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View Full Version : Why do you need so many AF points?


EcoRick
8th of June 2007 (Fri), 08:13
Might seem like a real newbie question, but why are so many AF points necessary? For 99% of my shots I use the center point. When I used the other focusing points, I've had terrible luck getting a sharp picture. Are the mutiple points used primarily for moving subjects? Thanks for your help.

PhotosGuy
8th of June 2007 (Fri), 08:26
Are the mutiple points used primarily for moving subjects? For portraits I like to have one on the eyes, & there never seems to be one right where I need it? :D

Mcary
8th of June 2007 (Fri), 10:41
99.999% of the time I use a single, user selected focus point, postioned on or near the subjects eye.

René Damkot
8th of June 2007 (Fri), 11:15
Because Focus Lock Recompose has certain drawbacks (http://visual-vacations.com/Photography/focus-recompose_sucks.htm), and because it's nice to be able to use AiServo, and actually focus on the subject ;)

tdp
8th of June 2007 (Fri), 13:42
Because Focus Lock Recompose has certain drawbacks (http://visual-vacations.com/Photography/focus-recompose_sucks.htm), and because it's nice to be able to use AiServo, and actually focus on the subject ;)

thanks for that link, I hadn't thought about it, but it makes sense.

Sathi
8th of June 2007 (Fri), 14:48
Because Focus Lock Recompose has certain drawbacks (http://visual-vacations.com/Photography/focus-recompose_sucks.htm), and because it's nice to be able to use AiServo, and actually focus on the subject ;)

Good read. I'll have to try manually selecting focus points...i'm real lazy though...manual...select..bleh. Can't canon work something out for me? ;)

I thaught this page of that link you threw in was pretty cool.

http://visual-vacations.com/Photography/aperture_and_lens_usage.htm

primoz
8th of June 2007 (Fri), 15:04
That's what I'm asking myself too, with all those "suggestions" about using center af point only.
Personally I use pretty much everything but center point. With wide open lenses, there is huge difference between focusing to athlete's eye or athlete's stomach. So that's reason why I use that one of 45 af point's, which is on athlete's face rather then center point, which is normally somewhere on his stomach.

CyberDyneSystems
8th of June 2007 (Fri), 15:07
I use "off center" points all the time. :)

pagnamenta
8th of June 2007 (Fri), 15:31
Just curious, the cross type center AF point on the nonprofessional models is only activated if using a f2.8 lens or lower, correct? Is that what makes the center AF more accurate?

I only have f4 and higher lenses so would all the other AF points be just as accurate?

EcoRick
8th of June 2007 (Fri), 15:35
I use the off center points occasionally, however what is the reason that Canon increases the number of AF points with each new model?. The Mk III has 45? Is this used in Al Servo? If you're shooting stationary objects, are so many focus points necessary? I'm asking because I feel I must be missing something.

primoz
8th of June 2007 (Fri), 15:39
I don't know about mk3, since they were changing something, and I still didn't get my hands on it (colleague got it 2 days ago, and in next few hours it was back in store due constant error 99), but mk1, mk2 and mk2n have 45 af points.
Are 45 af points necessary for shooting stationary (or any other) objects? No, but they do make life easier.

CyberDyneSystems
8th of June 2007 (Fri), 15:46
All those extra AF points aid in focus tracking of moving subjects. Even if you select only a single center AF point, in most models with AIservo, the other points can and will be used to aid overall focus tracking. (how they get used differs slightly depending on user settings)

The 5D has the best example of this where they don't even tell you the extra points exist (at least not in the viewfinder) there are 6 "invisible to the user" AF points that help with overall AF, but can not be selected for a focus position.

The 1D series has essentially had this all the way back to the EOS 3.
But they just did not make the points "invisible" instead you have the option of doing so.

Most all long time 1 series owners will have set a CF to limit the number of AF points that are "user selectable" down from the overwhelming 45 af points, to a more reasonable 17 or 11 AF points (if i recall the #s correctly)
this only means that when selecting AF points for off center shooting that only a percentage of them are available to use for that purpose, making it quicker to grab the correct area of the viewfinder to set an AF point,
It does NOT mean that the other non selectable AF points are "turned off" they still are there and used by the camera to aid in overall focusing, especially in focus tracking.

It functionally turns the other 28-34 AF points into the same sort of "invisible" points that the 5D has six of.

Longwatcher
8th of June 2007 (Fri), 17:08
As mentioned having 45 points means o\you have a better chance of having one where you need it, as long as where you need it is not out beyond the area of coverage (which when doing full body shots usually it is).
45 AF points are good, but I them to go out farther then they do now. At least to the rule of thirds point.

JeffreyG
8th of June 2007 (Fri), 20:31
I just have the XT and 7 points, but I rarely use the center.

If I'm holding the camera in portrait mode, I'm typically using the focus point on the centerline one or two spots above the center (on the subject's face).

In landscape mode I usually use the focus points on either side of the center. It's rare that I want the subject dead center.

If I have my 10-22 lens mounted then I tend to use all of the points at random. With a UWA lens I place a subject someplace in the view with an overall interesting background.

jayspec
8th of June 2007 (Fri), 22:26
Because Focus Lock Recompose has certain drawbacks (http://visual-vacations.com/Photography/focus-recompose_sucks.htm), and because it's nice to be able to use AiServo, and actually focus on the subject ;)
Oh my God! I've been doing that all this time and wondering why my focus has been off! Thank you so much for that link! POTN saves a another life...

Mark_Cohran
9th of June 2007 (Sat), 00:22
I use "off center" points all the time. :)

Same here, and the more the better as there is a greater chance one will be where I need it to be.

My off-center focus points give me just as sharp a focus as the center one.

Mark

Glenn NK
9th of June 2007 (Sat), 00:45
Oh my God! I've been doing that all this time and wondering why my focus has been off! Thank you so much for that link! POTN saves a another life...

I've read this article (it was posted a couple of months or so ago), and I agree with the concepts.

However I do Focus/Recompose, but I keep in mind the distance of my focus point from the camera, and adjust accordingly. If the difference in distances is significant, I will manually focus. While this is anathema to many, for someone that just got a camera with AF last year, manual focus isn't all that hard for me.

Wazza
9th of June 2007 (Sat), 01:27
I find my off centre points on 20D, aren't wide enough, and nor are they accurate. Still better on centre, and recompose. Sometimes I know I need to compensate, and will adjust by that 5cm to achieve the perfect focus

c71clark
9th of June 2007 (Sat), 11:47
Great link, thank you. I've had fuzzy problems too, and I almost always recomposed my framing after focus. I will make an adjustment and see what happens.

cosworth
9th of June 2007 (Sat), 11:48
45 points comes in handy when ETTL flash metering too.

EcoRick
9th of June 2007 (Sat), 12:54
Thanks for all the replies! I guess I'll start using some of the other focusing points more frequently. I'm looking forward to comparing the results.

pagnamenta
9th of June 2007 (Sat), 15:28
So I take it that the center AF point is still the most accurate? How do the points compare when at f4?