Auto Focus Sensor Placement for Action

A few years ago, I learned a technique from Rob Sheppard to make sure the critical part of my subject is in focus.  He suggested focusing precisely on the part of the image that must be sharp.  From that starting point, I could then calculate depth of field (DOF) and determine what additional parts of the image would be acceptably in focus.  In practice, I could also use the DOF preview button and see it real time.  This technique is great for every type of photograph I make.  It is sometimes a bit challenging though when photographing action.

In the example image, the eagle's eye must be sharp so it makes sense to focus precisely on the eye only.  I can make that happen by selecting a single auto focus (AF) point, placing it on the eye, and tracking the bird as it flies from right to left, easier written than executed when the bird is moving very quickly. 

Since this can be difficult, many photographers choose to use a zone AF system in which multiple AF points are active.  As long as the subject is under one of the active points focus is normally achieved.  This technique works well as long as there is sufficient DOF to cover from the point of focus (as determined by the zone) and the critical part of my image.

Looking at the example eagle again, notice the left wing tip is not sharp.  The shutter speed of the shot is 1/1250th so motion blur is not the culprit, lack of DOF is.  This demonstrates where the zone technique can hurt you.  If the zone had "locked" onto the outer portion of the left wing it would be sharp, but the critical area, the eye, would not be.   Fortunately, for this image I knew I had shallow DOF and needed to precisely place my single AF point on the bird's eye.   

So what should you do in the field, single or zone?  Single is much more challenging but it always works. The zone is much simpler but DOF can be an issue.  I use both methods but before I shoot I calculate my DOF with a  smart phone or tablet calculator.  This way I begin the shoot with the information that allows me to choose the appropriate AF technique. 

Approaching each action shoot whether birds, horses, or people, I follow the checklist shown below and always calculate my worst-case DOF so I know if I should use a single point or the zone. 

More info on this checklist and many others may be found at the following link.

Checklists

Of course making the choice between zone and single point is just one part of the process fpr action photography.  You must also practice your focus actuation technique, support, tracking, and exposure modification.   

Very few eagles where I am in California so I practice on any and every bird I can find.  If there are no birds around I make do with anything in the air. The shot below is a single AF point with the EF 800mm f/5.6L lens tracking a moth, needed a bit more shutter speed to freeze him.

When you are ready for eagles join Rick Sammon and me in Alaska and let's shoot! 

Alaska!

Canon 5D Mk III Auto Focus Performance

A quick post to describe my experience with the new auto focus (AF) system on the Canon 5D Mk III.  Bottom line up front, dramatic performance increase when compared with the 5D Mk II and equal to the 1D Mk IV. 

I spent a few days shooting with the Mk III and putting the AF system through some real world action shooting.  You can easily read all of the technical specifications, beeps, and squeaks on many other blogs, I will give you my simple man's take on the AF performance. 

The Mk III is a leap forward for the 5D series in terms of AF.  There are numerous modes and configuration options but you will find the best performance in demanding action situations when you select the Single Point AF mode.  In this mode you choose the appropriate AF point, engage AI servo with proper back button focus techniques and track your subject.  Although this mode does not engage some of the advanced AF logic and new tracking "assist" features, it will deliver the best performance. 

With that said, using a single AF point is not the simplest way to track and focus on a moving subject but, with practice, it is hands down the best.  As always, we do not shoot in a vacuum and the specific shooting situation may require or force you to another mode.  Depending upon the circumstances and variables you may get excellent performance with those as well. 

I consider the 5D Mk III's AF performance to be equal to that of the 1D Mk IV and clearly superior to the 5D Mk II's and 7D's.

I look at action AF performance looking at one main variable (assuming a relatively consistent subject path), track crossing angle or TCA.  TCA is the angular difference between where the camera is pointing (in aviation terms, where the camera is pointing is your effective "track") and the track of your subject.  TCA may be either high or low angle, from a shooters perspective low angle is when a subject is in front of your lens and tracking directly towards or away from you. High angle is when the subject tracks in a perpendicular manner across your shooting direction. 

Varying TCAs, when combined with subject aspect and speed, generates the multitude of tracking conditions necessary to get a good feel for what a camera can do.  I will do a more detailed post with images and some diagrams regarding these topics soon. 

For reference, the image at the top of this post is a 90 degree TCA at a high rate of speed.  The second image is low TCA. 

Fiat Lux!