20D focus points

Ok, I understand that the middle focus point is the strongest in terms of finding focus (especially in dark locations) with f/2.8 and faster lenses. For a while, most of my shooting included selecting the middle point which is great.

But what about selecting all points. But instead of releasing the shutter with random focusing. Instead keep focusing till at least the middle focus point lights up, and in this case,usually 2 or 3 other focus (non-middle) also focus and light up.

Isnt this the best of both worlds? Insure proper focus with the middle one, and a few other points too?

Lets say in the composure of the image, the middle focus point is on the edge of a wall that is about 10 feet away from the lens, 2 feet to the right , behind the right focus point is another wall that is 10 feet away from the first wall. Meaning it is 20 feet away from the lens. Can, while selecting all focus points, technically it focus on the middle point (10feet) and also focus on the 2nd wall (20feet away). Given that both are bright enough for the camera to focus on?

I guess not because , the focus (disregarding aperture for a minute) lands on a point that acts as an invisible wall of clear focus. So from top to bottom (on the same plane) everything will be in focus?
If so, the width of this wall depends on aperture obviously.

You're wording your question in a confusing way. What I think you're trying to say is "isn't letting the camera choose its own AF points better than selecting just one yourself?"

Unless the multiple points it (camera) selects are at the same distance from the camera - it cannot "focus" on more than one at once. It might average between the points it hit, but there's the bad aspect. When letting the camera choose the AF points you have no control over it unless you press halfway the shutter button many times until it selects the ones you want, if it ever does.

There are many posts on here about "why are my shots out of focus" and "soft images" and "which AF point to use" and the overwhelming answer it to choose your AF point yourself. If you need a large DOF, use a small appeture to acheive a lot in focus. Many new users think their focusing is bad or soft because the camera (when using full auto) is choosing AF points that don't hit your subject exactly where you need it to.

A very confusing question.

You can have the camera/lens focus on one point and one point only. Either you pick the point, or let the camera pick the point. Period. There is no autofocus option otherwise.

You can pick one point that is midway between the front and the back of the subject. You can examine the depth of field using the depth of field preview, and you can decide if you need to do anything about your depth of field via aperture, but you still have to pick a focus point.

You can go into A-DEP mode. There, the camera will attempt to pick an aperture that covers all of the bases. But, you still have to pick a focus point.

This is not an autofocus option, but your other option is to manually focus.

---Bob Gross---

These replies answered my question in regard to focus points falling on objects that are not at the same distance.

My other question is not about which AF point to select, Canon in the manual states that the middle point has a cross-hair feature which is definitely more accurate than the remaning focus points.

My question is, instead of always selecting the middle focus point as standard point. Why not have it on "all", where all the focus points can light up if focused. If you put it on "all", you will notice that often, more than one focus point is lit up. So I was saying, why not keep pressing the * (CF 4-1) focus button till the middle one lights up (you have achieved better accuracy) and watever focus point lights up is just extra confirmation of focus?

Again, a confused question!

If you set your autofocus system to allow the camera to light up all of the autofocus points where it can find contrast, then you are again allowing the camera to pick one of them, which may be right and may be wrong for what you are trying to achieve. The camera is not going to settle on "all" as the single resulting focus point. It is going to pick one of the multiple points lit, typically the closest one with the best contrast, and that might be the wrong answer. It might be the right one, also, but most of us like to avoid the uncertainty.

---Bob Gross---

So if I understand, lets say 4 AF points light up, then the camera selects the focus information from one of the lit-up points and uses it's information? Instead of using all 4 lit-up AF points' information for focusing?

What I was originally saying was, set it so that all focus poiints can light up (i.e dont select a FP), then focus on your subject, maybe on the first focus attempt, the left-most point will light up. Nope, not good enough, you focus ( * ) again, this time the left-most and the right-most FP light up. Nope,still not good enough. On your third attempt, 3 FP light up. The top most one, the bottom most one and the middle one. Perfect. Now you have achieved focus via the middle FP (most accurate) and confirmation from the remaning 2 FP's. Whats wrong with this method? (if time is not of the essense)

I answered your question. There's too much guessing when letting the camera "use" more than 1 AF point.

hellashot, im sorry, I didnt get it then. Care to elaborate?

Also, where is the guessing that you are talking about?

thanks

I don't understand the question, again.

With the current technology, you can't have the camera focus on more than one focus point at the same time. Period!

Apparently you choose not to use the depth of field preview, and apparently you choose not to use A-DEP mode.

You're really working yourself into a corner.

---Bob Gross---

Bob, lol calm down man. I do use DoFPreview a lot.

Then why does more than one Auto Focus point light up when I let the camera select a focus point and focus? (i.e: I have not selected a FP, all 9 focus points have the ability to light up after atttempting to focus)

I think you should take a little more time to write your questions more clearly.

If your question is "why not let the camera choose the focus point", my answer is because I want to choose the focus point, not the camera. When I shoot theatre I use about 4 different focus points, depending on what i'm doing. The centre point is the most sensitive, but with you have a narrow depth of field focus-recompose will throw your focus out.

If you let the camera choose it'll focus on what it thinks are important, and in auto modes it might adjust the DOF to compensate. I'm not really sure, I tend to control the camera, rather than let it control me.

That is a clear question.

More than one autofocus point can light up to tell you that those are the possible ones that the camera will choose from. In other words, if an autofocus point does not light up, that means that the camera can't find any contrast there, so it doesn't trust that point.

But then, in the split second as your finger is coming down on the button, it will pick the best one (according to its thinking), hit the lens's autofocus motor as necessary, and then capture the image. One shot, one focus point.

---Bob Gross---

Although you seem pretty confident about that (exclamation mark and all) I don't believe that's true. Chuck Westfall's (Director/Media & Customer Relationship Camera Division/Canon U.S.A., Inc.) posts in other forums, Canon's patent applications, and various Canon publications state otherwise.

Maybe you can explain why you believe "you can't have the camera focus on more than one focus point at the same time. Period!"

Ahh, you're right.

Canon probably does have such a trick up its sleeve, but not in the 20D, which is what this thread is about.

---Bob Gross---

I'm aware of that , and my comments and question are the same.