Nightclub
Very little light, lots of movement, colourful lights. Flash changes everything here.
Lenses that suit this: A bright 24mm–35mm or 50mm f/1.8 lens. Wide lenses help in crowded rooms.
The guide
Without flash
Good starting point: 1/80–1/160s, f/1.8–f/2.8, ISO 800–3200. Check your exposure and raise ISO before you drop below 1/80s.
With flash
Try 1/60–1/125s, f/2.8–f/4, ISO 400–800. The flash lights the people; the shutter speed controls how much of the room's own light you keep.
Direct vs bounced flash
Flash pointed straight at people is harsh but reliable. Bouncing it off a ceiling or wall looks softer, but it needs a surface close enough to bounce from.
Rear curtain sync
This fires the flash at the end of the exposure, so light trails appear behind the person instead of in front. It looks far more natural on a dance floor.
Focus in the dark
Use a single centre focus point and the focus assist lamp if your camera has one. Autofocus struggles in the dark, so aim at a bright edge such as a face lit by a light.