I am trying to decipher Canon documentation on their 61 Autofocus Points.
Here’s what I comprehend:
1. The 5 cross-points column in the center are the most precise points with a F2.8 or higher and with a F4 Lens.
2. The other F4 points would be also accurate for a F4 lens but less with a F2.8 lens or higher.
3. Only the vertical and horizontal lines of the F5.6 AF points would respond to a F5.6 lens.
4. I am out of luck with any F8 lens or equivalent ex. 600mm with a 1.4 multiplier.
There must be more to it; any information will be appreciated.
'Only the vertical and horizontal lines of the F5.6 AF points would respond to a F5.6 lens'.
The Lens aperture remains fully open (between exposures) when AF is taking measurements. For example f/2.8 lens, stopped down to f/5.6 - before exposure the lens remains fully open at f2.8, the AF takes readings at full aperture then the exposure is made using the aperture selected (in this example lens closes down to f/5.6 aperture). The operation is repeated.
F5.6 AF line sensors accommodate a plurality of lenses up to and including f/5.6 but are less efficient or may not operate with slower lenses, e.g. 600mm f/4 + 2X converter (=f/8). Some cameras which do not offer f/8 compatible AF point(s) may still AF at f/8 but are 'very' slow and less accurate, others may refuse to operate at f/8.
F/2.8 Af line sensors are known to be 2-3 times more accurate than f/5.6 types as they are provide almost double the baseline measurements, however due to their extra precision they are not very efficient detecting a subject with a large amount of focus blur - F/5.6 AF line sensors are far more efficient at initially tracking large amounts of focus blur - when the subject has been brought into better focus the f/2.8 line sensor(s) take over (if you have one selected). Canon overlap F/5.6 and F/2.8 sensors for this reason.
Contrary to widespread rumours, vertical only line sensors are as accurate as cross type sensors, PROVIDED they have sufficient contrast in the opposite direction (horizontal) to work with - try focusing a vertical only AF point on a vertical line (door or window frame), you will find the AF system cannot operate - to get around this you have to tilt the camera to obtain focus.
AF calculations are based on phase detection (contrast) and mathematical tables. It's critical to configure an AF system for different subject movements, speed and the type of obstructions involved, especially the tracking sensitivity.
A slower tracking sensitivity is useful if the subject you are focusing on becomes temporarily obstructed - focus point(s) in use blocked by obstruction. For example team games such as rugby where lots of players' may cross over your subject, also goal posts get in the way, or swimming, such as the breast stroke, where the subject's head routinely disappears from view under the water.
If the camera detects a noticeable change in contrast/focusing distance during tracking, a slower chosen tracking sensitivity instructs the camera to continue tracking using the same calculations as if the obstruction had not occurred. Hopefully by the time the intended subject reappears the AF will be still be on track or very close with only a very minor adjustment required.
The following may help http://learn.usa.canon.com/resources/articles/2011/1dx_af_precision_crosstype_article.shtml
- see diagramshttp://www.learn.usa.canon.com/galleries/galleries/tutorials/eos_1dx_tutorials.shtml
- video tutorials.