How to shoot portraits in low available light (intermediate)
Short version: I prefer to use a 50 mm f/1.8 prime lens on an APS-sized sensor (equivalent to short tele) . I use A-mode and select a large aperture. I prefer f/2.8 for a good balance between lens speed and depth of field. Use Auto-ISO set to maximum ISO 6400 or to your preference. Set the longest shutter speed to 1/80 or 1/100. Single mode, single point auto focus set to the middle point with cross type sensors. Focus on the models closest eye, recompose and shoot. For more than one model, try to move the camera for all subjects to be at the same distance from the camera and increase aperture slightly, to perhaps f/4.
Longer version: First, get a 50 mm f/1.8 prime lens if you don’t have one already. Nikon’s and Canon’s versions are among their least expensive (less than EUR200), most lightweight and best performers in terms of optical performance. In particular on APS sized sensors, this focal length yields very flattering portraits and comfortable distance to the subject.
In order to get that creamy, soft out of focus background, without getting too short depth of field, risking too many unsharp shots, I prefer using A-mode, selecting f/2.8. On Nikon, I use auto ISO set to maximum 6400, depending on camera body, and longest shutter speed 1/80 or 1/100, ensuring no handheld camera shake. In this way, when the light is dropping, the shutter speed increases until it reaches the preset shutter time. At even lower light, ISO will increase while shutter speed is constant.
Set autofocus to single point and Single (not auto or continuous tracking). Set your focus to the middle point or a neighbor, that has cross type sensor. These are the most light sensitive ones. Focus on the models closest eye, recompose and shoot. For more than one model, try to move the camera for all subjects to be at the same distance from the camera and increase aperture slightly, to perhaps f/4.
Stay curious and keep practicing! /Sven