Preditictive Autofocus and moving subjects

[ted_hendy]

I posed essentially the same question in the Canon EOS FAQ forum, but

received no responses. So I thought I'd try it here.

I've been getting frustrated in trying to use Canon's predictive

autofocus so I went back to the manual. The Elan IIe manual only says

"If the subject approaches or retreats from the camera at a constant

speed, the camera focuses the subject continuously and predicts the

position of the subject for the moment of exposure to obtain accurate

focusing." I thought that I had heard that Predictive Autofocus was

also for tracking a moving subject from the center point to one of

the side points. The Magic Lantern guide says "The...three AF points

make it possible to photograph a moving subject that is not in the

center...the object is automatically "handed off" from one cell to

the next, without the AF losing track." Maybe trying to track a

running deer, moving rabbit or jittery bird is too much, but I've not

never noticed my autofocus keep track from center point to a side

point. So I'd like to hear how some of you do it. 1. Does

predictive autofocus work when the subject moves across the field of

view? 2. Do I have to set the focus point selection to automatic

(i.e., no eye control) and go for the bulls eye effect? I guess my

bottom line question is: What's the best way to photograph a subject

moving across the field of view? My experience seems to show that for

me it is to select a single focus point for automatic focus and then

pan keeping that focus point on the subject. With eye control I find

the focus wandering as I "look" ahead of the subject. I'd like to

hear your solutions and opinions. Thanks. Ted

[cowan stark]

I can speak from the experience of an A2E user. I've been working on flight shots of birds-seagulls are my favorite practice subjects-they're fast,plentiful and- once in a while, actually photogenic. I've found the following when using AI servo mode: 1) turn off ECF 2) select one focusing point and lock on to it for fast moving subjects when panning 3) your target has to be reasonably large in the frame 4) the first shot is usually the worst, the focus doesn't lock on for me until the second shot in the sequence. 4) Using all the focusing points in my case 5, 3 for you does work when the subject moves across the field, but I save this for turtle photography! -OK I'm exaggerating a little, but I have found it slower. 5) Don't forget to re-acquire focus if you lose your lock on the target. 6) you didn't mention what lens you're using-this will be a factor. Finally, I highly recommend you read the section in Arthur Morris' book The Art of Bird Photography. It has really practical Canon-specific techniques for autofocus and telephoto photography, even if birds aren't you're main interest. Again, just my experience with the A2E, I haven't used an Elan II for this.

[lloyd_nakatani]

On my Canon 1N-RS in AI Servo mode, the center focus point must be used to acquire initial focus. Once the focus is acquired, the subject is automatically "handed off" to the other focus points to keep it sharp. Since the hand-off is automatic, I presume that ECF shouldn't be used on the Elan, and only the central focus point should be manually selected when using AI Servo mode. Remember that only the central focus point is a cross sensor; the other focus points only detect vertical lines. The central focus point lights up when the initial focus is acquired; thereafter the focus points do not light up to indicate hand-off even though it is in fact happening.

 

This worked beautifully for me at a figure skating show, but it was an easy situation because of the smooth movement with nothing in the foreground or background except ice.

[stanley_mcmanus]

Correct me if I am wrong, but aren't predictive focus and focus tracking two different things? Predictive focus is not designed, as I understand it, to track a moving subject across the screen. It is designed to allow for the fact that the camera to subject distance may change a bit between the time the shutter button is pressed and the shutter actually fires. Predictive autofocus is supposed to set the focusing for the distance when the shutter fires, not when the shutter button is pressed. Focus tracking, on the other hand, is designed to follow a moving object around the finder and hand off the focusing job to the next focus point, all the time keeping the subject in focus.

 

Correct me if I am wrong.

[ted_hendy]

Stanley,

I went back to the manual to find correct term. Under Predictive Autofocus, in the Elan IIe users manual, it only talks about approaching or retreating subjects. However, in the table below it labeled "Focusing Point Operation in AI Servo Mode" it says this under Automatic Selection: "The center focusing point focuses the subject first. If the subject then moves to another focusing point, focusing continues with predictive AF". That's all part of why I'm confused. In both the AI Servo AF and Predictive AF sections, on page 38 of the manual, it only talks about approaching and retreating subjects. But as I said above, and in the Magic Lantern book, it implies a lateral ability also. Ted