More specifically.... I am using a Sony a77ii and the 16-80mm Zeiss lens. I have been entertaining the idea of a Sigma 10-20mm 3.5 but am not sure if I need it.
What I want is to be able to use the camera at night in the city and get as much natural/environmental light without use of higher ISO settings (digital noise). I will post some photos when at my PC later today of examples for critiques and guidance.
What max aperture is the Zeiss.
Do you have a link to a specification?
I have a Sony A77 mk1 which is somewhat worse in low light than a mk2 so gives a guide.
In general terms the A77i is goodish at ISO 800, bearable at 1600 and usable at 2400 or 3200 if you value a usable image over noise. ie 800 would be daytime max if you cared somewhat. More at night may be OK.
If you use a tripod or solidly brace the camera then there is no sensible smallest aperture except for subject movement issues.
For night time street photography or static building shots etc f/1.8 is reasonably useful in a city with shop lights, good street lighting etc if you take great care.
f/2.8 about as good (1.8/2.8)^2 =~ 40% as much light
f/3.5 = 4x less light than f/1.8 = 2 stops and is getting reasonably marginal. Camera can be braced and you can Ninja breathe but any subject movement may reduce photos to more art-statements than records. You will sometimes get 'lucky' with moving subjects, but with the A77 you need as much light as you can manage.
Above that anything can work sometimes.
As for focal length. For street photography on APSC the 16-80mm is liable to work well in more situations than a 10-20mm. The latter of course has it's place but the wideness limits the range of things it is excellent at. – Russell McMahon 8 mins ago
I have put just a few night-time photos in this album as examples of either Sony A77 or the slightly noisier A700. All relevant settings shown where known.
These were selected by randomly jumping through a "RandomMaybes" folder and stopping at relevant night shots. No merit claimed for any except that I like them for some reason (in most cases :-) ).
What I want to do is be able to use the camera at night in the city and get as much natural/environmental light without use of higher ISO (digital noise)
Well you need to forget that idea. High ISO is a must regardless of lens choice for night shooting without a long exposure or flash.
If noise is a major issue for then get good noise reduction software and learn to use it to get the results you want. It's not perfect, but it's quite effective.
With night shooting of this sort you need to balance noise from high ISO, depth of field and slow shutter speed causing motion blur. The reality is that there is no magic bullet for this.
Ideally I'd prefer to shoot with a wide aperture fast lens at night in this way, just to have the option of a wide aperture. However having the option does not mean you can avail of it in all shots. Wide aperture gets narrow depth of field and this is not always acceptable.
If the goal is to capture moving people, then a decent shutter speed is required to avoid motion blur. But this will drive up ISO as it's a trade off.
If you're shooting static scenes or interested in scenes where motion blur is fine, then using a tripod and/or a longer exposure is the way to go to avoid high ISO.
Also note the often neglected technique of using flash to illuminate ( and freeze subject motion ) while capturing ambient background with a longer shutter speed. This called dragging the shutter and is a very useful general flash technique. It can be used at night for portraits.
