Pixiq

Autofocus is a great tool. It saves critical time required to focus on a subject, and in many circumstances it enables you to get pictures that you probably would have missed had you been focusing the camera manually. For example, I shot wildlife with a medium format (6x7cm) film camera for 25 years, and I can’t tell you how many pictures I missed because I couldn’t focus fast enough to catch fleeting moments in nature.

When subjects are stationary, autofocus is convenient but not crucial. You have time to focus on a grazing rhino in the Serengeti or a horse on a farm. However, what if you are confronted by a herd of horses galloping right toward the camera? Does normal autofocus work?

No, it doesn’t. By the time the camera focuses on the horses and you press the shutter, a fraction of a second has elapsed, and when the picture is taken, the horses will be several feet closer to you. If you are using a long lens and filling the frame with the charging horses, they will be almost sharp but not quite. Obviously, this is not acceptable.

Autofocus tracking (also referred to as predictive autofocus and shooting in AI servo) is a feature built into many digital cameras, and it is supposed to determine how fast the subject is moving toward or away from you and thereby hold it in focus as it moves. The theory is that at any point in time you can shoot and get a sharp picture. In my experience, this doesn’t happen. Predictive autofocus just isn’t that good if the fast-moving subject is coming toward or away from the camera.

The only time this feature works well is if the moving subject is going across your field of view from one side to the other. For example, a flying bird that flies left to right can be tracked successfully with AI servo. However, if the bird turns 90 degrees and flies toward the camera position, there is no way you can track it with confidence using any autofocus function.

This photo of the galloping horses in sunset light is an all-time favorite image of mine. It took two trips to France to get this. The first trip was made at the beginning of July to coincide with the lavender fields in Provence, but there wasn’t enough water in the marshes. In addition, my longest lens was a 200mm because I assumed the horses would be somewhat staggered as they ran toward me, and I was hoping that the fairly good depth of field characteristic of 200mm lenses would keep all the horses sharp even if they were staggered a bit. This was a mistake. The reason is I had to wait for the horses to fill the frame as they galloped toward me, but by that time they had dispersed, I was only able to get five horses at the most in any one picture.

My second trip to France was made in March. This time there was plenty of water in the marshes from spring rain, and I brought a 500mm f/4 telephoto. The horse wranglers forced the herd close together, and just as they started their forward motion I started shooting. The long lens enabled me to fill the frame, and I got what I wanted.

There were two major concerns, though. One was focus, as I’ve said. The other one was that once the herd of horses ran through the marsh one time, they became dirty. In fact, they were no longer white. They were muddy gray. Therefore, I had only one chance to get what I wanted at each sunset session.

Since I do not trust the AI servo mechanism in any camera, I had to use a different approach. Even though I used a 500mm lens, the horses were far enough away (about 300 feet) that I knew I had at least a small amount of depth of field at f/4. I used the largest aperture on the lens to get a fast shutter speed. As the horses were given the signal and they started churning the water, I focused (using a back button on my Canon 5D Mark II) and shot two or three frames on motor drive, and then I quickly refocused and repeated the burst of exposures. I did this several times — focus, shoot, refocus, shoot, etc. In just a few seconds the peak action was over. This picture is the best of the bunch.