Autofocus
Image: This JPEG image was taken at ISO 12,800 with the high-sensitivity noise reduction set to Standard. The details are softened a little and some chroma noise is visible, but the result is nevertheless impressive
As I mentioned earlier, Canon has introduced a completely new 45-point autofocus system for the EOS-1D Mark IV, with the primary aim of making it more stable than the EOS-1D Mark III’s AF system when focusing continuously in AI mode. While the EOS-1D Mark III has 45 AF points, only 19 of them are user selectable, whereas any of the Mark IV’s 45 AF points may be selected for focusing. Of these points, 39 are cross-type (the EOS-1D Mark III has 19 cross-type points) and function with all f/2.8 or faster EF lenses as well as some f/4 EF optics. In a bid to improve focus-tracking reliability and precision further, Canon claims the f/2.8-sensitive line sensors have been improved and some of the f/5.6-sensitive AF points have two lines.
There are 15 custom functions that govern how the AF system works. In the main, these are designed to tailor the camera’s response when shooting moving subjects and using the continuous AF mode. The most important functions allow the photographer to adjust the speed with which the camera reacts to a change in subject distance, determine which AF points are used to track it and to specify whether to prioritise using the main AF point or tracking the subject using the expansion AF points.
Canon attributes some of the reported continuous AF problems encountered with the EOS-1D Mark III to the fact that it has a very responsive system and this means that it is less likely to keep fast-moving subjects sharp if they cannot be kept within the AF frame. The manufacturer has reworked the AI Servo AF algorithm, calling the new version AI Servo II AF, to help make continuous AF more consistent and stable. EOS-1D Mark III users who upgrade to the EOS-1D Mark IV will also notice that the impact of the AF AI Servo Tracking sensitivity settings (Custom function III: Autofocus/Drive 2) has changed with a general slowing of the response time.
I used the EOS-1D Mark IV in a range of conditions designed to test the AF system. Not surprisingly, it struggled the most when I was shooting fast-moving cyclists at an outdoor velodrome after sunset and under fairly poor floodlighting, but I was still able to obtain sequences of sharp shots. When using an EF 70-200mm f/4L IS USM, which doesn’t support the full cross-type AF point functionality, I found the AF system a little sluggish (relatively speaking), and when Custom Function III: Autofocus/Drive 3 is set to give focus tracking priority (the default ‘0′ option), the shooting rate sometimes dipped below 10fps.
I noticed a significant increase in the speed of the AF response when the AF AI Servo Tracking sensitivity was pushed to its highest value so the camera reacted quickly to changes in the subject distance. However, when shooting fast-moving subjects at up to 10fps it is easy to be fooled into thinking that the active AF point is consistently over the main subject. It is only when the shots are played back on the LCD screen that it becomes clear that the AF point has often shifted to one side. Although there were a few short bursts where the camera failed to latch on to a cyclist when the light was at its poorest, the subject is acceptably sharp in 75-85% of my images.
Shooting the cyclists in brighter light and/or with an EF 300mm f/2.8L IS USM that supports the full cross-type AF point functionality improves the continuous AF performance significantly. As a result, I also noticed that the camera was able to shoot continuously at its maximum rate more often. It becomes much easier to get sharp ‘grab-shots’ of a moving subject without tracking it for a while in the viewfinder. With slightly slower moving subjects, such as footballers, the hit rate is higher, with only two or three shots in every 20 or so being
a little off target.
