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04-10-2008, 11:47 AM   #1
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Why is there front/back focussing with a closed-loop AF system?

To the tech gurus among you:

As I recently found out, Pentax AF is said to work as a closed-loop system, meaning that focus is re-checked and corrected before actually locking focus. Other brands are said not to re-check focus (open loop) and therefore to be faster but less precise.

If this is the case, shouldn't any malfocussing lens be corrected by this system? The Pentax AF system should notice that it hasn't focussed properly and correct that, right?

Could anyone of you experts explain that? Thanks!

04-10-2008, 02:07 PM   #2
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What the autofocus sensor sees is not what the picture sensor sees. If the image sensor is mounted a hair forward or backward to it's optimum distance from the lens, it might get a focus that is different from what the AF sensor sees. The reverse is also true. The AF sensor might not be located at the optimal distance for measuring focus. In the old days of film, there was a little bit more leeway, since the film surface is not perfectly flat, but the "silicon" sensor that are used on digital cameras are very "nasty" when it comes to focusing accuracy.
04-10-2008, 02:30 PM   #3
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Thanks for clarifying this point. I understand that calibration of CCD and AF sensors is crucial. But in this case back/front focus shouldn't depend from the lens used, at least as I understand.

However, people claim having lenses that focus correctly, while other lenses don't.
Is it correct to conclude that focus issues with Pentax AF do not (or at least less) depend on the adjustment of the lens, but simply on differing DOF between lenses? Or maybe other factors as well, like SDM vs. non-SDM?
04-10-2008, 05:28 PM   #4
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The depth of field behind the lens is paper thin compared to what you have on the other side, kind of like macro focus where you can see any focusing mistakes. In the old days of film, the film was held (still is, actually) more or less flat by a pressure plate at the back and guide rails at the front. Then, the emulsion itself on the film is not perfectly even, but more like a very fine sandpaper (I hope you'll forgive the "coarse"analogy), so lenses from film era where designed to work with those "flaws". If you'd compare them with "real" designed for digital lenses, you'd see that the latter give a sharper image at the "film plane" (or would that be "sensor plane"?). This is a must with digital since this one has a checkerboard pattern where 2 pixels are collecting green info vs 1 each for red and blue and everything in between is derived from the accuracy of the information collected from these pixels. Then, on color films, there are 3 layers of light sensitive material that are located in a pattern that allows the proper focusing dictated by the different wavelenght of each color, which also explain why there is purple fringing in digital vs almost none in film photography.
I hope you can figure out something from my explanations, because things are not as simple as they seem when it comes to light.

04-11-2008, 07:10 PM   #5
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Noisychip, you're wondering exactly the same things I've been wondering. In a closed-loop system, any problem that produces out-of-focus images should affect all lenses, not just some (though the problem may be more apparent the shallower the DOF is).
04-12-2008, 12:59 AM   #6
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QuoteOriginally posted by Noisychip Quote
To the tech gurus among you:

As I recently found out, Pentax AF is said to work as a closed-loop system, meaning that focus is re-checked and corrected before actually locking focus. Other brands are said not to re-check focus (open loop) and therefore to be faster but less precise.

If this is the case, shouldn't any malfocussing lens be corrected by this system? The Pentax AF system should notice that it hasn't focussed properly and correct that, right?

Could anyone of you experts explain that? Thanks!
The AF sensor is measuring phase differences between light entering the lens on the left and right side of the front element. If one of the lenses are misaligned, the sensor could see a signal which are in phase, but really isn't because of the misaligned lens.
04-12-2008, 02:54 AM   #7
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I think you are missing the point. If I undertsand things right and please correct me if I am wrong here.

Each lens mounts onto the camera with our trusty K mount configuration. There are machining tolerances and within those tolerances there will be variations. Some lenses will sit further back on the camera than others. We are talking small variations here, very small.

Also each lens, though supposedly accurate will have focuing tolerances to take into consideration. That is why they (repair techs) can fix your lenses if they are FF or BF.

If for example the camera AF can operate with a tolerance of +5 or -5 variation and your lens has a +7 variation then it will FF/BF until the Camera is adjusted (K20D) or the lens/camera is adjusted (K10D) by the techies.

My earlier life I was a Tech Elec and for example resistors had a value with a 5% tolerance, that was great if the variations were evenly spread out, but you would get spurious readings from a circuit when you had several components that were ALL at the extreme ends of their tolerances, the sum of the variations takes the total value outside the normal accepted and workable variations.

I know my lenses have variations both BF and FF and the K20D has dealt with most of them, bar two. The amount of FF of the lens is in excess of the adjustment range of the camera, so I have to get the lens adjusted or wait to see if Penatx brings out a firmware update that increases the AF adjustment range from the current +/-10 scale to about a +/-30 scale.

PK
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