Because the system needs contrast to work on, and dim light leaves it less to read. The two published figures that bear on it are the lens aperture at the focal length you are using and the low-light focus limit, where the body states one.
Autofocus works by finding edges. Where the light is low, the edges are faint, and the system searches rather than settles.
The figures that bear on it
The aperture at your focal length. The system reads the scene with the lens wide open, so on a variable zoom the figure that matters is the one at the end you are using — f/6.3 rather than f/3.5.
The low-light focus limit, where a body publishes one. It is quoted as a lowest light level at which focus is specified, and it is comparable between bodies that both publish it.
What to change first, in order
Give it an edge to find: aim at a boundary between light and dark rather than a flat surface. Then open the aperture, by zooming wider on a variable lens or by fitting a brighter one. Then use whatever assist light the body has, at short range. Then switch to manual focus with magnified view, which removes the question entirely.
What stabilisation cannot rescue
Stabilisation, which addresses camera movement and not focus. And a higher pixel count, which changes sampling and gives the focus system nothing further to read.
What people ask before they commit
Will a brighter lens fix it?
It helps, because the focus system reads the image at the lens’s widest aperture and a wider one delivers more light to read. The difference between f/6.3 at the long end of a kit zoom and f/1.8 on a prime is more than three stops, which is a large change in what the system has to work with.
What is an autofocus assist lamp actually doing?
Throwing a patterned beam onto the subject so the system has contrast to lock onto, at short range only. It is published as a feature rather than as a distance, so its usefulness is a matter of trying it — and it is no help at all beyond a few metres.
Last reviewed 12 September 2026