Wed 03-Apr-13 08:52 AM | edited Wed 03-Apr-13 09:12 AM by km6xz
There are too many unknown variables to judge the technique on the images you are concerned with unless we can see them. AF is complex on modern cameras and we often ignore that when most of the time it guesses correctly what we intended to be in-focus. When it guesses wrong based on our intended focus point, it is not just randomly picking a different target to frustrate us, it is picking one that the camera calculates as being more likely to be the desired target. To have the camera guess correctly more often we do better by intending a focus point that the camera would most logically pick instead of one we want it to pick. We can adjust how we pick focus targets easier than you can re-engineer the software of the AF system. We can make those adjustments easily on the fly by knowing what the camera can see and how it evaluates the targets. If for example we knew the focus sensor was larger in coverage area than the little box that represents the focus point in the view finder, we would be more careful about how much confusing possible focus points fall into the sensor's real detection area. Couple that with the traits of the focus object that are better for the focus sensor to make a decision on. For example we might see a wall, that nicely lit and large compared to the focus sensor cross section area and assume it is a great focus target. It probably isn't however if the wall is uniform in color or devoid of texture. If a fly was flying in the area between the wall and the camera, it would be possible for the camera to see a closer subject as being a more logical and higher quality focus target, and blur the wall in attempts to focus on the fly that was not your target at all. So thinking of what the camera see and what it thinks about the target choices it has within any chosen focus point, goes a long ways to helping the AF do what we intend, by wanting the same criteria as the camera has. What, according to the camera is a good focus target? One that fills the focus point with the same focus plane. One that has texture, instead of featureless or diffused scattered light. One with sharp demarcation between light and dark regions. One with well defined edges One that is well lit
What are bad focus targets? Those with seamless featureless areas. Those with several targets within the same focus sensor area. One which has lower contrast. One that is on varying focal planes.
So using this camera thinking, we have a few problems with birds that we humans much help the camera with by providing less ambiguous targets. The birds are often small in the frame, which means something else is also in the frame that is not the target desired. Birds often have soft diffused texture, feathers that have little contrast or detail texture seen at the scale we(the camera) sees it. Either fast moving and small in the frame or sitting still on a perch. Both are tough on AF systems. A perched bird in the wild is often using a thicket or branches as camouflage and their coloring is not by accident similar to the camouflage so there are lots of confusing information the camera focus sensor is trying to evaluate. The twig or branch behind the bird you mentioned could easily have been a more logical focus target than the bird since it has well defined edges, sharp texture elements, more reflective of light, and has portions with linear lines. Was the tree/bush in focus sensor area? Remember the focus sensor area of detection is larger than the area shown by the box in the VF. How to focus when the subject is confusing to the AF system? Try to pick a good target on the same focal plane as the desired subject that is not such a good target. Filling more of the sensor with the edges or high contrast areas of the desired subject. Use manual focus. This last one is not popular but is the only one that assures that what the camera focuses on is the same one you intended in the first place. You can help manual focusing by stopping down so your focus adjustments will not be so critical. You can install a special focusing screen like those from Katz-Eye which make manual focusing a snap like it was in the film all manual focus days....when it was rare to have an out of focus photo.
Regarding AF mode, if the subject is still, like a perched bird, you would probably get better results in AF-S mode and active birds, on the wing or ground feeding you will get better results with AF-C mode. Then select the number of focus points you would like the AF system to evaluate if the subject moves away from a focus point. The fewer the better if the subject's fast movements are constrained to a small area around your chosen focus point that you are attempted to keep centered n the focusing target.