2012-12-12

Canon EOS 1DX Autofocus Review Part II - Fine tuning AI Servo by Garry Ridsdale ARPS

Swiss AF Eurocopter, Switzerland.
Auto AF. Canon EOS 1D X & 800mm f/5.6
Handheld. 1/30 sec @f/11, ISO 100.
Continuing from Part I which looked at the AF Area, Part II considers fine tuning AI Servo.

Fine Tuning AI Servo
Canon has introduced example settings as a result of apparent misunderstandings and complexities on previous bodies.  While the 6 case studies could be useful actually understanding what each of them are doing goes a long way in telling the camera what you are pointing it at (remember the camera has no idea!) 


Tracking Sensitivity
This function sets the sensitivity for the AF to move onto another subject if for example an obstacle comes between the subject and camera, or perhaps if the AF point(s) moves off the subject momentarily. The subject matter and environment should dictate how this is set. For my low level aviation work, and when I am not using Auto AF, there is a risk that I might not be able to keep the jet within the chosen AF point(s).  So to reduce the chance of the AF moving onto say the background it is set to 1 or 2 notches down from 0, or to Locked on. If I was photographing a downhill skier and waiting for him/her to suddenly come into view from over the crest of a hill, this would be set to Responsive.

Accel/Decel Tracking
This is a new setting and is one example of the upgraded AF on the 1DX.  To understand what this setting is doing it's worth remembering what AI Servo, in its default setting, is doing.  The camera has to predict where the subject will be based on previous information it has captured, so for a subject with constant movement the default 0 is appropriate. What Canon seems to have done with this setting is recognise that while the data collected from previous information is fine for a constantly moving subject it might not be for one that suddenly accelerates, or stops - clearly the previous data will be inaccurate.

What the AI Servo actually does I do not know.  It seems a little counter intuitive that the scale is 0, 1 and 2 when the subject may decelerate rather than accelerate, a scale from minus 2 to plus 2 would seem more relevant given what Canon has described.  Essentially, my take on this setting is to tell the camera "don't expect this subject to be consistent, it will change direction or speed".  Areas where I can see it's use is perhaps athletics events such as the high jump or long jump, a cheetah chasing and catching its prey, a jet at an airshow performing fast manoeuvres -  anything where the movement isn't going to be consistent.

AF point auto switching
This comes into play when the AF points are not on Single or Spot (see Part I).  It alters the sensitivity of the AF points to switch and is appropriate for subjects with a lot of/erratic movement, such as birds in flight, or a jet displaying at an airshow.  This should not be confused with tracking sensitivity - the key to remember is that this setting is all about the speed with which the AF points switch.

AI Servo 1st & 2nd image priority
This is a crucial setting and is effectively Custom Function III (3) from the MKIV but with more permutations.  With this setting the photographer is able to tell the camera to release the shutter even if focus is not acquired.  Alternatively, the camera will not allow the shutter to release unless focus is achieved.  I would urge users to test this in non-crucial circumstances so as to experience what it is doing and the results it gives.

 For example when priority is given to Focus and the subject is difficult to track and lock on the shutter will stutter and not release continuously.  Switch it equal priority or shutter release priority and the camera bursts into life even if focus is not attained.  I've tested this on the middle (Equal priority) setting and been surprised as to how many images were not critically sharp.  My default setting is Focus priority, but for a Press photographer I can see the benefit of ensuring the picture is obtained and priority given to release the shutter.  Be very careful with this setting!

Short Eared Owl Hunting. Centre AF Point only.
Canon EOS 1D X & 800mm f/5.6 plus 1.4x @1120mm,
1/640 @f/8, ISO 1000. 
Shutter release time lag
The 1D X has a shutter lag of only 55ms and with the Shortened Release Lag custom function enabled, certain lenses mounted and certain aperture settings (typically wide open) in use, it can be shortened to an even more impressive 36ms. compared to the 5dmkiii at 59ms.

Putting it all into practice
While the Case studies provided in the AF menu are useful I believe they should only be taken as a guide and the photographer should be trying to set their own.  If someone has bought this camera and is relying on case studies to set the AF they should have bought a much cheaper model! 

RAF Hercules, Wales. Auto AF.
Canon EOS 1D X & 400mm f/2.8, handheld.
1/100 sec @f/7.1, ISO 200.
I have placed Tracking Sensitivity, Accel/decel tracking, AF pt auto switching, Selectable AF point and AI Servo 1st image priority into the 'My Menu' tab.  This way, and before every time I use the camera, I go into My Menu and think about each of these settings for the subject I am taking.  Personally, and while the AF Menu is a big improvement over previous layouts, this method gets the photographer to think about what to tell the camera.  Moreover, it rapidly increases the photographers understanding of what these modes do.

Garry Ridsdale
www.aircanon.co.uk

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