certain areas — it's as if a low quality JPEG has been rendered from the image. The same region, when viewed in the camera's LCD, has plenty of detail in place.

The image is in RAW format and the lower ISO mentioned above was compensated for by using a flash.

Answer

High ISOs don't cause noise, lack of light causes noise. If you decrease the ISO without doing something to address the level of incoming light (note if you shoot in one of the auto modes your camera will do this for you) your signal to noise ratio will get worse.

There are a few other contributory factors, for example if the sensor is left on for a long period of time and becomes hot e.g. using live view then noise can increase.

Your post processing regime also contributes, if you pull the shadows up a lot then you will see a lot more noise.

But the major contributory factors in noise are "shot noise" due to the random emission of photons from a light source and not collecting enough photons to even out the randomness, and electrical noise, from the

Both of these get better with more light (by capturing more photons the signal produced is stronger with respect to the electrical noise). The fixes are to use a longer shutter time (and tripod if necessary), open the aperture more, or use an external source of light such as a flash.

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