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07-30-2009, 09:44 PM   #1
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What point does camera actually focus on?

I was wondering if any of you knew... Say, when I choose center point as focus point, then focus - I played around a bit, and found out it's definitely not the place where red box flashes, and it's definitely not center of the frame. It almost looks as if the whole center circle is used for focusing... and when you point the camera away from current subject so it's no more in that circle, only then camera refocuses (AF-C used). Is that right?

Thanks,
Bo.

07-30-2009, 09:53 PM   #2
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This should have been in 'DSLR Discussion'.
07-30-2009, 10:13 PM   #3
pbo
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Huh? I thought I put it there... Weird....
07-30-2009, 11:19 PM   #4
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QuoteOriginally posted by pbo Quote
Huh? I thought I put it there... Weird....
Yeah,this place can be a bit like the twilight zone sometimes,it's always best to double check.

07-31-2009, 02:17 AM   #5
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yes, the AF points are larger than the red spots. The center being kind of the same size as the center circle. Someone once posted an image of the actual sizes of the AF points here. Make a forum search for it...
07-31-2009, 08:15 AM   #6
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I reckon the focus area is smaller than the centre circle...have taken pics with very little space between foreground objects and the camera still picked up the item I wanted to focus on. I can put the red dot on a branch some distance away with foliage behind and it will immediately focus on that branch. Try it.....
07-31-2009, 09:13 AM   #7
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It's easy enough to test - put a small black dot on a piece of white paper and see where you need to point the camera to get it to focus. On my K200D, I find it slightly larger than the circle in the viewfinder - definitely *much* larger than the red square. Meaning the camera has the right to focus on anything it chooses withint that area. If there are multiple obejcts t different distances within that area, sometimes it will happen to guess right, sometimes it won't.
07-31-2009, 09:43 AM   #8
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in case there are multiple objects within that focus "circle"
what algorithm does the camera use to determine which object to focus on?

is it always the closest object?

07-31-2009, 10:10 AM   #9
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QuoteOriginally posted by opiedog Quote
in case there are multiple objects within that focus "circle"
what algorithm does the camera use to determine which object to focus on?

is it always the closest object?
first you would need to map out the focus sensitive area, using the black dot on a white sheet test suggested earlier, then set up a specific test. My gut feel is it prioritizes on closest, only because when I take shots of distant animals on the ground or in grass, the lens always close focuses, yet when there is no competition (for focus) the lens focuses correctly.
07-31-2009, 11:52 AM   #10
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QuoteOriginally posted by opiedog Quote
in case there are multiple objects within that focus "circle"
what algorithm does the camera use to determine which object to focus on?
Ancient Chinese secret.

Seriously, there's no way to know for sure. It seems to me it "often" goes for the closest, unless it's moving and the background isn't, or something in the background has sharper edges or higher contrast, or the background is already closer to being in focus, or if it's the second Tuesday of the week and the moon is full. Only thing to do is hit the focus button and see for yourself in the viewfinder what the camera did, and try again - or override manually - if it isn't what you wanted.
07-31-2009, 01:09 PM   #11
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Tried this with the K-m and it always seems to focus on the thing with most contrast (that usually means any object with sharp/hard/well defined edge). If there are multiple well defined objects, it usually goes to the closest one.

Closest =/= nearest!

For example, if the last focus point was 15m away, the camera moved and saw another two objects, one at 3m and other at 10m, it would lock on the one that is 10m away, because the distance from 15m to 10m is shorter than the distance from 15m to 3m.

Note that it is very uncommon to have multiple objects in the focus circle, unless you're shooting very small or narrow objects (usually tree branches without leaves).
07-31-2009, 05:03 PM   #12
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Thanks everybody, this is helpful. The other day i was focusing on 3 baby seagulls on a concrete beam. The baby chicks were all out of focus when i got home, but the edge of the beam was perfect. I just love autofocus - not the parent seagull was in good focus, so guess 1 out of 4 isn't a bad accomplishment for AF
07-31-2009, 09:19 PM   #13
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AF sensors definitely not like a laser sighting system

QuoteOriginally posted by philbaum Quote
Thanks everybody, this is helpful. The other day i was focusing on 3 baby seagulls on a concrete beam. The baby chicks were all out of focus when i got home, but the edge of the beam was perfect. I just love autofocus - not the parent seagull was in good focus, so guess 1 out of 4 isn't a bad accomplishment for AF
Phil .. your example here demonstrates the downside of AF systems. The AF sensors simply cannot be used like an AimPoint sighting system or a laser pointer. Pentax had the good foresight to offer a work-around .. Quick Shift Focusing.

I use Quick Shift Focusing all the time; I let the camera's AF system do its' thing to get my focus fairly close and then I tweak it manually to where I say sharpest focus should be. This capability with Pentax DA series lenses is a significant feature if your photographic style often uses selective focus and shallow depth of field.
08-01-2009, 12:08 AM   #14
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Ironically I have a split-screen prism, which makes the center part always in focus. So, I tried to rely in some complicated situations on camera's focus confirmation - well, that didn't go that well. Thus the question.
I was also wondering what "cross type" focus point means? I was kinda thinking (just intuition and such) that it'd mean the single point where "cross" crosses would be in fact the point camera focuses on and then relied on the red square as a guide to that point's position... But then yesterday (or was it day before yesterday?) I found out that was wrong.
08-01-2009, 03:04 AM - 1 Like   #15
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This is how wide each AF sensors cover according to my own test, the actual patterns are unknown, but gives you an idea how much they cover in practice.


Notice how it appears off centre. Quite likely most Pentax DSLRs are, but the actual placement is different for every sample. The AF sensor of my DS shifts upward, but the K-m downward. What I have found is that the actual spot meter placement is based on the AF sensor as well, not the focus screen. That means if the AF sensor of your camera shifts upward, so will the spot meter. This might not be obvious until you tried to pin point the exposure using the centre bracket and cannot make sense of the reading (because the spot meter is reading something else). This is where and how much my DS spot meter covered.



The good news is the spot meter could be adjusted to match the centre bracket of the focus screen, but the AF sensor placement is pretty much a wild guess and not much can be done really if you try to match it with the focus screen. This has much to do with how the Pentax DSLRs were designed structurely. There is simply no mechanism for the AF sensor and the focus screen matching each other. I don't know about the K-7, but previous models have rather dumb AF which is a contrast magnet. If you point the camera to a person standing in front of a high contrast background, quite likely the camera will focus on the background w/o taking the distance into consideration. I once tried the Nikon D200 which has different options in the menus guiding how the AF behaved in such sitiation, but I am not aware Pentax has the same feature (nor is my 40D).
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