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Old Jan 29, 2008, 8:28 AM   #1
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I'm confused about the number of autofocus points in different cameras. When I bought my Oly E-510, one of the criticisms was that it had only three autofocus points - compared to 9 on the XTi and 11 on the D80.

My confusion concerns all those autofocus points being "averaged" to focus the lens. It seems to me the only time one would want an "average" focus would be at the most stopped down apetures where one was getting maximum DOF. And then, in that case, why would you need a bunch of points to average since the DOF was so great? At wide open apetures I would think one wouldn't want "averaged" focus - that would be counter to the intent of narrow DOF.

Sooo... could someone explain why many points are better than just one? - in laymans terms, please. Or, if you agree with me, misery loves company.

Thanks, Dennis
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Old Jan 29, 2008, 9:42 AM   #2
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The people who need 9, 11, 45 focus points are the guys shooting the images you and I see in Sports Illustrated and need superior focus tracking. For me, whether the camera Iam using has 1, 3 or9 (the most I've ever had)focus points, I only shoot with ONE activated...the center one. If I was using a camera with 45 focus points, I'd be using ONE.... the center one.
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Old Jan 29, 2008, 11:41 AM   #3
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I disagree with Gregg's assessment.

The value of multiplepoints isn't for tracking - it's for composition. The value is realized inseveral types of situations:

1. you don't have time to focus/recompose

2. DOF is very shallow and you incur focus issues with focus/recompose

I'm sure there are others.

But a a sports shooter (although not a SI level shooter) I can say most sports shooters are NOT using all 45 or 51 points. But having multiple points to choose from helps with both points above. Additionally if you're using center point, the framing is poor - for sports shooting you want to focus on a face not the middle of the body so if using center point your image is framed wrong - and if you frame loosely so you can adjust the crop in PP then your image isn't as sharp.

Additionally, l'll add that focus points work on contrast. Let's say you're using only a single focus point - if that focs point isn't over an area of contrast you'll have focus issues. So, if I enable all 51 points for just a standard shot and I get 8 points on my subject the camera has a better chance of accurate focus.

There are all sorts of photographers that choose a non-center focus point - it's use isn't limited to sports shooters. Let's say you're using a tripod - you don't want to focus / recompose it's much easier to select a focus point that is over your subject - having more pointsg gives you more options in that framing.

Also, since Gregg brought up sports shooting - it isn't just the number of focus points but their ability that matters to a sports shooter. Whether a focus point is cross-type (detects contrast differences along both axis), whether or not a focus point is high-precision (higher degreee of accuracy with wide aperture lenses) makes a huge difference. Not all focus points are created equal.

In the end, for many types of simple photography you won't encounter errors using the focus/recompose as long as you keep in mind it's risks and limiatations.
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Old Jan 29, 2008, 12:45 PM   #4
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First of all I agree 100% on the focus-recompose issue. On my 50 f1.2 the DOF is so shallow that there is no room for error and centre-point focus-recompose is a guaranteed backfocus at most distances and f1.2. Using a side point is much better when that's where you want the focus to be.

Even if you only generally use the centre point, it can be very handy to have a bunch of AF assist points nearby.

On my 5D even when I am using the centre-point I find AI-focus (with the 6AF assist points enabled) helps because if the subject moves a little bit the camera will attempt to track it and adjust. Once again with wide apertures something as innocuous as the subject swaying backwards or forwards an inch or two can throw off the focus. Having AF assist on helps get more keepers.

And the real point is that when you have 45 or 50 AF points then a lot of them are used for AF assist, not AF selection.
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Old Jan 29, 2008, 2:42 PM   #5
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erutcip wrote:
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My confusion concerns all those autofocus points being "averaged" to focus the lens.
The autofocus points aren't averaged. Only one autofocus point is ever used, and it's the one with the highest contrast. In most modes of operation, the camera picks the focus point it will use, but most cameras allow you to select a focus point that you want it to use, and it will use that point to determine the correct focus distance, and ignore the others.

More points means there is a greater chance that the camera will focus on a subject that may not be centered. This is a lot faster than the 'focus/recompose' operation, but it also reduces the possibility that the camera focuses on something other than your subject. And since most cameras will not take a photo until the camera has focused on something, having more focus points means there is a greater likelihood that it will find something with enough contrast to focus on, and therefore, allow you to take the shot. This can be especially important with low light photography as well as fast moving subjects.
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Old Jan 29, 2008, 3:44 PM   #6
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JohnG wrote:
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I disagree with Gregg's assessment.

The value of multiplepoints isn't for tracking - it's for composition. The value is realized inseveral types of situations:

1. you don't have time to focus/recompose

2. DOF is very shallow and you incur focus issues with focus/recompose

I'm sure there are others.

But a a sports shooter (although not a SI level shooter) I can say most sports shooters are NOT using all 45 or 51 points. But having multiple points to choose from helps with both points above. Additionally if you're using center point, the framing is poor - for sports shooting you want to focus on a face not the middle of the body so if using center point your image is framed wrong - and if you frame loosely so you can adjust the crop in PP then your image isn't as sharp.

Additionally, l'll add that focus points work on contrast. Let's say you're using only a single focus point - if that focs point isn't over an area of contrast you'll have focus issues. So, if I enable all 51 points for just a standard shot and I get 8 points on my subject the camera has a better chance of accurate focus.

There are all sorts of photographers that choose a non-center focus point - it's use isn't limited to sports shooters. Let's say you're using a tripod - you don't want to focus / recompose it's much easier to select a focus point that is over your subject - having more pointsg gives you more options in that framing.

Also, since Gregg brought up sports shooting - it isn't just the number of focus points but their ability that matters to a sports shooter. Whether a focus point is cross-type (detects contrast differences along both axis), whether or not a focus point is high-precision (higher degreee of accuracy with wide aperture lenses) makes a huge difference. Not all focus points are created equal.

In the end, for many types of simple photography you won't encounter errors using the focus/recompose as long as you keep in mind it's risks and limiatations.
Wow...it's wonder anything shot with a 400mm f2.8 prior to, say, 2001 was ever in focus....
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Old Jan 29, 2008, 4:12 PM   #7
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Hi
My d40x has 3 points autofocus and very often camera made focus where I do not want, so one focus point and recompose is faster and easier way for me. I have played with multypoint focus camera and same problem. When use multypoint you can not recompose, so you have to refocus again and again. I am very happy with one point focus and I would like to find some bether way to use multypoint.
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Old Jan 29, 2008, 4:13 PM   #8
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...very often manual focus is the fastest!
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Old Jan 29, 2008, 4:18 PM   #9
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emes wrote:
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...very often manual focus is the fastest!
Unfortunately,focusing screens don't have the focusing tools (microprisms, split-images, etc.) that were popular in the days of manual focus.

And acurate manual focus on an EVF is near impossible.

So, while manual focus may be faster, it's a lot less accurate.
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Old Jan 29, 2008, 6:05 PM   #10
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I know what you mean about having the camera focus on the wrong thing, when you let the camera decide what to focus point to use. I tend to use center focus/recompose because that's what I'm used to and it works quite well for me. However, my camera also allows me to select which point I want to use, and I'm now playing with it more, but I'm not convinced its any better or worse(it takes a couple of seconds to push the button tocycle through the focus points to get the one you want).It really does depend on the particular situation.

I rarely find manual focus faster - though it is sometimes more accurate (again, depends on the situation). I take lots of macros so have developed a reasonable eye for "seeing" the focus even without getting a katz-eye or other third party split focusing screen, and if I'm not sure I listen for the camera to beep it's focus indicator. It's easier to see the focus if the lens is sharp to begin with.

My conclusion was that one should try various techniques and decide for themselves what works for them.
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