GUIDE TO EXPOSURE

GETTING SHARP FOCUS FOR 5D MARK III

By : Hadijanto Jogi Basoeki 

Reference : CANON EOS 5D Mark III Digital Field Guide ( Charlotte K. Lowrie


For anyone who had the 5D Mark II and waited for the Mark III, one of the top items on the wish list was an upgraded autofocus system. That upgrade came in spades with the 5D Mark III, which has 61 AF points. Fully 41 AF points are cross-type with f/4 or faster lenses. So if you are shooting action, focus is more precise for even low-contrast  subjects, and subject tracking is more reliable.

Canon spared no engineering expense in designing an autofocus system that is versatile and customizable. With all  the new features come a plethora of autofocus options. This chapter helps you set up the autofocus system so that it suits your shooting.

With the 5D Mark III, you have plentiful options for setting up autofocus to suit your specific shooting needs. For this image Common IORA, I used AI Servo mode with manual AF-point selection. Exposure: ISO 640, f/5.6, 1/320 seconds with +1/3-stop of exposure compensation. 

Understanding The Autofocus System

It's important to know that sharp focus ultimately involves three factors:

1. The resolving power of the lens
2. The resolution of the sensors 
3. The resolution of the printer

Printing is beyond of the scope of this topic, and it is assumed that you're using high-qualty lenses. That  leaves one remaining factor - THE SENSOR AND INTERNAL AUTOFOCUS SYSTEM. And it's safe to say that CANON's 61-point High-Density Reticular autofocus system is excellent. The system delivers high focus accuracy for still subject shooting as well as improved subject tracking.

One step in improving autofocus and tracking performance was to incorporate more cross-type and high-performance dual-cross-type AF points in the 5D Mark III. If you understand what cross-type AF points offer, you can use them to get the best performance from the autofocus system. The result is that when you combine the new autofocus system with high-quality lens, you get tack-sharp focus.

Cross-type Sensors

On a very elementary level, for an autofocus system to focus, it has to align vertical and horizontal lines in the scenes so that they are in phase, or perfectly aligned. That's where cross-type sensors come into play. To understand cross-type sensors, it helps to go back to the first autofocus system. Early systems used a pair of single-line auto-focus sensors positioned horizontally in the centre viewfinder. Light coming in from the lens was separated into two beam striking the AF sensors. Based on where beam stroke the sensors, the camera could tell if the focus was accurate. An, if the focus wasn't accurate, then this early system could quickly bring into focus vertical lines in the scene - line that perpendicular to the sensors. However, it was  much harder for it to bring horizontal lines into focus - lines that ran parallel to the sensor themselves.

As autofocus systems evolved, another pair of sensors was added and positioned at a 90-degree angle to the existing sensors. This created the first cross-type sensor in the shape of a cross. Although the sensor were still only in the center of the viewfinder, focus accuracy improved because if the horizontal sensors couldn't establish focus, the vertical sensors most likely could.

High Precession and dual-cross-type sensors

Today, Canon uses a variety of cross-type sensors as well as high-precission autofocus sensors that offer two to three times greater autofocus accuracy at the sensor with a wide aperture lens than standard sensors.