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03-06-2009, 01:13 PM   #1
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AF Question

Ok, first off I apologize for the poor attempt at humor in my previous post (free focus test), but I now ask for your opinions and assistance regarding some auto-focusing issues I am facing.
Question: If I use AF center focus, how large relative to the "red dot" or even relative to the frame should the object of my intended focus be for reliable auto focus? Does that focus area change depending on focal length of the lens? distance from the lens? I just recently bought a Tamron 70-200 2.8, while I have had no problems in the past with other lenses ( although under different conditions), I have really noticed with this lens that my subject must be nearly 2-3 times larger than the "red dot" to focus correctly. The lens focuses quickly, just not correctly. Based on the DOF calculations, its not that its close or back focused, its clearly focused on the objects directly behind my subject, with a DOF that correctly matches the calculations. The reason I posted the "free focus test" was that at one point I questioned whether the camera or lens had failed, but when I snapped that referee shot it was obvious the problem was clearly operator error. Here are a few more examples
EX1: I could focus all night long on subjects that filled the frame to this size,
EX2: These shots were hit or miss as to correct focus, notice the subject is much smaller that in EX1. More often than not I would be focused on the cheerleaders behind the players (try and explain that to your wife)
EX3: I had a heck of a time trying to focus here, am I expecting too much in this shot, the subject itself has very little change in contrast while the surroundings are filled with vertical lines. Thanks in advance for any advice

Ex1: ()
Ex2: ()
Ex3: ()

03-06-2009, 03:22 PM   #2
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No way AF will work well with the deer pic. Too many vertical saplings in the way grabbing the AF sensors attention. I would use AF and then quick shift to nail it on my DA* 300. Or go manual entirely.

Jack
03-06-2009, 03:38 PM   #3
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The actual focus area is a lot larger than the dot - maybe twice as wide, four times as tall? So if anything else is in that area, the camera might focus on that instead. That's why in scenes of any complexity, MF is far more reliable.
03-06-2009, 03:50 PM   #4
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If you want to check the size of the area covered by the AF sensors, just draw a black line on a white sheet of paper and put that sheet on a wall. Then, set your AF on center sensor and continuous AF, then aim on a white part of the sheet and move slowly toward the black line. When the AF "locks" focus (as far as you can call lock on continuous AF) is where the sensor starts detecting the line. Then, turn your sheet 90 deg., and do the same thing. After that, you'll know the boundaries of the AF sensors. Quite straightforward, and no need to go to the university.

03-06-2009, 04:04 PM   #5
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I wish I bookmarked it, but a while back, someone posted the actual size of the cross sensors vs. the focus points. The cross sensors almost touch! They're that big...

I'd agree w/ the deer comment as well...way too much stuff to focus on and the camera will lock on the branches. That's why wildlife photogs use manual focus to get it to lock right...
p.s., just tell your wife those cheerleaders aren't good enough to take photos of compared to her But really, they're not very cute... :P
03-06-2009, 04:08 PM   #6
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as others wrote, the AF areas are much bigger then the dotts in the VF so as you found out, it behaves just as you described in your OP. It does matter on the size and cotrast of the target...

BR
PS: good luck explaining the cheerleeders...
03-06-2009, 07:35 PM   #7
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BTW, although I mentioned MF, it is not the only solution, of course. In #2, had you first locked focus at the players' *feet* then recomposed (or selected a focus point near the feet) that would have been likely to have worked.

Last edited by Marc Sabatella; 03-07-2009 at 02:22 PM.
03-07-2009, 02:14 AM   #8
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Does the sittuation worsens only by relative subject size or by distance. I had a similar problem with my K100ds/Sigma 30/1.4 but it was more distance than size related. For example at distance 5m (~16 feet) or more my camera would focus on background object even if the foreground one was solid wall covering all the focus points.
https://www.pentaxforums.com/forums/pentax-dslr-discussion/45604-k100ds-focus...naccuracy.html
Few of my samples:
Inbox Foto (foto.inbox.lv)
http://content8-foto.inbox.lv/albums87168631/tamrac/Sigma2/IMGP7547.jpg
Now my camera is sent to service but i dont know if they will fix it.

What can show if its a problem, is that if you try 10 shots in a situation where it should be easy for camera to focus, but it tends to miss. If most of the shots (>7..8) are bad that might be a problem.
How the camera act if there is no bacground to focus on? Does it lock correctly, randomly or goes to infinity.
When shooting object with large depth but shallow DOF (wall/floor at 45 deg. with some pattern for example), wich part of the object is sharp. The one at focus points center or somewhere else. Try at different angles and distances.
Go to the shop and try with different camera/lens (similar type like DA 50-135).


BTW, in the second picture, why one of the players has dark blue lips?

03-07-2009, 02:18 AM   #9
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The sensor area is 3 times the size of the markings.
03-07-2009, 11:46 AM   #10
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QuoteOriginally posted by ytterbium Quote

BTW, in the second picture, why one of the players has dark blue lips?

Looks like he is "chewing" on his mouth guard.
03-08-2009, 07:32 PM   #11
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Thanks for all the input, I guess I didnt realize how many situations call for MF. I should order my split-prism this week.


QuoteOriginally posted by flyer Quote
If you want to check the size of the area covered by the AF sensors, just draw a black line on a white sheet of paper and put that sheet on a wall. Then, set your AF on center sensor and continuous AF, then aim on a white part of the sheet and move slowly toward the black line. When the AF "locks" focus (as far as you can call lock on continuous AF) is where the sensor starts detecting the line. Then, turn your sheet 90 deg., and do the same thing. After that, you'll know the boundaries of the AF sensors. Quite straightforward, and no need to go to the university.
This sounds like a good idea, I'll give it a try sometime. Before I bought the 70-200 2.8, I took all of my indoor Bbal with a 50mm 1.4, and I don't remember having any problems. I think I'll do this test comparing the two lenses. Thanks

QuoteOriginally posted by kenyee Quote
I wish I bookmarked it, but a while back, someone posted the actual size of the cross sensors vs. the focus points. The cross sensors almost touch! They're that big...

p.s., just tell your wife those cheerleaders aren't good enough to take photos of compared to her
But really, they're not very cute... :P
I did a quick search before posting, I'll have to dig a little deeper

Oh, and she'll only accept that answer if its accompanied with dinner and flowers

not that cute? ouch



QuoteOriginally posted by Marc Sabatella Quote
BTW, although I mentioned MF, it is not the only solution, of course. In #2, had you first locked focus at the players' *feet* then recomposed (or selected a focus point near the feet) that would have been likely to have worked.
I see your point, I'll have to give it a try. One of the more important shots I missed that night (senior night) was a mid-court introduction of a graduating cheerleader and her Mom and Dad. From my angle, what I saw behind them from the mid -section up was very busy, lots of stuff to confuse the camera, from their mid-section down (towards their feet) was simply the court, I think the camera AF would have done fine with that advice, thanks

QuoteOriginally posted by ytterbium Quote
BTW, in the second picture, why one of the players has dark blue lips?
Mouth guard, one of the few that night to wear on, dad must be a dentist.

QuoteOriginally posted by stgmgr Quote
Looks like he is "chewing" on his mouth guard.
yup.
03-09-2009, 01:45 PM   #12
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QuoteOriginally posted by MaKettle Quote
This sounds like a good idea, I'll give it a try sometime. Before I bought the 70-200 2.8, I took all of my indoor Bbal with a 50mm 1.4, and I don't remember having any problems.
If you're using the same aperture on both lenses, the 70-200 will have shallower DOF, which would be more likely to expose any focus errors. Even if you were taking advantage of the wider apertures available on the 50, and thus making DOF smaller again, the fact that it's taking in a wider angle changes the composition enough that relatively speaking, more objects in the scene might be closer to the same distance away, making it less crucial for the AF system to pick the exact target you intended. Hmm, not sure that makes sense when I put it that way, so let me turn it around: the more you zoom in, the more likely you are to encounter situations with a single foreground object and a lot of background, and the chances that the camera will choose the background goes up accordingly. With a wider view, there's more to choose from that's more or less at the same distance as your subject. Not that this would always work in your favor, but I can how it might at times.
03-09-2009, 02:15 PM   #13
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QuoteOriginally posted by Marc Sabatella Quote
If you're using the same aperture on both lenses, the 70-200 will have shallower DOF, which would be more likely to expose any focus errors. Even if you were taking advantage of the wider apertures available on the 50, and thus making DOF smaller again, the fact that it's taking in a wider angle changes the composition enough that relatively speaking, more objects in the scene might be closer to the same distance away, making it less crucial for the AF system to pick the exact target you intended. Hmm, not sure that makes sense when I put it that way, so let me turn it around: the more you zoom in, the more likely you are to encounter situations with a single foreground object and a lot of background, and the chances that the camera will choose the background goes up accordingly. With a wider view, there's more to choose from that's more or less at the same distance as your subject. Not that this would always work in your favor, but I can how it might at times.
I generally shot the 50mm above 2, maybe not as high as 2.8, but I assumed close enough to disregard the difference in DOF.

Before I read your post my thoughts on the wider field of view with the 50 vs the 70-200 were completly opposite of yours, but suddenly it hit me, your right. I assumed that as I zoomed in I would be "increasing" the amount of area in my desired focal plane that was landing on the "dot" or focus point. Actually because of the size of the "dot" as I zoom in, those extra areas in my focal plane ( the other players) are moving outside the perimeter of the "dot". What is increasing in area is the contrasty stuff in the background. Does that make sense?
I think the next time I go I'll try shooting at different focal lengths just to see the effects.

Thanks Marc, I'm feeling alot better about the money I spent on this lens
03-09-2009, 10:19 PM   #14
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QuoteOriginally posted by MaKettle Quote
Before I read your post my thoughts on the wider field of view with the 50 vs the 70-200 were completly opposite of yours, but suddenly it hit me, your right.
Well, to be honest, I'm sure it could go either way, depending on the scene. AF is kind of a crap shoot no matter how you slice it.
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