The 5D Mark II's relatively poor performance in the shadows at Low ISO (below ISO 400) has been well documented (http://diglloyd.com …shingTheBlacks/index.html) and multiple threads on various forums
Naysayers and 5D Mark II fanboys have countered the criticism with statements like "Don't underexpose" (as if one deliberately underexposes...:rolleyes and other theoretically obvious, but often impractical suggestions like that.
For me, it has never been a huge problem because I largely photography at ISO 1600+, however the other night I was trying some night photography for the first time. Looking at my exposures on my Calibrated screen at home, all is good.
Today I am looking at these photos on an uncalibrated monitor at my work, and the amount of shadow noise, banding and cross hatched posterisation is incredible (look at the skyline). You might not see it with a proper monitor, but the general public without proper monitors will! The thing is, photos from camera phones don't even look this bad on my uncalibrated office monitor (Dell 2408WFP)
This is an especially bad shot, note it has not been pushed at all (no exposure adjustment whatsoever). And this is not even a 100% crop (it is the full size frame). I think most reasonable people would conclude this photo is not underexposed either.
Again, you may not see this issue if your monitor is good, but turn up the brightness a bit, and you should be able to see the crud that forms a thick cloudy region around the buildings and skyline:
I think it has been exacerbated by the flare of the Tokina 12-24, as this type of shadow noise is most evidence in the low contrast areas
I posted this basically to assert that whilst the low ISO cross hatch pattern of the 5D Mark II is often of not of practical relevance, at the same time it is for some applications of photography. Basically be careful and use Noise Reduction and darken the shadows if necessary.
I think I'll start using my 400D (Rebel XTi) for night photography from hereonin