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08-15-2010, 09:17 PM   #1
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How can different lenses have different focus adjustments?

I had a great day today because I finally spent a few hours and figured out the adjustments necessary to correct the front focusing problems I have had with my AF lenses. At first I thought I just was a bad photographer, but as I learned about the issue I realized that my lenses needed adjustment.

So today was the first day I shot with the focus adjustments dialed in to my K20D and I must say, everything was in focus, finally. I don't understand something though, so I was hoping the forum could help.

If the AF sensor is the same distance from the lens as the focal plane/sensor, how can different lenses require different adjustments? If the distance between the lens and the sensor is slightly off, wouldn't the correction be the same for every lens? Any effect the lens itself might have I would think would be compensated for by the camera, thus causing every lens to be off the same amount. Or am I missing something?


Last edited by GregK8; 08-15-2010 at 09:18 PM. Reason: Typos
08-16-2010, 01:25 AM - 1 Like   #2
rkt
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You may want to take a look at this article ...

https://www.lensrentals.com/news/2008.12.22/this-lens-is-soft-and-other-myths
08-16-2010, 02:29 AM   #3
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I don't know the physical cause of the effect, but I can confirm I had some lenses with slight FF and some with FF.
It may be connected to focal length of the lens ( I had different type of inaccuracy with telephoto than with wide lenses) or with focus shift effect when the aperture is closed.
But the lenses can be physically adjusted for proper focus - recently I brought few of my lenses to service center along with my K10D (which do not support different adjustment for different lenses) and the service engineer adjusted them all perfectly.
08-16-2010, 04:28 AM   #4
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QuoteOriginally posted by rkt Quote
Great article.

It still doesn't answer the question though. If the act of focusing changes the location of the plane with the sharpest image, then if a lens is out of alignment, wouldn't the body compensate for it to bring it into focus. An example is that let's say a body is perfec +-0 where the AF sensor is the exact distance from the lens that the sensor is. I have a lens that front focuses a little and is -2. Wouldn't the camera be able to compensate for this in the AF process to just change the focus slightly?

Alternatively, let's say I have a perfect lens and a body that front focuses. I think this is the situation where the distance between the AF sensor and the lens is less than the distance between the sensor and the lens. In this case, it would seem to me that every lens would front-focus by the same amount because the AF sensors location doesn't change in relation to the image sensor.

Complicating the second example, if I have a body of +2 and a lens of -2, you'd think that they would cancel each other out, but because the sharpest point of +2 and -2 converge on the AF sensor, the image is actually not in focus because the distance to the image plance is different.

Does any of that make sense?

08-16-2010, 04:59 AM   #5
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I believe the problem comes from the fact that the camera and lens work together to determine the "in-focus" position. for instance, if the camera decides it needs to go to 5 meters to reach focus, the lens will tell it "where" 5 meters is for that lens, and the camera will move towards that position. If, along the way, the communication gets messed up, you need to adjust the lens.

This is just a general feeling, not a certainty. What IS certain is that it's more complicated than just moving the lens drive until the camera feels focus is OK.
08-16-2010, 05:17 AM   #6
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The AF sensors for phase detect auto focus are not on the sensor so that we can autofocus with the mirror down. Live view focus should be pretty much spot on since it uses the sensor for the slower contrast detect autofocus.
08-16-2010, 10:24 AM   #7
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QuoteOriginally posted by VaughnA Quote
The AF sensors for phase detect auto focus are not on the sensor so that we can autofocus with the mirror down. Live view focus should be pretty much spot on since it uses the sensor for the slower contrast detect autofocus.
True, but they are supposed to be precisely the same distance from the the lens that the image sensor is, so that when the image is in focus for the AF sensor, it should also be in focus on the image plane.
08-16-2010, 11:54 AM   #8
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There are a couple of different possible casues for FF or BF. Having the focus sensor physically misaligned is one, but that might manifest differently depending on focal length. It's also possible the lens itself is overshooting or undershooting the amunt of adjustment calculated by the camera. Then there is the whole issue of focal point changing as the lens stops down - I forget what that is called - and that could a source for variation between lenses.

08-16-2010, 11:58 AM   #9
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I think bdery is on the right track.

It's true, and you're absolutely right GregK8, that the AF sensor and the image sensor should be the exact same distance from the lens mount, and if that's not the case, then a single correction should make sure all lenses that are focused correctly on the AF sensor are also identically focused correctly on the image sensor.

BUT the AF sensor isn't just a focus confirmation sensor. When an unfocused image projects onto it, it's also capable of measuring exactly how far and in which direction to move the focus mechanism to arrive at correct focus. That's why SLR focusing is so much faster than point-and-shoot camera focusing -- P&S cameras, in general, can only confirm correct focus, and have to hunt around a bit to figure out the right place to focus.

So, back to the phase-detection SLR autofocus sensor. Like I said, it can be used to drive the focus mechanism to the correct point. I believe (though I haven't done the math) that this calculation, done by the camera's processor based on measurements from the AF sensor, depends on a lot of things, like the focal length of the lens, how the lens focus mechanism is geared, and the measured current position of the focus mechanism. And it can all be done without taking additional measurements from the AF sensor.

Frankly, to me it's almost magic that it works as well as it does! But any of those additional factors can be mismeasured or miscalibrated, so luckily we get a bit of lens-specific focus adjustment to help correct it.
08-17-2010, 05:13 AM   #10
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One thing we must be wary of is the concept of "precisely"... There is no such thing as a perfect assembly. Any manufactured device has tolerances, which ensure that each device is better than the garanteed spec, but nothing is perfect.

Let's say Pentax garantee that camera X will have a battery life of 1000 shots. That means that they took a sample of cameras, tested them, and got an average. Then they calculated the standard deviation, and substracted that from the average. Then they calculated the uncertainties of their measurement, and substracted that too.

THAT result was better or equal to 1000, or they would not garantee that value. But some cameras will still be better than others, and that might or might not take into account such things as air temperature, file format, processing parameters, etc. So there is a strong uncertainty implicit with any manufacturing process.
08-19-2010, 09:48 AM   #11
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QuoteOriginally posted by Marc Sabatella Quote
There are a couple of different possible casues for FF or BF. Having the focus sensor physically misaligned is one, but that might manifest differently depending on focal length. It's also possible the lens itself is overshooting or undershooting the amunt of adjustment calculated by the camera. Then there is the whole issue of focal point changing as the lens stops down - I forget what that is called - and that could a source for variation between lenses.
Marc's got it in a nutshell.
08-19-2010, 10:01 AM   #12
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I would add that under most conditions none of us will really care that much. The tolerances built into the mechanism of the camera mirror box and the AF sensor system combined with whatever errors that may be presented by the lens should be more than accounted for by the DOF of even a fast prime lens. I suspect most of us manually focus a lens when shooting close-up work such a macro shot which, while it doesn't prevent focus errors caused by tolerances in the mirror, does rule out AF errors.
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