We explain how focusing works
Test your AF limit
In a room where the ceiling and walls are of a similar colour, try focusing on the angle where they meet, first manually and then with autofocus. Repeat in poorer and poorer light to find the AF limit, and see whether it’s better than your eye.
Test your AF speed
With the lens cap on, manually set the focus to infinity, then switch to auto. Half press the shutter release and the lens focusing will move rapidly to its closest distance – and possibly back again to infinity. Some lenses prefer to start from the closest focus. This shows how fast the AF moves and allows comparisons to be made with other lenses.
Test the displacement between the viewfinder image and AF zone display
Try autofocusing using the central zone on a small black & white cross about the same size as the zone in the finder and pinned to a stick in the garden. If the cross records out of focus and the distant objects are sharp, it means the viewfinder image is displaced relative to the AF zone display – which is something worth knowing.