Hi I'm wondering why the high ISO performance in a 5Dmk2 or 1Dmk4 is so much better than a 600D or 7d? I've been trying to photograph the milky way galaxy and when I use an ISO of 6400 I have an incredible amount of noise on the 600D, but I've seen the same ISO used with the 1Dmk4 and 5dmk2 to achieve wonderful photos without any apparent noise. I'd have to say the same for ISO 3200 which I can't get picturea without bad noise, ISO 1600 works but that causes noticeable star trails as the exposure becomes longer than the calculations I make using the declinations. Is this because of more pixels in the sensor size? Or do different cameras just have different ISO performance? Thanks for any insight.
The bigger the pixels, the less noise there is. This is a matter of physics. More light gets accumulated in each pixel and so it take more noise to appear significant.
The 600D and 7D have APS-C sensors which are small and have a high megapixels count. This makes their pixels comparatively smaller than the 5D Mark II which has a larger sensor and hence larger pixels since the resolution is not much higher than that of the 7D.
You can see the same thing in the Nikon range by comparing the D3X and D3S which are both full-frame cameras but one has twice the number of megapixels. The one with fewer megapixels shows less noise because each pixel is larger.
The extreme example of this is the poor performance of compact cameras which have tiny sensors. The last factor is technology because newer sensors show improvement over previous generation but physics cannot be beaten.
the two things two distinguish are overall noise and per pixel noise.
overall noise is the same - whether 4 pixels occupy a space or 1.
per pixel noise will be higher on 4 individual pixels instead of 1 larger pixel which will contain the same noise as the average of the four pixels.
the APS-C cameras have tiny pixels compared to the full frame models. and with current technology, 18MP is most likely the ceiling for acceptable noise performance and pixel size.
now over time technology improves, and smaller pixels can be less noisy per pixel, but the 7D, 600D, 550D and 60D share their sensor. the 7D sensor is also only slightly younger than the 5D MK II sensor, and hence just a tad worse, though the pixels are a lot smaller than the ones on the full frame sensor.