Aperture and Focus Points
The maximum aperture of the lens has an effect on the ability of the autofocus system and which autofocus points are actually available. The autofocus system needs light to work, which is why it has trouble in situations with very low light.
You need to understand how the camera and lens work together when you take a photo. When you mount a lens on the camera, the camera opens the aperture in the lens to the widest possible opening, allowing the most light through the lens. This helps the camera focus. When you press the shutter release button all the way down, the camera adjusts the aperture to the needed values for the proper exposure, then raises the shutter to record the image.
For example, if you are taking a photo in bright sunlight with the exposure setting set to 1/500 second, f/8.0, and ISO 400 using the 24–70mm f/2.8 lens, the camera will have the aperture set to f/2.8 until right before the shutter is raised, when it adjusts the aperture from f/2.8 to f/8.0. Right after the image is taken, the camera opens the aperture to f/2.8 again, allowing the maximum amount of light into the camera.
This gets a little more complicated if you are using a variable aperture lens for which the maximum aperture changes depending on the focal length. The camera still keeps the aperture as wide open as possible, but that changes depending on the focal length. For example, if you are using the 70–300mm f/4–5.6 lens, the camera will open the aperture up to f/4.0 when at 70mm and f/5.6 at 300mm.
Depending on the camera, some focus points don’t work if the aperture is not wide enough. That means that even if the camera has more than 50 focus points, if you use a lens that doesn’t open wide enough at the widest aperture, some of those points won’t work. The usual cutoff for this is between f/5.6 and f/8 and won’t affect most photographers, because the widest aperture on most lenses is at least f/5.6. At times, however, especially when using a long lens coupled with a tele-extender, some of the AF points in the camera are just not going to work. If you find that this is happening to you, stick to the center points, which are usually the more sensitive ones and work even at the smaller apertures.
For example, the professional-level Nikon D4 DSLR has 51 AF points, and all of them work with lenses that open up to a maximum aperture of f/5.6, with 15 of them acting as cross-type sensors and the rest as linear sensors (Figure 4.11). When you use a lens that has a maximum aperture of f/8.0, 11 of the sensors work with the center AF point acting as a cross-type sensor (Figure 4.12). When you have a lens that has a maximum aperture slower than f/5.6 but faster than f/8.0, there are 15 AF points, with 9 acting as cross-type and the rest as linear (Figure 4.13).
Figure 4.11 These D4 focus points are active when using lenses at f/5.6 and faster. The red AF points are cross-type, while the rest are linear.
Figure 4.12 These D4 focus points are active when using lenses that are faster than f/8.0 and slower than f/5.6. The red AF points are cross-type, while the rest are linear.
Figure 4.13 These D4 focus points are active when using lenses that have a maximum aperture of f/8.0. The red AF point is a cross-type, while the rest are linear.