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11-24-2010, 09:24 AM   #1
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K5 auto focus adjust

I just upgraded my trusty (if the light is good) K10 for a fantastic (no light required) K5.
Spent the last couple hours dialing in the focus of my lenses. The results are fantastic.

I understand the concept of front and back focus but why, if the focus sensor is in the camera, do I need to adjust for my individual lenses?

Thanks in advance
Joe

11-24-2010, 09:30 AM   #2
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Curious what lenses you dialed in? Also how hard it was? Also does it hold the setting when you add the lens or do you have to do it each time you switch lenses?

Sorry lots of questions but just got my K5 the other day and not got as far as dialing in yet.

Thanks

and congrats :-)
11-24-2010, 09:41 AM   #3
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QuoteOriginally posted by joelovotti Quote
...
I understand the concept of front and back focus but why, if the focus sensor is in the camera, do I need to adjust for my individual lenses?
...
Because it's not the sensor that is 'out' but the lens (or better, the combination).

QuoteOriginally posted by vievetrick Quote
...
Also how hard it was? Also does it hold the setting when you add the lens or do you have to do it each time you switch lenses?
...
The camera can remember info for a certain amount (not sure how many) of Pentax lenses. And one global setting for all lenses (regardless if it's Pentax or something else).
So if you have 2 Sigma lenses, you must not forget to change the correction after you swap lenses.
And it's not hard

Maybe not the perfect method but I used a newspaper against a wall, camera on tripod with 2 sec delay and cable release. Set correction, take photo, adjust correction, take photo etc. Afterwards compare the shots, determine the best one and set that correction.
Repeat for all your lenses.
11-24-2010, 10:53 AM   #4
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You really notice if focus is off when you use some fast lenses i.e. f2.8 or less, where the depth of field is razor thin.

I'm on a K10D and I have to enable the hidden debug menu to apply a global focus correction for my FA35 f2 lenses so I can't wait to upgrade to the K-5.

11-24-2010, 11:31 AM   #5
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Having also done the adjustment in my K-5 as well as the k20d, I can recommend using a target that lets you see the whole depth of field instead of the 2-dimensional view from newsprint on the wall. There are many focus targets in PDF format on the internet that provide a good way to go when shot at about a 45 deg angle. THis allows you to place the center of the DOF where you want. For example, I adjusted all mine to give about 1/3 in front and 2/3 behind the target.
11-24-2010, 12:05 PM   #6
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QuoteOriginally posted by sterretje Quote
The camera can remember info for a certain amount (not sure how many) of Pentax lenses. And one global setting for all lenses (regardless if it's Pentax or something else).
So if you have 2 Sigma lenses, you must not forget to change the correction after you swap lenses.
That is incorrect. 3rd-party AF lenses are recognized just fine as long as they have different "lens id". For instance, Sigma has sometimes reused an ID from older lenses or from other 3rd-party makers (eg. my Sigma 17-70 is sometimes labeled as a Tamron 24-90 or somesuch by some software). But it should not be a problem unless you have a quite extensive lens collection.
11-24-2010, 12:52 PM   #7
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QuoteOriginally posted by vievetrick Quote
Curious what lenses you dialed in? Also how hard it was? Also does it hold the setting when you add the lens or do you have to do it each time you switch lenses?

Sorry lots of questions but just got my K5 the other day and not got as far as dialing in yet.

Thanks

and congrats :-)
It's easy. Go to menu C26 and follow your nose. I dialed in a Sigma 18-50 2.8 and a 70-200 2.8 along with my DA35 Limited. The camera remembers each lens when you attach it so it's a one time deal.
11-24-2010, 12:55 PM   #8
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Can someone help me with my original question?

Why since the focus sensor is in the camera and the camera drives the lens focus directly does the lens effect the auto focus?

11-24-2010, 01:23 PM   #9
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QuoteOriginally posted by joelovotti Quote
Why since the focus sensor is in the camera and the camera drives the lens focus directly does the lens effect the auto focus?
The AF mechanism in the lens is built to a certain tolerance. Therefore, it may not necessarily move to the exact spot the camera requests.
11-24-2010, 02:16 PM   #10
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QuoteOriginally posted by joelovotti Quote
Can someone help me with my original question?

Why since the focus sensor is in the camera and the camera drives the lens focus directly does the lens effect the auto focus?
The issue is real enough (as evidenced by the existence of a lens specific setting), but I'm yet to see a convincing explanation of this myself: one would certainly assume that one setting would be enough to compensate for the AF sensor being slightly off as the camera should handle the rest as a part of the AF process. I suspect the reality of this is probably more subtle than my mental model of it though :-)
11-25-2010, 06:31 AM   #11
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QuoteOriginally posted by RBellavance Quote
The AF mechanism in the lens is built to a certain tolerance. Therefore, it may not necessarily move to the exact spot the camera requests.
If this were true the camera's sensor would still register an out of focus condition and request another focus adjustment. We all have heard our lenses hunt for focus.

So the question remains, why do individual lenses have front and back focus errors?
11-25-2010, 08:23 AM   #12
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QuoteOriginally posted by joelovotti Quote
If this were true the camera's sensor would still register an out of focus condition and request another focus adjustment. We all have heard our lenses hunt for focus.
I don't have a valid counterpoint to that, except that we know FF/BF happens, so it must have a cause

The answer would require a complete technical descripton of the inner workings of SAFOX, which I don't have and can't find. I would *love* to learn, however.

All I can come up with is that the AF system is not a "pure" closed-loop system, i.e. it does not endlessly recheck the achieved AF position. I also believe the AF system does not "see" the same DOF as the lens (eg. I believe SAFOX "sees" with an effective aperture of f/5.6).
11-25-2010, 08:38 AM   #13
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Looks like there are three areas for potential focus errors:

1. Lens mount/rear optical element distance to focal plane
2. AF focus sensor distance to focal plane
3. Focus screen/sensor distance to focal plane

The AF focus adjustment in the menus will take care of #1 and #2. Shims added to the focus screen will fix #3 (so that it correctly 'shows' being in focus when the lens + sensor actually are in focus).

On my K5 I needed the same "-3" AF fine tune offset for four primes and one zoom lens which suggests item #2 was the issue. Different numbers would suggest the lens(es) were off, and it can happen. Why do lenses have BF or FF issues? Manufacturing tolerances of camera or lens, damage, grease viscosity in moving bits, etc., anything that can cause optical elements to move affecting the focus point of the rays.

Last edited by Kubicide; 11-25-2010 at 08:46 AM.
11-25-2010, 08:56 AM   #14
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Something I wondered about when I was doing this on my K-7 was whether or not the camera-wide and lens-specific adjustments combine. i.e. would a +3 to the camera wide and a +7 for the lens you're using result in a +10 adjustment, or does the camera simply prioritize one of the settings when that lens is attached?
11-25-2010, 09:33 AM   #15
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QuoteOriginally posted by v5planet Quote
Something I wondered about when I was doing this on my K-7 was whether or not the camera-wide and lens-specific adjustments combine. i.e. would a +3 to the camera wide and a +7 for the lens you're using result in a +10 adjustment, or does the camera simply prioritize one of the settings when that lens is attached?
They don't combine. If you choose the camera wide adjustment, the per-lens adjustments are ignored.
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