When you cannot focus, you cannot take a picture.
And to focus, you need
- A subject (lines/contrast)
- Enough distance (depending on your lens(
- Enough Light
There is often insufficient light. And that can make focusing very difficult.
One thing that can help is your flash. Look at the photographer here, a colleague at a recent shoot:
As you can see on his hand, the flash is emitting some red lines. Those red lines are thre “focus assist” lines. The flash emits these when there is insufficient light. You can even use this function when flash firing is disabled: the flash can be useful in more ways than you thought!