If the flash cannot supply enough light and you need to capture low-light scenes, you might want to consider getting a 'fast' lens if you take such scenes often.
'Fast' lenses are so called because of their ability to absorb light faster due to their larger apertures. In the lens, the aperture is the hole where light reaches the sensor.
The aperture controls both the brightness of the image(along with the shutter speed and ISO), and the depth of field of the image.
When the aperture is thrown wide open(thus letting more light reach the camera in the same period of time), the image is brighter but has a shallower depth of field. Depth of field is how much of an image which remains sharp. The deeper it is, the more in-focus an image is.
Although more of the image will be 'blur' if the aperture is opened wider, you can use this to interesting creative effects.
An example of a 'fast' lens is a 50mm f1.8 lens. Note that many of such lenses have images of slightly lower quality at too wide apertures.
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