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03-02-2012, 02:27 PM   #1
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All the fuss about AF point size

Everybody sais that the AF points are so much larger than indicated. In a thread (that I read recently but cannot find at the moment, would appreciate if somebody points me there) it was found that the center AF point was larger than the one in the VF. That it was about as large as the half-circles in the VF. Hello? NOT INDICATED?


I think it can be seen quite clearly that the AF points are very well indicated. Just overlay the ( ) over any point and you can see it corresponds to the square bracket pattern on the VF.

//rant mode off

03-02-2012, 02:46 PM   #2
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Not quite sure what you mean.

Do you mean that if you take the two half circles in the centre and make them a complete circle then that circle is what the camera actually sees to focus on? And if you then overlay that circle over any other AF point then that is what the camera sees to focus on?
03-02-2012, 02:48 PM   #3
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Well if they put a few more on them they have to get smaller, wich would be great.
03-02-2012, 03:06 PM - 1 Like   #4
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In my experience, the center point is considerably larger than the peripheral ones. About the size of that circle, if you completed the brackets to form one, maybe a bit smaller than that. I've had much better luck using it since I started consciously noticing the behaviour of it. Sometimes it's necessary to place just the edge of that circle on the point you want, and then recompose a little if you're taking a centered shot.

The peripheral cross type points are smaller, but not quite as sensitive as a result. The two regular (non cross type) points I avoid using.

I think the rant is a bit misplaced. People see the red square light up and expect that to indicate the size of it. They interpret the brackets to be a framing aid, not a focus point indicator. It's a reasonable assumption that Pentax does not correct in their documentation.

03-02-2012, 03:11 PM   #5
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What we really need then is a focusing screen that has the actual sensor limits scribed on it, although that would tend to get a bit confusing with all 11 of them.
03-02-2012, 03:12 PM   #6
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What it is, is that most cameras, the focus point is the red square itself. in the K5 the autofocus square does indeed stretch to the idicated semicircles !
This simply meens the the autofocus point is indeed quite large. Anyone beleiving the focus point is just the red square will get an awful lot of focus errors !
03-02-2012, 03:21 PM   #7
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QuoteOriginally posted by westmill Quote
Anyone beleiving the focus point is just the red square will get an awful lot of focus errors !
I'm not admitting anything to anyone, I'm just going to say that this thread has been quite informative about the subject of auto focussing and what those points actually mean.
03-02-2012, 03:24 PM   #8
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QuoteOriginally posted by mtansley Quote
I'm not admitting anything to anyone, I'm just going to say that this thread has been quite informative about the subject of auto focussing and what those points actually mean.
LOL thank you !

03-02-2012, 03:55 PM   #9
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If you wish to test it, its very quick and easy. Place a coat hanger on a peice of white paper. use the tip of the metal coathanger for your focus point.
The lens will hunt and not focus until it hits just inside the circle.
In comparison, when I tested the D3s and D300 focus zaps on the little squares.
03-02-2012, 03:58 PM   #10
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I'll give that a try, good idea.

After all, instead of complaining about the limitations of his equipment, a pro will determine the limits and work with them or around them. (I'm not a pro but at least I can be an informed amateur!!)
03-02-2012, 05:02 PM   #11
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Im a pro of 30 years, but i can assure you, there is always something to learn.
Its my first pentax so im still very much learning. And this is something Ive only just learnt myself.
I had heard quite a few people say that the sensors were larger, but it was a real shock to discover just how large.
it was another guy on here that gave me the idea for the test. I carried out this test with my D3s and D300 first so I had a genuine comparison.
The Nikons focussed exactly as you would expect. Very precisely on the square ! This allows for very accurate focus.
The k5 was a shock for sure. It realy is genuinly huge in comparison.
It explains so many oomph shots with this camera that made no sense watsoever. Ive always foucused on the eyes. the eye is very close to the side of your head.
A sensor that size can easily and will pick up on anything in the background that has higher contrast as there will often be quite an overlap.
Up close improves things a lot since its bigger in the veiwfinder. i have now resorted to focusing on the face rather than trying to focus on the eye unless im close.
If you need critical focus, I would recomend manual focus.
Somthing Ive not tried and tested.... but.... I would expect there could be one advantage of these huge sensors. Thats focus tracking using all focus points !
In this secenario your camera is not so steady and little time to be precise on a fast moving object. I beleive this is an area that the K5 should excel.
03-02-2012, 05:17 PM   #12
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Good to hear you confirmed this, westmill. I think I was the one that mentioned that to you. It's a *huge* difference if you've come from another system, and I'm fairly certain it's a big source for the "missed focus" complaints. If you're used to locking a focus point on an eye, you'd be pretty annoyed at the Pentax AF system :-)

Hopefully, they've addressed this w/ the K-01 since it has a lot more AF points, but I haven't seen anyone confirm yet...
03-02-2012, 05:26 PM   #13
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Something positive to say about AF on the K-5 westmill? I am stupefied!

If you are not careful you will lose your status as the forum's K-5 curmudgeon!

And yes, this is very helpful knowledge about a subtle point of how this camera functions.
03-02-2012, 05:37 PM   #14
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QuoteOriginally posted by kenyee Quote
Good to hear you confirmed this, westmill. I think I was the one that mentioned that to you. It's a *huge* difference if you've come from another system, and I'm fairly certain it's a big source for the "missed focus" complaints. If you're used to locking a focus point on an eye, you'd be pretty annoyed at the Pentax AF system :-)

Hopefully, they've addressed this w/ the K-01 since it has a lot more AF points, but I haven't seen anyone confirm yet...
Yes ! i cant tell you how grateful i am to you too ! I did a shoot today and took 512 model pics without a single missfocus !
Its an eye opener for sure if people actually start to realise the implcations.
All these front and rear focussing problems ! I wonder..... rear focus.... on the ear maybe ? front focus.... nose maybe !
Its taken me two months to crack it. But judging from today, I do beleive its cracked.
I thought too that my lens was front and back focussing inconsistantly. That doesnt make sense though.
Ive no doubt in my mind that this is the main cause of the vast majority of complaints.
It explains almost everything.... it even explains why some people have zero problems too, since its down to subject matter.
03-02-2012, 11:17 PM   #15
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QuoteOriginally posted by mtansley Quote
Not quite sure what you mean.

Do you mean that if you take the two half circles in the centre and make them a complete circle then that circle is what the camera actually sees to focus on? And if you then overlay that circle over any other AF point then that is what the camera sees to focus on?
Bingo.
Someone should make a sticky post about this.
I agree with mtansley, just highlight the selected AF points with squares or something.
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