How do I get the 300d to constantly autofocus in Tv mode?

That sounds like the "AI Focus" mode on the 20D, perhaps.. which is like I mentioned earlier, where it will start to refocus itself automatically when it sees the subject start moving... so basically it has AI Servo built into it, in a way. I'm not positive it's the same thing, though, someone with this hack know for sure?

In cannon software AIFocus is not the same as AIServo I believe you are looking for AIServo. Who knows what the hacks actually produce. Often there are unusual side effects. Both the 10D and the 20D have AIServo, I enjoy the feature a lot in following action.

Those were not requirements in his message.

---Bob Gross---

Nope.

---Bob Gross---

So what, what if other users did not know this? Should they just guess?

True but sometimes users does not provide all information either because they forget or don't think it is relevant.

In your scenario, I would not use AI Servo. I shoot a lot of sports and I do not use AI Servo much eventhough it is available on my 20D. AI Servo, in my humble opinion, is better suited for continual fast moving action such as tracking a moving race car. Having AI Servo on when trying to track eratically moving objects (boxers) may result in focus being acquired in the wrong place. I have had this happen to me when I tried to use AI Servo with a baseball batter. Since the camera is constantly forcusing, if my focus point move off the batter momentarily I may end up shooting an out of focus shot. I find it easier to focus on the player and hold down the shutter half way until I get the action that I want and release the shutter.

Again, this is my preference but you may want to consider just using one shot focus mode (available with the hack) and not AI Servo (which does not work well with the hack).

Ovi

As Oviv mentioned, AI Servo may not perform too well for the eradic movements of a boxer. If you do want to use AI Servo, you can install the 300D (unofficial) firmware which adds a bit of support. However, it doesn't work as it should.

1) It requires you to hold the shutter down half way so it will continually focus (normal)
2) When you want to take a picture, you have to completely release the shutter then press it all the way to release the shutter. (no normal for a working AI Servo)

It works, you just have to be fast on step 2. And I'm not sure how well it would work. Another option would be to use manual focus, and use an f/stop so that the majority of the area of the boxing ring is in focus. Of course, lighting and distance from subject may not allow this...

Good luck, and I'm interested in seeing how your shots turn out

A rundown on the different focusing modes
AI Focus: Obtains focus, but if it senses movement it will refocus on the moving subjuct
AI OneShot: Obtains focus and keeps that focus until you refocus
AI Servo: Continuously refocuses on moving object

I don't remember off of the top of my head, but in TV mode, it is either AI Focus or AI OneShot. It will tell you in the manual. AI Focus sounds good, but it's behavior is often erradic (doesn't always refocus on moving object, sometimes refocuses when you recompose the scene).

300D / Digital Rebel (without hack) always uses AI Focus in creative zone modes (M, P, TV, AV). The problem that I found with AI Focus is that if you focus and recompose your shot the camera will sometimes refocus eventhough this is not what you may want. With AI One Shot you can focus, recompose while half holding shutter and not have the camera refocus. This is what I use most of the time.

Regards,
Ovi