Following are two extensive 100% crop comparisons between the 5D II and several other Canon EOS camera bodies. If you have read any of the site's other recent Canon EOS DSLR camera reviews, you will recognize the following color block test that clearly shows and compares sensor noise.
Read about the Camera ISO noise tests in the help section to learn more about the tests and how they are conducted. A key take-away from that page is that noise reduction is completely off unless otherwise specified.
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Noise is a very hard characteristic to compare and to put a number on. It has size, shape, quantity, brightness, color ... and noise reduction can be done at any point in the image processing pipeline. Applying noise reduction reduces the amount of noise in an image - usually at the cost of image sharpness/details. As I've said before, my personal preference is for the camera to allow me to choose the amount of noise reduction applied during post processing of RAW images. I usually opt for no NR until ISO 1600 or so and then select a low NR setting until the highest ISO settings are reached.
I've spent way too many hours looking at these comparisons, but here are some of my observations.
Since I've already made comparisons to the 1Ds Mark III, I'll continue comparing the 5D II image quality against this body. I've already established that these two bodies have similar resolution and sharpness - at ISO 100. In the color chart comparison, the 5D II shows a small advantage over the 1Ds III in the noise category. In the fabric example (from same shots as the color chart), ISO 400 and 800 appear identical between these two bodies. At ISO 1600 and 3200, the 5D II samples appear sharper - but also appear noisier. A small amount of noise reduction added to the 5D II images would probably equalize this difference. Obvious in this comparison are the three additional stops of high ISO settings available in the 5D II. For my taste, the last two ISO settings (12800 and 25600) are for emergency use only. ISO 6400? I'll avoid it, but it is usable - especially if the resulting image is downsized.
Comparing two cameras with different resolutions is not easy. And there are many ways to make the comparison. Thus, I have provided samples from the native resolution of the Canon EOS 5D and the Canon EOS 5D Mark II as well as samples from both resized to the other body's resolution. Even the method of resizing and the amount of sharpening applied can affect the results. So, in this case, I simply used Canon's standard - Digital Photo Pro - for creating these samples.
For my eyes, the 5D II delivers obviously more resolution and/or less noise. At their native resolutions, noise results from both cameras are very similar with the 5D having slightly less noise at ISO 3200. The 5D II shows much more detail in the fabric. Up-size the 5D images to the 5D II dimensions - noise characteristics are enlarged and sharpness suffers. Detail is not magically created by this process. Down-size the 5D II images to the 5D dimensions and the comparison is closer. The biggest 5D II advantage in this comparison is lower noise levels. Thus, even if the 12.8 mp images from the 5D are large enough for your purposes, the 5D II is still going to deliver better image quality at this final image size - especially at high ISO settings. I think 5D owners will be especially happy with the increased 5D II resolution. I'll add that in-camera high ISO noise reductions can now be applied to 5D II JPG images and of course RAW images can be tweaked as desired during post processing.
The comparison samples clearly show the 5D II as a significant upgrade in image quality from the 50D (the advantage is not lost in the price of course). The 5D II's images are noticeably sharper and show more detail. The 5D II has noticeably lower noise levels - and the 50D's ISO 12800 makes the 5D II's ISO 12800 look good.
As made obvious by the above comparisons, the 5D Mark II includes two sRAW (small RAW) formats. The primary reason to use these is to get reduced file size while retaining RAW post-processing options. The original, moderately detailed RAW images for the above samples vary greatly in size among camera bodies and the 5D benefits in this regard in that it is not storing 14-bit data (a 5D image quality disadvantage).
Back to the Canon EOS 5D Mark II Review.