I just came back from the horse race track and looking through some of the photos, the camera/lens (of course not the photographer 🙂 ) couldn't get the horses in sharp focus when they were bunched together racing by.
I have a Rebel XSI (2008) and the manual stated to keep the automatic AF point selection (all 9 points) when shooting in AI servo.
I would like to know your thoughts?
I tried the center only AF point and it worked good as long as I kept it on the horse as it raced by 🙂 even in bunches.
For cameras such as yours that have only 9 points, I think you should have only the center point. Having all nine points will mean that the camera will choose a random one for you. When you have a bunch of horses and you wish the lead horse to be in focus...the camera will most likely choose the one in the middle or at the end...
As you have seen, tracking moving objects with only one point is difficult. Cameras that have more than 9 points will generally allow you to use the center point plus several more around the center to make tracking the target a bit easier.
Depending on what the camera allows (Rebels likely don't offer this) use center point or expanded center point & put it on the horse that you specifically want as the main interest in the photo. IF the camera offered picking an expanded point other than the center area you might want to use the one opposite the centre point from the direction they are heading so the lead horse is the main focus but you have more horses in the frame behind it. When I frame an R/C model with the smoke on I want the plane on 1 side of the frame with the smoke trail going across to the other side rather than 1/2 the smoke trail because I had the plane in the center of the frame.
The correct technique is to use the center point first on the subject to acquire focus. Then, if the camera moves away from the center point one of the other points will pick it up and focus. If you don't have AF Point selected, I.e. only have center point, if the subject leaves the center point the camera will choose the next thing the center point sees.
If if you move the camera to keep the center point on the subject then center point only is OK.
Yes, I previously had the center point on and panned the horse/s and it seemed to work fine, then reading the manual (I'm a newbie still learning) I decided to use the AF automatic ponits... bad choice, haha.
Here's a bad example, but you get the idea.
Yes I was far away and I only have a 100mm prime lens, but here you see the brown railing in focus, the horses, no way. It's a throw away.
For subjects like this, where subject is moving parallel to camera, I get better results with One Shot Continuous. No need for camera to refocus since distance isn't changing.
AI Servo benefit is predictive focus based on subject motion changing distance between camera and subject.