Why does pushing in-camera ISO look better than exposure correction in post?
Most likely it's because the ISO is an analog process which is applied to raw sensor data (not to RAW digitized data).
If you take a photo, you a fixed number of tones (very few dark tones) to boost in post, which results in posterization (or large areas with the same tone - black or white).
But a higher ISO analogically amplifies the signal-noise and gets you the same number of tonal intervals at the corrected exposure (just like at the original exposure), albeit the noise makes the signal look more and more random (because the same signal gets distributed over a wider tonal range, that is, it's spread out more).
You can test this by simply trying to boost the exposure in post with, say, 4-5 stops (from ISO 100 to 1600-3200). Basically, you get garbage.
I don't believe that ISO 800 is any sort of analog limit in these times. For one, the new Cinema sensor has a native
ISO of 850 (according to the Canon FAQ).http://learn.usa.canon.com/resources/misc/cinemaEOS_faq.shtml
What is the camera's native ISO?
The EOS C300 has a native ISO of 850. It is at this setting that dynamic range, especially in highlight areas, reaches its maximum. At lower ISO settings, the dynamic range tends to shift more toward shadow detail.
Why 850 ISO?
ISO 850 on the EOS C300 provides an optimum balance between dynamic range and noise levels. At higher ISO or gain settings, dynamic range remains essentially the same, but digital noise tends to increase.