Shooting in low light can be tricky. As light fades increasing ISO and opening up the lens is a tricky game. It’s easy to fall victim to a wide open shutter only to find the depth of field was a little too shallow. It may look sharp enough on the LCD, but enlarge it to reveal the true horror. Equally, cheap to mid range lenses generally won’t be able to open up to their maximum at longer focal lengths. At least here the only problem (albeit a major one), will be under exposure.
Increasing the ISO will regardless of camera quality, increase noise. The scale of the noise and the ability of software to minimise the impact is the difference between a well captured low light shot and a painterly disaster.
So what’s the answer? Planning for the shot and having a contingency up the sleeve. If it’s candid photography (parties etc), and the hardware doesn’t allow wide apertures & high end ISO performance then the unfortunate answer is on camera flash (eeek!). To achieve a decent exposure in low light without flash requires getting as much light as possible to the sensor (wide aperture), high ISO and as much image stabilisation as possible. This may still yield a shutter speed too slow to be useful.
The highest I have allowed myself to go with ISO is 2500. The situation was indoors and required mid to long focal lengths. I used the 70-200 and kept it at f 2.8. Dangerous but I had little choice. The results were barely suitable to give to the client, but they were by far and away better than anyone else managed to capture.