20D focus points

My EOS-3 will allow me to chose more than one AF point but for metering averaging only, not for focusing.

On p. 65 it says "The AF point which achieves focus flashes briefly. At the same time the focus confirmation light <*> in the viewfinder is displayed." From that, one can equally infer that if more than one AF points light up, the camera's achieved a focus solution for all of them, and will use that solution, not that it's going to pick one at random. The use of the singular (AF Point) in no way implies that one and only one point will be used.

can someone close this useless thread?? if one does not know how to read the manual - too bad. If ppl who know how to read it answer and get yelled at...what;s the point?

No, Canon never claimed that the system will pick a focus point at random from multiple point candidates. It does pick one, and it is usually the closest one with good contrast, but it is not random.

That is also correct that the use of the singular AF Point in no way implies that one and only one point will be used. It is also possible that no point will be used if there are no point candidates.

---Bob Gross---

I'm a bit confused.

I have noticed in the past that more than one focus point lit up when focusing. I assumed that this means that with this specific distance set on the lens, one focus point was selected, but another one is on the same focal pane, so is also in focus.

Best regards,
Andy

Okay having read through the rather amusing to'ing and fro'ing between Bob and Iron.... I still find myself asking the question.

Where more than one focal point is highlighted in autofocus mode, which one is used for the final focus point when shutter button is depressed.

Also ive noted that while it is possible to select a focus point in manual mode, depress and hold the focus button (custom function 4) while adjusting focus manually to achieve focus blink in viewfinder mode of selected focus point, it does not result in numerous blinks of more than one focus point where a number of objects are at equal focal distances.

Never mind, i read Bobs last response and it seems to make gooooood sense...

Where lies the truth..

So even when more than one AF point lights up, the camera selects a point with best contrast. Got it!

kufel, yes it did help! thank you

If two or more autofocus points light up, then those are candidates for the final choice. That means that the camera found _something_ contrasty there that it could deal with. Then, at the instant your finger goes down on the button, it picks one, which is typically the closest candidate point with good contrast.

I don't know exactly when or why it would pick something other than the closest, but I doubt that it is random.

---Bob Gross---

Bob, can you clarify which button press you're talking about? Assuming a CF4-0 configuration, do you mean the half-press to lock focus, or do you mean the full press?

Surely you're not claiming that after focus lock, there is still ambiguity about focus and that the 20d will change the lens's focus again the moment you do the full-press, are you?

I thought the fact that a focus point was lit up was because the camera had achieved focus on that point(s). I use the center focal point on my 20D only, most of the time, but when I had all 9 enabled, I often would get multiple points lit up. It seems pretty straight forward that those multiple points were in the same focal plane.

I don't understand your statement that "when you finger goes down, it picks one, etc...". Again, hasn't the camera already achieved focus?? No further adjusting is necessary at this point, as far as I know. But my post count is low, so perhaps I'm a noob.

William

I don't use any custom button configurations. The button is the shutter release button.

---Bob Gross---

Bob - understood that part already. Trying to understand the sequence you are explaining. The shutter release button can be pressed halfway (to lock focus), and fully (to take the picture). Going through the sequence of pressing halfway, and then pressing fully, when is the camera in the "display contrasty potential focus points" state you mention, and when does the camera decide on the actual focus point?

Thanks.

When you half-press the button, if the camera has a single focus solution, then it will drive the lens to that point and leave it. Then when you full-press the button, it thinks "Hmmm, only one focus point, I think I'll go with that." It may re-check the point and re-hit the lens if necessary.

When you have a multi-candidate situation with the camera automatically deciding the point, it is more complex. Half-press only causes the viewfinder to light up with the candidates, and the focus goes to "something." Then full-press forces it to make its decision, typically the closest candidate with contrast, and that drives the lens properly for that chosen candidate.

At least that is the way mine works in P mode.

---Bob Gross---

perhaps this link will help. though written about the 1d, most of the discussion of focus point algorithms applies to the 20D as well. see page 8, "AF focus point selection methods".

It's important to emphasize that some of that document applies to 20D, but not all, and it was intended for 1-series bodies.

The first time I read it, maybe six months ago, I was thinking, "Wow, this is good stuff." Then I realized that it does not apply to all bodies 100%.

---Bob Gross---