Awesome, with my Sony Nex 7 it is up to ISO 400 without noticeable noise, if not underexposed much. As I use long expo from 5 to 20 minutes mostly, I can afford to use ISO 200 with f no. of 16, 18 usually at 18 mm or 55 mm for sphere pics (the f 18 I usually use to get more depth of field in near field and long focus and not such a strong bokeh in far field), if more is used than there is too much interference from laser light on camera aperture. However I still consider it somehow as part of the photo, if it is not disturbing much, kind of characteristic of technology, where the reality is measured, but that is my approach. Usually laser grain is more significant than noise.
The more I'm worried about the dust in optics and sensor, so I also appreciate zoom lens as I do not have to change it in terrain. But my camera is like 4 and half year old, therefore it experienced a lot already and dust in internals is somehow function of time and use what I see. If this mess is on surface you can clean it as DYI, but sometimes it gets on internals of lens or between layers of sensor. Still, if it is not much it can be considered as part of photo IMO, if you do not go for kind of super perfect photo. It is not that much issue with classic film as the dust was taken away with new film frame. It is neverending fight with interchangeable lens digital cameras.
If you do not need, do not change your lens. If you need to change it try to prevent getting mess inside by hiding it from wind, holding body with sensor facing to ground and make sure you spend as little time as possible with internals exposed to external environment. Also do not forget to cover lens not in use ASAP. I'm sure you know it, but for the others these might be usefull tips.
BTW: 12 mm focal length corresponding to 24 mm full frame is crop factor 2. My camera does not have full frame, crop factor of 1.5, therefore with 18 mm I get it like 27 mm full frame equivalent.
And thanks for recommendation.
