We put these two cameras through their paces with noise tests shot at the same time, same lens, same tripod and same subject.
Comparison using our night test subject. For this comparison I am just going to use the NR off results. Both NR on and off tests are available for download on the camera test page. Exposure was 8 seconds at varying aperture.
100ISO
350D
400D
The 400D seems to do better at night.
200ISO
350D
400D
Same at 200ISO
400ISO
350D
400D
By 400ISO the noise actually seems lower on the 400D than on the 350D
800ISO
350D
400D
Noise is roughly equal.
1600ISO
350D
400D
Noise is pretty equal.
Final conclusions
Well, overall, I am impressed. I can see a slight increase in noise at many ISO settings but it is very close. Overall I think the 400D is an excellent successor to the 350D. I wouldn’t rush out and replace your 350D with a 400D, unless, as I do, you have another use in mind for the 350D. The increase is resolution is slight, but at least we now know that you are not paying a huge price in increased noise to get it.
Download the RAW files and examine them yourself. I see the results one way, you may see them another, which is why I love being able to offer the RAW files for download and personal examination.