20D AF Point Determination

[Mark_48]

Canon 20D

I'm trying to get a better grasp on the AF points and using the EOSViewer utility I've been able to see what point was active during the shot. Some of the points shown active have further confused me.

I am led to believe that the active point uses a difference in contrast within the point to determine focus. I have occasional shots where the AF point selected rests in an area that to my eye has very little difference of contrast levels and this was the only AF point selected in the shot and the shot generally has alot of other edges and distinct shadows it could have selected AF points for. I do have the camera set to use all AF points and Evaluative metering. Is the AF point strictly defined to be within the rectangle of the point or is there some slop that it could catch something with contrast slightly outside the box?

Just this evening I took a couple of pictures of a white house that filled about a 1/4 of the frame. They were shot in manual, one metered right on and the other opened up about 1 stop. The one metered right on selected several AF points directly on the house, edges of clapboards and window frames. The shot I took opened up a stop selected one single AF point off in a wooded area adjacent to the house. Both were framed about the same and both appear to be relatively in focus. This was my first trial at this and I'm not sure how repeatable it would be. Would the camera select an AF point on that which it believes to be properly exposed, rather than an intended target which is metered slightly over?

Mark........

[tim]

AFAIK most people use the single centre point to focus, which would mean you didn't have to worry about this question. I often use non-centre AF points too, I prefer that to focus recompose which throws focus off when you're using a large aperture lens.

[PacAce]

Canon 20D

I'm trying to get a better grasp on the AF points and using the EOSViewer utility I've been able to see what point was active during the shot. Some of the points shown active have further confused me.

I am led to believe that the active point uses a difference in contrast within the point to determine focus. I have occasional shots where the AF point selected rests in an area that to my eye has very little difference of contrast levels and this was the only AF point selected in the shot and the shot generally has alot of other edges and distinct shadows it could have selected AF points for. I do have the camera set to use all AF points and Evaluative metering. Is the AF point strictly defined to be within the rectangle of the point or is there some slop that it could catch something with contrast slightly outside the box?

Just this evening I took a couple of pictures of a white house that filled about a 1/4 of the frame. They were shot in manual, one metered right on and the other opened up about 1 stop. The one metered right on selected several AF points directly on the house, edges of clapboards and window frames. The shot I took opened up a stop selected one single AF point off in a wooded area adjacent to the house. Both were framed about the same and both appear to be relatively in focus. This was my first trial at this and I'm not sure how repeatable it would be. Would the camera select an AF point on that which it believes to be properly exposed, rather than an intended target which is metered slightly over?

Mark........
When you use the auto AF point selection, the camera tries to find the AF points which have the most contrast and which are usually closer to the camera. However, the camera will not always select the same points each time you press the shutter button, even if you're looking at the same scene and the camera is mounted on a tripod. I'm not really sure what algorithm the auto AF point selection uses but I do know that you'll have a hard time making the camera select the same AF points twice in a row.

BTW, as far as the AF selection point is concerned, the camera doesn't care whether the image is going to be exposed correctly or not and, hence, there is no correlationship between the two.

[Mark_48]

Here's a sample shot. Note that the AF point is in woods which is out focus, yet the intended subject is OK. So I'm not sure what knowing the active AF point is really telling me.

Mark.........

[tim]

Mark - no idea what's going on in that shot. Did you focus recompose? The woman looks a lot more in focus that the trees.

[Bob_A]

Mark - no idea what's going on in that shot. Did you focus recompose? The woman looks a lot more in focus that the trees.

Yup, sure looks like focus recompose to me. Sometimes it's easy to do without even thinking about it. :)

Bob

[robertwgross]

When you use the auto AF point selection, the camera tries to find the AF points which have the most contrast and which are usually closer to the camera. However, the camera will not always select the same points each time you press the shutter button, even if you're looking at the same scene and the camera is mounted on a tripod. I'm not really sure what algorithm the auto AF point selection uses but I do know that you'll have a hard time making the camera select the same AF points twice in a row.

BTW, as far as the AF selection point is concerned, the camera doesn't care whether the image is going to be exposed correctly or not and, hence, there is no correlationship between the two.

I couldn't agree more. When I shoot, my first preference is to manually select only the center focus point for best results. My second preference is to manually select some non-center focus point, but still I am the one choosing. My last preference is to let the camera auto-select a point, because it will sometimes be a good choice, and sometimes it is some point chosen only for good contrast in the near field. Also, of course, good contrast in a focus target typically requires it to be moderately well lit.

---Bob Gross---

[I Simonius]

I only use the central point now after some dissapointing results from the others
I only let it use the others if I use A-DEP mode.

[Mark_48]

Very likely it could be a focus/recompose. As Bob said it's easy to do without thinking about it.
I've had good luck using all focus points and allowing the camera to decide, but as most recommended I'm going to give the single point method a try.

Mark.......

[PacAce]

Very likely it could be a focus/recompose. As Bob said it's easy to do without thinking about it.
I've had good luck using all focus points and allowing the camera to decide, but as most recommended I'm going to give the single point method a try.

Mark.......

I don't really see auto AF selection as being a good thing or a bad thing, as long as in the end, it is you who decides whether the AF points selected is what you want or not. Of course, it may take several half-presses of the shutter button until the camera comes up with points you like, but then again, maybe it never will... :mrgreen:

I never use auto AF selection in SingleShot mode. However, I rely on auto AF selection a lot in AIServo mode. In AIServo mode, when the AF mode is set to auto AF selection, the camera uses the center AF point first until it achieves a lock on the subject. Once focus is locked in, if the subject moves out of the area covered by the center AF point, the camera will use any of the other AF points to track the subject and keep it in focus as long as the subject is large enough to be covered by at least one of the AF points at any given point in time.

[Phil V]

I don't really see auto AF selection as being a good thing or a bad thing, as long as in the end, it is you who decides whether the AF points selected is what you want or not. Of course, it may take several half-presses of the shutter button until the camera comes up with points you like, but then again, maybe it never will... :mrgreen:

The camera might never come up with the focus points you like, but you still can't decide whether it's a bad thing?:confused:

The only time I set the camera to choose the focus point is when I'm handing the camera to a relative (or child), and it's too much hassle to give proper instructions.

To the original poster;
You now know enough about photography to decide where you want the point of focus to be, use the tools you have to your advantage. Either select the focus point each shot, or select the centre focus point and focus recompose.

[Mark_48]

You know...........there are just some days I think about getting the old Pentax Spotmatic out of the closet, loading up with film and just relaxing. Life was simple back then. Manual focus on a split screen, no automated exposure, thumb film advance, and no Err99's....30 years and still tickin' and clickin'....

Ah, life was good !!! :)

Mark.......

[Tom W]

Where the multiple focus points are really helpful is when you're tracking a moving object in A1-Servo mode. In still one-shot shooting, sometimes it's better, and sometimes not. That's why the choice is there, I guess.

[PacAce]

The camera might never come up with the focus points you like, but you still can't decide whether it's a bad thing?:confused:

The only time I set the camera to choose the focus point is when I'm handing the camera to a relative (or child), and it's too much hassle to give proper instructions.

To the original poster;
You now know enough about photography to decide where you want the point of focus to be, use the tools you have to your advantage. Either select the focus point each shot, or select the centre focus point and focus recompose.
I never said that I can't decide whether using the auto AF point selection mode is a good thing or a bad thing. All I said was I don't see it as being either a good thing or a bad thing. There's a world of a difference in the meanings of the two sentences, isn't there?

[lancea]

You know...........there are just some days I think about getting the old Pentax Spotmatic out of the closet, loading up with film and just relaxing.
I just did - well, just did get my XD7 out of the closet. Wow! Ignoring the gunk around the microprism circle and the split-prism, focusing is so bright, quick and precise. Surely it's only a matter of time before one of the high-end manufacturers puts out a retro model - 25 megapixel sensor but otherwise ALL MANUAL. :mrgreen:

As to the focusing points, I always start on auto-select then if I don't like what is being selected I'll specify a point. Works well. Having the option to manual-focus the USM lenses (without switching to MF) also comes in handy.

[robertwgross]

You know...........there are just some days I think about getting the old Pentax Spotmatic out of the closet, loading up with film and just relaxing. Life was simple back then. Manual focus on a split screen, no automated exposure, thumb film advance, and no Err99's....30 years and still tickin' and clickin'....

Ah, life was good !!! :)


Why don't you get out the black and white television to watch Sid Ceasar?

---Bob Gross---

[markubig]

I'm surprised that more people aren't using CFn13-1 to use the joystick to manually choose AF points. I'm always changing AF points to get the one closest to my subject.

[lancea]

I prefer to use the dial. Much quicker and more conveniently placed than the joystick.

[Mark_48]

Why don't you get out the black and white television to watch Sid Ceasar?

---Bob Gross---
I do have a B/W television kickin' around. Do you know which network carries Sid? :) Actually would rather watch Red Skelton...

Mark.......

[wiselion]

Could you please explain what focus recompose means? I'm a newbie. I think it means that you focus, let up on the shutter and then focus again to quickly causing blur. Am I close?

Mike

[robertwgross]

Could you please explain what focus recompose means? I'm a newbie. I think it means that you focus, let up on the shutter and then focus again to quickly causing blur. Am I close?

Suppose you aim your camera toward subject number one and two. Now aim it a little toward subject number one only. Half-press the button to get a focus lock. Now, with the button still half-pressed, move the aim back toward subject one and two together. Now fully press the button.

If you had left the aim on subject one and two, the actual focus spot might have been halfway between the two, which might have been fifty feet behind them and in the wrong place.

---Bob Gross---

[wiselion]

Suppose you aim your camera toward subject number one and two. Now aim it a little toward subject number one only. Half-press the button to get a focus lock. Now, with the button still half-pressed, move the aim back toward subject one and two together. Now fully press the button.

If you had left the aim on subject one and two, the actual focus spot might have been halfway between the two, which might have been fifty feet behind them and in the wrong place.

---Bob Gross---

Thanks Bob. Makes sense.