Autofocus

Autofocus is a system relied on by 99% of photographers in the modern era. To achieve proper AF you first need to analyze your subject. Do I want to show motion or stop it? What do I want to show in this image? What is my main subject? Once you have answered these basic questions, you will know which AF to select. You will see that learning these is very easy.

One shot or single AF (AF-S)

This is the AF system you want to use for STATIC subjects. In portrait photography you will use this AF to properly achieve your focus and select the desired point for the proper composition. You will use AF-S or One Shot with architectural photography, macrophotography, portrait photography, landscape photography and more.

Ai Servo or Continuous AF (AF-C)

This is the AF system you want to use for CONTINUOUS autofocus. This means when you want to shoot a subject that is moving towards or away from you. In sports, this AF system will be depended on. With the proper autofocus point selected, you will be able to achieve a great composition while capturing fast sport images.

TIP: When shooting sports, learn to use your AF-c or Ai servo autofocus and your Af-on button near your thumb behind the camera. This will make the autofocus work continuously without having to hunt for focus after a picture is taken.

AF SENSORS and the importance of fast lenses.

There are high-precision (“cross type”) autofocus sensors among the total autofocus sensors of your camera. For example, the flagship model of Canon, the 1D markIV has 39 cross type sensors out of a total of 45. The Nikon D3s has 15 cross type sensor out of a possibility of 51.

Nikon D3 AF points compared to Nikon D90 AF points.

The high precision sensors are sensitive to both horizontal and vertical lines, while the remainder of the “assist” sensors are sensitive only to horizontal lines. This is very important to keep in mind if you are focusing using a lens that uses few or no high precision sensors. High precision sensors are about twice as sensitive to vertical lines as they are to horizontal, and all other sensors can only respond to horizontal lines. A judicious composition when setting autofocus can exploit that bias. If you have access to high precision sensors, prefer vertical detail with them, while if you are “stuck” with other sensors ensure that you are providing them some horizontal detail.

Please re-read and re-read that last paragraph until you understand it fully.

Lenses with maximum apertures of f2.8 allow the camera to use all high precision sensors. In low light or other situations that are hostile to autofocus, that’s a big deal. Lenses with a maximum aperture of f4.0 use only the center focus sensor in its “high precision” mode, and use the other sensors in their “horizontal line” only mode. Lenses with a maximum aperture of f5.6 use all sensors in their “horizontal-only” mode, and lenses with a maximum aperture of f8 use only the center sensor point, and that with horizontal sensitivity only.

Thus, while shooting with a f4 lens, it does not harness all of the camera’s high precision autofocus sensors. Your decision to use it instead of a  f2.8 lens for a low-light shoot must be made weighing this and a variety of other factors.

TIP: Use your flash to achieve better focus in low light and with slower lenses. The modern DSLR flashes have a AF assist light that will help you to focus faster and with more precision without using the light of the flash.