The Effect of Lens Aperture
No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens. Remember, during autofocus the camera automatically opens the lens to its widest aperture, only closing it down to the aperture you’ve chosen just before the shutter curtain opens. Phase detection autofocus is more accurate when the light beams are entering from a wider angle. In the schematic below beams from an f/2.8 lens (blue) would enter at a wider angle than those of an f/4 lens (red), which are still wider than an f/5.6 lens (yellow). By f/8, only the most accurate sensors (usually only the center point on the more expensive bodies) can function at all, but even then focus may be slow and inaccurate. This is the reason our f/5.6 lenses stop autofocusing when we try to add a teleconverter, which changes them to f/8 or f/11 lenses.