D40 + Sigma 30mm does low light

Hi Guys,

I've been lurking around and watching discussion about Sigma 30mm f/1.4 lens, and finally got one last week. And last night I went out to take Belly Dancers pics in my favorite resto. One thing amazed me, I can get a VERY FAST shutter speed (1/2000s, ISO1600, f/1.4) in an extreme low light condition as you can see blow.

However, I noticed sometimes the focus hunts unable to lock focus, and about 5% of my shot was missed, is this normal in a light condition like this?

I really like this setup, combined with D40 the size is pretty much compact & easy to carry whenever I want to get out after dusk.

1/15s, f/1.4, ISO800

1/1600s, f/1.4, ISO1600

1/1000s, f/1.4, ISO1600 (focus missed!)

1/320s, f/1.4, ISO1600

1/2000s, f/1.4, ISO1600