For the first time in ages I have taken concert photos again in low light. You can find a small report of the concert here . What was also a challenge was that you are not allowed to flash during the concert and cannot move freely. So you must immediately find the best possible position. In addition, I had made it a bit more difficult for me because I wanted to take photos that came a bit closer to the singers than the average photo of a choir to get a different approach. That meant that I had to work with a good telephoto lens and a correspondingly fast shutter speed. That also means a large aperture but that in itself is not a disaster because you also get a nice bokeh.
A few photos were taken with a bright 17-50 mm lens, but that was only at the end of the concert when no more daylight entered the building.
The title photo was taken with the telephoto lens at the 70 mm position. A shutter speed of 1/100 sec and ISO 6400 says enough about the light conditions. Fortunately the lens has good stabilization.
A lot of photos were taken at the full 300 mm position so it was important to keep the camera stable under poor conditions. The set-up to make more or less portraits has also succeeded perfectly. As an example the portrait below at 300 mm, 1/100 sec. and ISO 6400. You can click on the photo then you can see it on Flickr.
Fortunately, the camera and my RAW processing software have excellent noise reduction. That is certainly necessary, even if you want to give the photos a grain later or convert them to black and white. An example of this is shown below, a black and white with a light grain made in Silver Efex with one of my portrait presets. Of course I have adjusted some here and there.
So don't be deterred by bad circumstances, but make the right choices that suit your equipment and the software you use.