Depth of Field
2) Aperture
In bright situations, the aperture is smaller--and that creates a large depth-of-field. Conversely, in dim situations, the aperture must open up, and that means a shallow depth-of field.
Sometimes filmmakers will purposely cut back on the light to get their aperture larger—in order to get a smaller depth-of-field.
For low-light scenes like this, the aperture opens wide to an f-stop of 2 or perhaps 2.8. That means a very shallow depth-of-field. Notice how the middle candle in is focus, but the candles closer and further are out of focus. If this scene was outside in bright sunlight, all the candles in the shot would be in focus equally.
