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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 26 Feb 2015 (Thursday) 19:14
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To Focus or Not To Focus

 
MikeWa
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Feb 27, 2015 18:31 |  #31

Thanks LJ3Jim you seem to have nailed my question pretty well.

I think I am ready to put forth a supposition as to the cause.

As this sometimes happens when shooting a BIF I have always put it off as the bird moved between the time focus locked and the picture was taken. On still objects I just assumed focus locked onto something else. This was the first time I ran into the issue in a closed environment.

My actual knowledge of the inner working of focus sensors is quite limited. So my logic can be flawed but here goes. Looking at my OOF picture, I was photographing near the TV and DVD player, both of which have infra red transmitter receivers. I am thinking somehow the camera sensor picked up some stray light interference. Light not in our normal visible rang but seen by the sensor such as infra red. I believe in this case the stray light overwhelmed the visible light at the sensor causing the miss focus. And causing the camera to believe it had achieved focus. Which it may have. Just not on something I could see. I also believe this stray light could (rarely) occur naturally out in the field. Causing the camera to lock focus on something other than what we see. This might account for the unusual occasion when the camera refuses to focus on any object but gives a focus lock indication.

Mike


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LJ3Jim
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Feb 27, 2015 20:21 |  #32

MikeWa wrote in post #17452941 (external link)
Thanks LJ3Jim you seem to have nailed my question pretty well.

I think I am ready to put forth a supposition as to the cause.

As this sometimes happens when shooting a BIF I have always put it off as the bird moved between the time focus locked and the picture was taken. On still objects I just assumed focus locked onto something else. This was the first time I ran into the issue in a closed environment.

My actual knowledge of the inner working of focus sensors is quite limited. So my logic can be flawed but here goes. Looking at my OOF picture, I was photographing near the TV and DVD player, both of which have infra red transmitter receivers. I am thinking somehow the camera sensor picked up some stray light interference. Light not in our normal visible rang but seen by the sensor such as infra red. I believe in this case the stray light overwhelmed the visible light at the sensor causing the miss focus. And causing the camera to believe it had achieved focus. Which it may have. Just not on something I could see. I also believe this stray light could (rarely) occur naturally out in the field. Causing the camera to lock focus on something other than what we see. This might account for the unusual occasion when the camera refuses to focus on any object but gives a focus lock indication.

Mike

I don't know anything about how focusing works. But I am a retired computer programmer. Therefore my guess is a software bug. :)


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ppmax
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Feb 27, 2015 20:52 |  #33

LJ3Jim wrote in post #17453052 (external link)
I don't know anything about how focusing works. But I am a retired computer programmer. Therefore my guess is a software bug. :)

I have a question that I have thought a lot about but have not been able to wrap my head around...perhaps you can shed some light.

Given a "black box" containing both hardware and software, is it possible to design a test that will enable one to identify the cause of any given behavior? In other words, as it relates to focus issues, how might one devise a test to prove that hardware (perhaps misalignment or a faulty sensor) or software (perhaps a bug in the AF algorithm) was to blame? The catch is that you don't get access to any source code, or any schematics...you can only use the system through the various interfaces that have been provided to the user.

I'm not challenging your comment...I'm just genuinely interested to know if this question is answerable.

Thx
PP


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LJ3Jim
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Feb 27, 2015 21:12 |  #34

ppmax wrote in post #17453079 (external link)
I have a question that I have thought a lot about but have not been able to wrap my head around...perhaps you can shed some light.

Given a "black box" containing both hardware and software, is it possible to design a test that will enable one to identify the cause of any given behavior? In other words, as it relates to focus issues, how might one devise a test to prove that hardware (perhaps misalignment or a faulty sensor) or software (perhaps a bug in the AF algorithm) was to blame? The catch is that you don't get access to any source code, or any schematics...you can only use the system through the various interfaces that have been provided to the user.

I'm not challenging your comment...I'm just genuinely interested to know if this question is answerable.

Thx
PP

In theory, the answer is absolutely "yes." In practice, it's usually prohibitively expensive. In my career, when difficult bugs arose, we generally focused on how to reproduce the bug somewhat repeatedly. That means trying to get a bug that shows up once a week to show up once a day or once an hour. Once we get an environment where the behavior is repeatable, it becomes much easier to find and fix the bug (either hardware or software).

If I were a Canon software engineer, I would be particularly interested in Mike's environment. It seems as though he has found a place where he can reproduce the "camera says focus when there clearly isn't focus" scenario. That's the ideal situation in which to deploy hardware and software that have both been highly instrumented to see exactly what's going on. The data obtained from that testing would almost certainly show where the problem is. From there, it's just a matter of cost vs benefit to determine if a bug will be fixed.

Regards, Jim


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Submariner
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Feb 28, 2015 04:09 |  #35

MikeWa wrote in post #17451642 (external link)
The focus spot was actually quite a bit larger than shown, covering the lower edge of the cabinet shelf. I added the red spot as a reference point since it is not on the picture. I also tried on the edge of the DVDs with the same result. I know the camera needs contrast to focus. But in this case the camera indicated it had locked focus while the image was this blurry. The focus point in the viewfinder turned red. The green dot in the lower corner lit up and there was the audible beep. All this even though the image was clearly out of focus. Again I don't think there is a defect with the camera. I believe this is an inherent issue with AF digital cameras. Although I am still not sure of the exact cause. I am not so surprised by the missed focus as by the camera indicating it had focused.

Mike

May not be your problem. But my fist 7D was rubbish. Luckily I came across a very well researched article where a chap proved the AF boxes were not where the AF point was. Some were to the left , some to the right, and some above and some below all by varying degrees.
A classic example would be an image of my nephew at his graduation ceremony. The red square was on his face, focus confirmed etc. but his face was totally OOF. The wall some 15 ft behind him was in brilliant focus.
In your example maybe instead of focusing say on the edge of the TV rack it had focused in between and was actually focused on the back of the cabinet where it was really dark!
You see it says its focused and locked but where its suppposed to focus is OOF!

As its a major sensor fabrication issue I returned it and got a great copy without that fault.
I did spend 2 days solid trying to find out what was going on.
A lot of other folks had this problem, and most of us were told we were morons and it was user error. Er Canon knew about this - and lied their ass off!

Hence when I saw all the problems with the 7DII I thought no thanks never again - thank god the 5D3 is perfect no issues.


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Feb 28, 2015 08:36 |  #36

ppmax wrote in post #17453079 (external link)
I have a question that I have thought a lot about but have not been able to wrap my head around...perhaps you can shed some light.

Given a "black box" containing both hardware and software, is it possible to design a test that will enable one to identify the cause of any given behavior? In other words, as it relates to focus issues, how might one devise a test to prove that hardware (perhaps misalignment or a faulty sensor) or software (perhaps a bug in the AF algorithm) was to blame? The catch is that you don't get access to any source code, or any schematics...you can only use the system through the various interfaces that have been provided to the user.

I'm not challenging your comment...I'm just genuinely interested to know if this question is answerable.

Thx
PP


In the true black box example, with zero knowledge of what systems, and how they work, then no it would not be possible to tell what part of such a system was causing the error. All you could know is that you have good data going in, and bad data coming out. At which point you would replace the box. With detailed knowledge of what was in the box, and how exactly it worked, then yes you could make a very educated guess, and it would only be a guess, at what exactly caused the fault. That knowledge might mean that you were correct considerably more than half the time. Remember that the knowledge of the systems would also include the details of previous faults, and what caused them. The only way to be sure of the fault though would be to open it up and carry on fault finding within the box.

I spent 9 years in the RAF as a ground radar technician, working on complex air defence radar systems. Our basic 18 month trade training course meant that we could, given the manuals, probably fix most systems, down to the "box" level on any radar. The radar system that I mostly worked on though took a six month further course to be able to work down to the component level. This was a system that went into full service in 1966, so was mostly Valve technology, with the control logic systems using 50V post office relays (as used in old telephone exchanges). This meant that we could actually work down to the component level, modern PCB's and surface mount technology mostly makes that impossible these days. The transmitter hall was about 50m by 25m and the processing hall, upstairs was the same size, but our systems only filled just over half that area. Fortunately the system was multi channel, so we only really had to know around eight cabinets (7 foot tall, five bay 19" racks) worth of systems. Most of the system was duplication, as there were a total of 12 channels. Fortunately most of the control systems that we had were aimed at telling us technicians just where any particular system may have a fault. So as at least to be able to know which cabinet to go start looking in. Even out cameras have this sort of failure monitoring systems, along with the error codes, which tells the technician what sub systems to go look at.

As far as the alignment between the location within the image area of the actual phase detect system, and the indicators in the viewfinder, well that is not always going to be precise. Especially on cameras with interchangeable viewfinder screens. The viewfinder indications should really only be taken as an approximation, which should allow you to place the AF point reasonably close to the point you want to focus on in the image.

Alan


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To Focus or Not To Focus
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