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Thread started 03 Jul 2018 (Tuesday) 17:34
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Canon AF zone focus algorithm issue

 
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Jul 10, 2018 01:13 as a reply to  @ post 18659660 |  #16

I don't have quite the same experiences, through my many years of shooting pro basketball. I have used the 7d, 1d3, 1d4, 5d3 and now the 5d4/7d2. I currently use off center AF points and lock on a players face through a drive for a dunk, a fast break and pull up for a 3 pointer, etc. It took time with each to find the correct settings, but invariably it ended up being my ability to keep the AF point tracked on the subject that seemed to be the key. I do get periodic locks on undesirable areas, and get a shot where the camera seemed to lose its mind, but those are few and far between. The 5d4 AF system has performed very well for me.

We have a basketball thread here somewhere where people share their settings and images, there are many good suggestions on how to work around the AF complications we encounter.

Ah found it: https://photography-on-the.net …/showthread.php​?t=1457450


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Jul 10, 2018 11:43 |  #17

That is the best method. Get your desired AF point point locked on and track yourself. One day AI will take over but until then.

I posted this before. I wanted to see how well the expansion points worked. First I tested One Shot on a white wall and if the system was not happy with the centre points contrast an outer point would light up. However right after an outer point is called upon the centre point lit up right after and you can see both later in DPP. This was consistent. Canon says in all the stuff I read the AI Servo works the same way as One Shot.

I read a lot of threads where and expansion point focused on an undesired area like the crowd behind a soccer player. The centre point and 3 other expansion points were on his black shorts for example. So the system did get a little confused as there was no contrast at all on the centre and 3 expansion points. If one point had been on his checkered shirt he would have been in focus.

So to compare apples to apples I did my tests with two equal high contrast targets, one in front of the other.

1st image = defocused.
2nd image = centre and 4 expansion points on the front target.


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Jul 10, 2018 11:46 |  #18

3rd image = I then moved the Moved center point onto back target and kept right assist point on the front. I could hold it there all day and it would maintain focus on the front target.

4th image – Moved all points off front target. System refocused on the back target. TS -2 had a delay. TS +2 was instant. You could clearly see the difference between TS-2 and TS+2.


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Jul 10, 2018 11:47 |  #19

Here was the interesting one. I defocused again placed the centre point on the back target while right assist point was on the front target. System focused using the centre point.


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Jul 10, 2018 12:02 |  #20

At least these examples are according to how Canon states it works.
If, in the last example, you had used zone AF instead of single point with assist points, according to Canon, it should have focused on the nearest target instead.


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Jul 10, 2018 12:21 |  #21

apersson850 wrote in post #18659922 (external link)
At least these examples are according to how Canon states it works.
If, in the last example, you had used zone AF instead of single point with assist points, according to Canon, it should have focused on the nearest target instead.

In my experience if the target is closer than other objects it is pretty good at it. When I was out practising on seagulls in flight and using full Auto it did a great job. Even with trees, buildings, etc in the background it always focused on and tracked the bird. It is when your target is within other objects it can get confused. I have not tested it at sports where the target is clearly closer than other players but I can see that it can drop it if of within the group.

I had more videos at one time. Example at minute 9:15. Like I said with BIF without iTR it will isolate the bird if it s clearly in alone and in front.

https://www.youtube.co​m/watch?v=PYhBjb-01KQ (external link)


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Jul 10, 2018 14:28 |  #22

digital paradise wrote in post #18659909 (external link)
3rd image = I then moved the Moved center point onto back target and kept right assist point on the front. I could hold it there all day and it would maintain focus on the front target.

4th image – Moved all points off front target. System refocused on the back target. TS -2 had a delay. TS +2 was instant. You could clearly see the difference between TS-2 and TS+2.

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Thanks for sharing those! In my (admittedly cynical) opinion, because the AF points are significantly larger than the squares, image 3 still may have part of that center point on the near target. I've seen this happen enough that I feel there's a lot of "wiggle room", especially when you're moving the camera after locking on a target. In image 4, even the right assist point might be close enough to the near target but they don't seem to hold very long after the center point has moved away.

May I ask where your tracking sensitivity was set for these shots?


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Jul 10, 2018 14:33 |  #23

TeamSpeed wrote in post #18659667 (external link)
I don't have quite the same experiences, through my many years of shooting pro basketball. I have used the 7d, 1d3, 1d4, 5d3 and now the 5d4/7d2. I currently use off center AF points and lock on a players face through a drive for a dunk, a fast break and pull up for a 3 pointer, etc. It took time with each to find the correct settings, but invariably it ended up being my ability to keep the AF point tracked on the subject that seemed to be the key. I do get periodic locks on undesirable areas, and get a shot where the camera seemed to lose its mind, but those are few and far between. The 5d4 AF system has performed very well for me.

We have a basketball thread here somewhere where people share their settings and images, there are many good suggestions on how to work around the AF complications we encounter.

Ah found it: https://photography-on-the.net …/showthread.php​?t=1457450

As always, right on the money, TS. Thanks! Yes, my settings are based on being able to hold the center point well enough to outweigh any "edge cases" and have been tweaked a lot until they work most of the time. And I don't want it to sound like I have tons of mis-focused shots - when I can keep the center point on a high contrast area, the AF is great on all my cameras. I'm just OCD enough that any OOF shots drive me up the wall, and the alternate focus point options Canon provides do not suit my use cases very well.


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Jul 10, 2018 14:42 |  #24

TeamSpeed wrote in post #18656925 (external link)
Yes spot AF shrinks sensitivity down somehow, and I use it quite a bit. However you sacrifice AF speed when using that, so there are always compromises.

Might agree with you in low light but in good light AF speed using single point spot is quite good/fast.


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Jul 10, 2018 15:09 |  #25

PNPhotography wrote in post #18660031 (external link)
Might agree with you in low light but in good light AF speed using single point spot is quite good/fast.

I think that would depend on your particular use. Canon goes as far as to recommend against using spot AF for not only moving subjects but hand holding in general. Spot AF hasn't proven particularly effective for me. Way too much hunting compared to a full spot.


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Jul 10, 2018 18:11 as a reply to  @ PNPhotography's post |  #26

Not with fast sports, even slower children sports, and there is little to be gained in that scenario anyways that regular simple point af won't capture. I have tried that with baseball, softball, etc, and spot af didn't gain me anything additional over the normal mode.


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Jul 10, 2018 19:36 |  #27

GregDunn wrote in post #18660024 (external link)
Thanks for sharing those! In my (admittedly cynical) opinion, because the AF points are significantly larger than the squares, image 3 still may have part of that center point on the near target. I've seen this happen enough that I feel there's a lot of "wiggle room", especially when you're moving the camera after locking on a target. In image 4, even the right assist point might be close enough to the near target but they don't seem to hold very long after the center point has moved away.

May I ask where your tracking sensitivity was set for these shots?

I did several times and it was consistent until the right assist point fell onto the back target. It was on a tripod and moved the camera slowly.

TS-2 which gave me about a second after the right assist point fell onto the back target before it refocused on it. I also tried TS+2 later. When the right assist point fell onto the back target it refocused onto it instantly.


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Jul 10, 2018 19:41 |  #28

PNPhotography wrote in post #18660031 (external link)
Might agree with you in low light but in good light AF speed using single point spot is quite good/fast.

I only use spot when I'm shooting through a tight opening and focus is bouncing back and forth between the obstruction and the target like a chain link fence.


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Jul 11, 2018 00:51 |  #29

Just as a reference (and most of you have probably seen it), Canon offers this image as representative of the actual area covered by a standard AF point on the 1Dx. Having looked at images of the physical AF sensor chip, I'm of the opinion that there is no "hard edge" to the sensor's light sensing area - in other words, beyond the area shown, sensitivity trails off at some unknown rate. Looking at how closely the points are spaced, it seems likely that there is overlap between the coverage of adjacent points (not necessarily the spot sensor, of course).

I just looked at an image I took tonight at practice, and this is just one example of the AF points being significantly sensitive to data well outside of the square. I placed and held the center point on one skater for at least a couple of seconds before shooting, and pretty clearly it still liked the background better. Shots before and after this one were fine. I was using TS -2 and the speed of the skater was nearly constant - toward me, and slightly to the left during this interval . Am I 100% certain that I held the point right in the center? No, it's still possible that I was too close to the edge - though I'm not sure what it saw in the background. Out of 50 photos like this, that's the only one which failed though.  :p Oh, FWIW the lighting was about EV+7, so plenty bright enough for action shots. I intentionally set ISO high enough to get 1/500 shutter speed so the focal plane would be unmistakably sharp, and tolerated the noise.

I guess I should be happy - this is far better than I would have gotten before tweaking the AF parameters and acclimatizing myself to the behavior. Even my 7D2, which focuses sometimes when you think it can't, isn't quite as good as the 1Dx2.


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Jul 11, 2018 01:46 |  #30

Several of the AF points are actually the same sensor strip on the AF sensor. You can look at the sensor strip as long enough to cover multiple AF points. Then software simply selects to look at the left, center or right part of the sensor strip (if it implements three AF points). In the 1DX, some strips implement more than three points.


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