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08-06-2012, 04:07 AM   #1
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focus point/s thoughts

Following comments on the thread Military wedding with the K5 -- the good and the bad post#10 linked image.

I never change from the single point as I assume what is important is in the middle of my frame so why would I want the camera to look and try to get every thing right via 5 or 11 points?

If I am concerned about the camera making a mistake I will often focus on something larger say a persons chest and lock that then recompose rather than try to focus on their face especially if I am far enough away to suspect the single spot might get confused with a the face and say a tree behind as in the example image from FrancisK7.

But what I am now wondering is in that circumstance where there is an other object behind the subject that the single point can also recognise would using the 5 point help to stop the tree rather than the face being the focus point?

Now of course we don't know how far away the camera was from the bride, how far the tree is behind her and what aperture /dof is being used but lets assume for arguments sake that it was f2.8 to deliberately give a shallow dof.

in that case do you think that using the 5 point have stopped the camera focusing on the tree rather than the couple? no probably not but;

Since I would guesstimate that in the viewfinder the 5 points would have covered the couple and the three large trees at least the red indicator might have warned the photographer that he would have a problem.

I have been trying in my garden to replicate the problem with that image but of course I don't have a couple to hand so I used a garbage bin with a large plate standing in the top as a face but no matter what I do every time it is the centre single point that lights up. I put some dark cloth over the plate to cut the contrast and only once did one of the other indicators light up, which at f2.8, would have definitely make the "face" out of focus and the tree behind in focus.

so back to my original question before I rambled on,

What I don't understand is why the auto with a multi point system is even there ?
under what circumstances would it give you a better, or if you like more correct , focus point than the centre focus only option?

Has anyone had spent the time and effort try to work out a plan/ program [call it what you will] as to when to use the 5 point versus the single point focus system ?

08-06-2012, 04:12 AM   #2
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Use centre point only, and ensure an entire "+" drawn in your mind inside the "( )" symbols only cross lines that are the same distance from the camera.

that, or buy a S-type screen so you can see and correct AF misses in the OVF
08-06-2012, 05:31 AM   #3
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This is a great question. I'm going to try a few different things when I'm out shooting this weekend. I've been very fustrated with focus lately, but figured I was doing something wrong.. DOF, reading the confirmation wrong etc. I usually use the centre point. When taking close ups of my dog (I focus on his eyes like everyone says) they end up being out of focus, but his shoulders are in focus...
08-06-2012, 06:33 AM   #4
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Sometimes when you do things like fast sports where you can't seem to pan correctly quick it's great to have multi-points.

08-06-2012, 06:57 AM   #5
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Focus/recompose is not always an appropriate approach. You may want to read these: Don't recompose! and Why Focus-Recompose Sucks
08-06-2012, 07:08 AM   #6
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QuoteOriginally posted by bxf Quote
Focus/recompose is not always an appropriate approach.
There is a lot of controversy about that view, of course. Those who shoot wide-open with fast lenses all the time in particular like to go negative on focus/recompose. But not everyone spends all their time shooting portraits at 2m using an 85mm at f1.4.
08-06-2012, 07:39 AM   #7
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QuoteOriginally posted by bxf Quote
Focus/recompose is not always an appropriate approach. You may want to read these: Don't recompose! and Why Focus-Recompose Sucks
Yes point taken and very interesting and informative , well worth remembering , but, since the majority of shots I have had gone wrong do not fall into the "The error introduced by focus-recompose is greater with shorter camera-to-subject distances." argument .

The majority are more likely to be in the " If you are using a telephoto lens and focus-recompose on a football coach on the other side of the field, the error introduced would only be a small fraction of an inch and would be dwarfed by many other factors such as subject movement, photographer movement," argument and I wonder if this is why the 5 point set up is there?

Could it be the camera is clever enough to look at a 5 points and hit a average even if it does only flash one up on say the edge?

Could it also be that using 5 points with the central one selected would get you over the self induced error amply demonstrated in the links?

How can one set up a test to try it out I wonder ???:confused

You see I think my orginal question still has to be answered, why do Pentax give us a choice of auto and why 5 or 11? there has to be a reason.

To be able to select either centre point or to select a non centre one makes lots of sense however.

Last edited by adwb; 08-06-2012 at 09:31 AM.
08-06-2012, 07:50 AM   #8
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QuoteOriginally posted by rawr Quote
There is a lot of controversy about that view, of course. Those who shoot wide-open with fast lenses all the time in particular like to go negative on focus/recompose. But not everyone spends all their time shooting portraits at 2m using an 85mm at f1.4.
There is no need for controversy. It's math, and any given combination of FL, aperture, distance, and angle of recompose either results in a significant focus error, or it does not. I do it myself all the time, but one should be aware of the possibilities.

08-06-2012, 07:56 AM   #9
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QuoteOriginally posted by adwb Quote
... why do Pentax give us a choice of auto and why 5 or 11? there has to be a reason.

To be able to select either centre point or to select a non centre one makes lots of sense howver.
Well, you CAN select a non-centre point, hence you could view the AUTO option as simply saving you a step in having the camera do it for you. If it chooses the right one, fine. Otherwise, you can change it manually.
08-06-2012, 08:45 AM   #10
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Speaking for myself, I have never gotten the Auto-11 or Auto-5 point focus methods working on my k-x to my satisfaction. And recently I've been having trouble shooting my 50 F1.4 with center focus and recompose.

But I work around it, since it's a k-x.
08-07-2012, 01:20 AM   #11
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QuoteOriginally posted by adwb Quote
What I don't understand is why the auto with a multi point system is even there ?
under what circumstances would it give you a better, or if you like more correct , focus point than the centre focus only option?
5 point auto can work fine when you have a fast moving object with an "empty" (or very far away, blurred/oof) background. The most obvious example is birds in flight. The clear sky cannot confuse the AF system so it can select the correct AF sensor from the 5 and you don't need to try to keep a pre-selected sensor (center or any other) on the bird. Same goes for fast moving cars or any fast moving object you are trying to track.
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