How autofocus works
As light passes through the lens, it hits the main mirror and is reflected up to the focusing screen, where the image can be seen through the viewfinder.
Some light passes through the semi-silvered surface of the mirror and is reflected down to the AF module.
The dedicated AF sensor measures the image contrast in specific areas of the picture (which area depends on which AF point is selected).
The system works on the principal that a sharp image has a higher contrast than a blurred one, and the lens is adjusted to the point at which the highest-contrast image is achieved.
How to set your autofocus
1 Lens focus mode
Autofocus will only work if the lens is set to AF (or A or M/A). If the lens is set to M (for Manual) then the autofocus motor in the lens becomes disengaged, and the lens has to be focused by hand – even if the camera is set to autofocus.
2 Camera focus mode
Some cameras have a switch to set the focusing mode. In this case, we’ve selected S (for single shot autofocus). Others require you to press a button marked AF and turn the camera’s dial to switch between the settings.
3 Focus point selection
Now decide which autofocus point(s) to use. You can manually select a single point from all the ones you can see through the viewfinder, or you can let the camera automatically select from all of the available points.
4 Activate autofocus
Press the shutter release halfway down to activate the camera’s autofocus system – then fully press it to take a picture. When single shot autofocus is used, the camera won’t shoot until it calculates that the subject is in focus.
11 common lens errors (and how to avoid them)
Manual focus: what you need to know to get sharp images
Annoying problems at common aperture settings (and how to avoid them)
Using Autofocus: 9 situations when AF will fail you
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