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I have a Canon Rebel XTI now, and am considering upgrading my camera body at some point. Aside from the increase in megapixels have there been other improvements to the CMOS sensors of the newer bodies like the 7D, 50D, or 60D?
I know they go up to higher ISO values then my XTI, but do they have the same level of noise as my XTI at the same ISO? Has there been any improvement to the dynamic range that can be captured?
Ask yourself why you're considering upgrading your body.
Are you unhappy about its output? Or is it something else? Small changes in sensor technology no doubt still happen, but for most users they'll not yield any different results on their own from the sensors used in their current cameras (and neither will more megapixels, not unless they go from a sub-6MP camera to a 10MP+ camera at least).
Differences in metering, AF logic, ergonomics (control layout, form factor, etc). are far more important. And that's always been the main difference between the lowend and midrange/highend cameras. To some it's not worth the investment, to others it is (to me for example, the differences in the viewfinder size of the Nikon D200 and D70 alone were worth spending the extra money on the professional body, the higher spec sensor and metering were very welcome icing on the cake).
According to DXO mark there's a slight improvement in noise and dynamic range between the 400D and 60D (about half a stop at base ISO to a stop at high ISO).
There's a much bigger improvement in the latest Pentax and Nikon bodies (k5 and d7000 respectively) in terms of dynamic range at low ISO, worth about three stops. This is mostly due to a reduction in the noise floor so there's no reason Canon couldn't match this if they pulled their finger out!
If noise is a problem to you I would first look to faster lenses if possible not to a new body.
I have compared the sensor of your camera (Canon 400D) with its latest upgrade (Canon 600D) and the Canon 7D, all data taken from dxomark.com (you can see the comparison here). All images are copyright DxO Labs.
Also attached are MTF plots showing the edge blur for the three cameras. Analysis conducted on images from DPReview.com.
The bottom line is that the newer bodies show some improvements in dynamic range and significant improvement in low light situations. The MTF graphs show only some improvement in resolution which is probably hardly noticeable.
I won't comment on other features of the newer bodies since that was not part of your question other than to say it might be an important factor in your calculations.
Rather than saying more MP = better, one should take the sensor size into account thus the transistor/inch ratio.
The higher the density the smaller the transistor will be, the higher the temperature will be, therefore the more noise and lower quality you will get. Thats why a point and shoot camera with 12MP compared to an 10MP DSLR is not necessarily the better camera. Also, a bigger sensor means more area that can catch light, hence better low light abilities.
Modern cameras have either CCD (charge-coupled device) or CMOS (complementary metal–oxide–semiconductor) sensors, whereas both types have their trade-offs and there is no apparent winner at first sight. Since CMOS is cheaper to produce and doesn't tend to produce funny results while dealing with heat or very bright spots as the cheaper CCDs this is a popular design indeed.
High quality CCDs won't suffer of the same problems as their cheaper siblings, in fact if a high quality camera, film cameras in particular, is used, often they come with a 3CCD, which means that instead of having 1 sensor that has to deal with RGB color information, the color is split up into red, green and blue with some kind of a prism and therefore each color is handled by its own sensor. Needless to say that this often delivers very good results and helps improving noise and dynamic range indeed, but unfortunately I don't know a current DSLR that is using 3CCD.
After all, there is more to a camera than just a image sensor. It is the careful fine tuning and orchestration between several hardware parts, the software and last but not least the lenses All these little delicate properties will tell you if a camera will deal better with lowlight than others. But as this is one of the main fields of development, as a rule of thumb you can expect modern cameras being better in lowlight situation (see that I am not saying sensor?)
Before focusing too much on a sensor itself look at the bigger aspects of a DSLR and rethink if you really need extreme low light features so that you have to keep track of the current sensor development.
