Evaluating and comparing DSLR image quality is very difficult - in part, due to the various imaging pipelines involved. My goal, when evaluating camera noise, is to compare image data coming off of the sensor with as little image manipulation applied as possible. Removing all software enhancements shows what the camera itself can do. I'm much less concerned about in-camera enhancements since these can typically be made on a computer as well as in the camera.
Noise reduction is of course a primary in-camera image manipulation and noise reduction is typically turned on by factory default. My first pass through a new camera's menu system includes turning off this feature.
I also refer to noise reduction as detail destruction. As you will see in the example below and throughout the reviews on this site, noise reduction indeed reduces visible noise in an image as desired. But there is a big downside – the destruction of image details – and sharpness along with it. Almost daily I hear a claim that this camera or that camera delivers noise-free images at some extremely high ISO setting. I have to bite my tongue, because I know that there has been a penalty paid for the low noise.
Noise reduction has its place, but it is a tool that you need to apply in the right amount. And my taste for that amount is "just-enough". I personally generally find the factory default noise reduction setting too aggressive. For the noise tests on this site, noise reduction is turned off unless otherwise stated.
I also find the factory default sharpness setting too strong. Too much sharpening applied to an image causes another form of image detail destruction – in the form halos around contrasting pixels and aliasing (stair-stepped straight lines). So, you will find a low standard amount of sharpening added to the color block noise comparisons – a sharpness setting of "1" on a 1-10 scale.
Of course, that sharpness setting only makes sense in context of the RAW image processing sofware being used. The tests on this site utilize Canon's Digital Photo Professional software. This software is free with EOS DSLR cameras (and some PowerShot models as well). It is not the most powerful software (consider Adobe Lightroom for that title), but it is easy to use (especially easy to get proper color balance), stores recipe information in the raw image file and creates excellent quality JPG conversions. Another benefit to using DPP is that the latest Canon camera models are always supported – and upgrades are always free.
Many other parameters must be identical to get comparable image quality. The color block test images utilize an identical camera setup, identical targets, identical lighting, identical framing and identical processing with all noise and other optional in-camera processing turned off.
RAW images are captured with Auto White Balance, the "Neutral" Picture Style (processed to "Standard" Picture Style) and again, absolutely no noise reduction (a key factor). RAW images are converted to 16 bit .TIF files and Photoshop "Save for Web" is used to create the 70% quality .JPG crops shown on the site.
Lighting is from a crazy-hot 4000 watts of Photoflex Starlite tungsten lights in a pair of daylight-balanced Photoflex SilverDome softboxes. Tungsten lights were chosen for their near-best-available full spectrum output.
The subject for the color block images is a Kodak Gray Scale and Color Control Patches target. The Kodak target is part of a much larger test target. Ideally, the camera test images will show perfectly solid colors for the various color and gray blocks/rectangles in the target. Reality is that noise will be apparent, especially at higher ISO settings.
Here is an example pulled from the Canon EOS 5D Mark III review:
Click on the labels under the test image to select the results to view. The rule is that the higher the ISO setting you use, the more noise you will see. And the highest settings available in the camera are usually not worth using.
By alternately clicking between the with-noise reduction and without noise reduction samples, you can see the effects of noise reduction.
Many of the DSLR reviews also include a sample crop from a piece of fabric for a different look at noise and resolution. The details in the fabric better-hide noise and more-clearly show resolution advantages.
Find the camera review that most-interests you and then look for the color block samples available for that model.