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02-26-2015, 06:05 AM   #1
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AF accuracy with DA Limited lenses

I have two limited lenses most of the time not 100% in focus, either with PDAF or CDAF, because of inaccurate mechanical transmission of the AF move. I sent the lenses for repair to the address provided by Ricoh. The repair center checked the lenses (I hope they actually did) and said that the lenses are within spec, although on the DA 40 ltd, there is between 0.5mm and 1mm mechanical play on the AF ring. I can only reach the sharpness of the Tamron 17-50 by manually adjusting the AF to compensate for the mechanical play. It seems that Quick Shift is vulnerable over time by the necessity to allow some mechanical play between the AF screw and AF ring, hence a linear position error.

Do you also observe similar issues on the AF accuracy of your DA limited lenses ?
How is this possible that the repair center states that the lens is in spec , and I can easily see how much error is present without any lab equipment ?

02-26-2015, 06:45 AM   #2
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what camera do you use.
I have many issues with k3. all my lenses has issues with the camera i cant get a proper focus .
my k5iis does well.
02-26-2015, 07:24 AM   #3
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Yes, all DA limiteds have a completely broken autofocus lol just kidding, of course not!
There are many thread about AF, but to be honest, often we find out the actual fault is with the users, once they post some sample photos. Blur can be caused by handhsake, pressing the shutter button, even slightly long exposure (slow shutter), shallow DoF, mirror slap, SR, moving subjects, and so on. It also often happens that the AF works, but it locks on an object the user didn't want, for example if the background is contrasty, or if auto AF point selection is enabled. PD AF and CD AF are completely different from each other, and it is unlikely that CD AF would have a consistent front or back focusing. But CD AF has a bigger chance on locking focus on something that is technically the most contrasty object, but is not what the user wanted. With PD AF, the AF points are much bigger than those little overlays you see in the VF. The centre AF point is (depending on camera) about as big as the ( ) in the middle. So it is very possible that focus is locked on one part of the AF point, but not the part the user wanted.
It is possible you got a bad lens, but it is not a likely thing to happen. Some play with the AF ring is no problem, as it doesn't necessarily move the focus, its just the space between gears. And that wouldn't affect AF performance much, anyway.
But like I said, there are many threads about AF. About AF calibration, using focus charts and many different techniques. Feel free to look them up. There is an in-camera setting to help with AF adjustment. Thing is, there are certain tolerances in the factory, and if the specific lens and specific camera you have are on opposite sides of the tolerances, this might lead to a consistent small error. So it is possible that camera and lens are within spec, but need to be tuned together. AF calibration can fix this. And its also possible the problem aren't lenses, but the camera. Maybe you already have a faulty AF correction setting.
If you want perfect calibration out of the factory, go buy Leica gear. That is the difference in cost.
I can't give any more info than that without seeing sample photos with exif data.
02-26-2015, 07:39 AM   #4
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My DA21mm is among the most accurate and quick focusers I have. I think it has a pretty short focus throw, though, so it would be quick.

I'm having some issues with my DA55-300 WR though, and need to really run some controlled tests with it. The (broken) 55-300 it replaced was always pretty accurate.

02-26-2015, 08:55 AM   #5
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QuoteOriginally posted by zmohie Quote
what camera do you use.
I have many issues with k3. all my lenses has issues with the camera i cant get a proper focus .
my k5iis does well.
Interesting, I have a K-5 and a K-3 and also noticed differences with all lenses. I found out what they have done one the K-3 is to enlarge to AF threshold within which the AF confirm locks. So, with my K-5 + DA*300 , the K-5 refuses to lock if not perfectly in focus, and the K-3 is accepts to lock with the same unperfect focus as observed with the K5. The K-3 AF lock is more tolerant from lens to lens (relative to K-5) but slightly less accurate.

Regarding the problem with the DA limited, if I photograph a tree at 7 meters, the K-3 AF locks but the tree is blurred and the far ground (building at 100meters) is tack sharp. I can only have the tree sharp by refocusing by hand.
With cheap Tamron 17-50, or Tamron 18-250, or DA 18-55, the tree is tack sharp, as it should be.

---------- Post added 02-26-15 at 05:04 PM ----------

QuoteOriginally posted by Na Horuk Quote
Yes, all DA limiteds have a completely broken autofocus lol just kidding, of course not!
There are many thread about AF, but to be honest, often we find out the actual fault is with the users, once they post some sample photos. Blur can be caused by handhsake, pressing the shutter button, even slightly long exposure (slow shutter), shallow DoF, mirror slap, SR, moving subjects, and so on. It also often happens that the AF works, but it locks on an object the user didn't want, for example if the background is contrasty, or if auto AF point selection is enabled. PD AF and CD AF are completely different from each other, and it is unlikely that CD AF would have a consistent front or back focusing. But CD AF has a bigger chance on locking focus on something that is technically the most contrasty object, but is not what the user wanted. With PD AF, the AF points are much bigger than those little overlays you see in the VF. The centre AF point is (depending on camera) about as big as the ( ) in the middle. So it is very possible that focus is locked on one part of the AF point, but not the part the user wanted.
It is possible you got a bad lens, but it is not a likely thing to happen. Some play with the AF ring is no problem, as it doesn't necessarily move the focus, its just the space between gears. And that wouldn't affect AF performance much, anyway.
But like I said, there are many threads about AF. About AF calibration, using focus charts and many different techniques. Feel free to look them up. There is an in-camera setting to help with AF adjustment. Thing is, there are certain tolerances in the factory, and if the specific lens and specific camera you have are on opposite sides of the tolerances, this might lead to a consistent small error. So it is possible that camera and lens are within spec, but need to be tuned together. AF calibration can fix this. And its also possible the problem aren't lenses, but the camera. Maybe you already have a faulty AF correction setting.
If you want perfect calibration out of the factory, go buy Leica gear. That is the difference in cost.
I can't give any more info than that without seeing sample photos with exif data.
I use different lenses, 18-55 @ 40mm f5.6, 17-50 @ 40mm f5.6, 18-250 @ 40 f5.6, DA 40 Limited @ f5.6, exactly the sames camera settings for all lenses and a tripod. The variable parameter is only the lens. All lenses works fine with K-5 and K-3; except the DA 40 that give a blurred target (tree) placed a around 7 meters, and tack sharp background. Same problem with CDAF and PDAF , fine AF adjust has no effect (so I assume it is not a problem of front focusing/back focusing).

On this Pentaxforum web site, I found the comparison between the DA 35 2.4 , DA 40 Limited ( https://www.pentaxforums.com/reviews/da-35mm-40mm-limited-xs-shootout/image-q...sharpness.html) and I get similar unsharp photos as Pentax forum, but the repair center said that my lens is fine !!! So , either the repair center did not bother spending time on this lens because they had more valuable things to do (in this case they are fooling me...) or I don't understand. Should I complain to Ricoh that their third party repair center is unprofessional ?

Last edited by biz-engineer; 02-26-2015 at 09:11 AM.
02-26-2015, 10:59 AM   #6
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QuoteOriginally posted by biz-engineer Quote
Regarding the problem with the DA limited, if I photograph a tree at 7 meters, the K-3 AF locks but the tree is blurred and the far ground (building at 100meters) is tack sharp. I can only have the tree sharp by refocusing by hand.
You said it happens with both CDAF and PDAF. Do you get the focus confirmation indicator even when things are not in focus? That will tell whether the camera is falsely detecting things as being in focus, or if the camera is just giving up before focus has been achieved.
02-26-2015, 11:02 AM   #7
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I have the 15, 21, 40, and 70 Ltds. I've never had a focusing issue with any of them on my K20D or my K-5iis.
02-26-2015, 11:20 AM   #8
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QuoteOriginally posted by biz-engineer Quote
I have two limited lenses most of the time not 100% in focus, either with PDAF or CDAF, because of inaccurate mechanical transmission of the AF move. I sent the lenses for repair to the address provided by Ricoh. The repair center checked the lenses (I hope they actually did) and said that the lenses are within spec, although on the DA 40 ltd, there is between 0.5mm and 1mm mechanical play on the AF ring. I can only reach the sharpness of the Tamron 17-50 by manually adjusting the AF to compensate for the mechanical play. It seems that Quick Shift is vulnerable over time by the necessity to allow some mechanical play between the AF screw and AF ring, hence a linear position error.

Do you also observe similar issues on the AF accuracy of your DA limited lenses ?
How is this possible that the repair center states that the lens is in spec , and I can easily see how much error is present without any lab equipment ?
The K-3's faster AF motor may introduce inaccuracies if there's play in your AF ring. I'd get the 40mm serviced as there shouldn't normally be any play.


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02-26-2015, 11:43 AM   #9
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QuoteOriginally posted by biz-engineer Quote
.... fine AF adjust has no effect (so I assume it is not a problem of front focusing/back focusing)
don't be so quick to rule out FF/BF. I owned an FA 28 that calibrated fine with my K5 (no AF Adjustment). When I tried to calibrate it on my K3 it was so far BF even a global 10+10 could not bring in. I needed a tape measure to figure it out.

but that must be a really large building 100 meters away to be tack sharp on a 40mm lens, that's 25x the focal length, especially with a tree at 7 meters being the subject. your depth of field at f2.8 would be from 6m front to 9m back, so your lens is off by a factor of 10. the hyperfocal distance at f2.8 is only 28 m... haven't heard of a lens being off by that much

I'd be curious to see the test image. I'm assuming you are using fixed center point focus and the tree is centered? have you visually verified the distance scale marker when correctly focus versus building in focus? if the distance scale says 7m and a building 100m away is in focus, then that's definitely BF. but if distance scale doesn't say 7 meters when tree is correct you potentially have a registration issue.
02-26-2015, 12:31 PM   #10
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QuoteOriginally posted by DeadJohn Quote
You said it happens with both CDAF and PDAF. Do you get the focus confirmation indicator even when things are not in focus? That will tell whether the camera is falsely detecting things as being in focus, or if the camera is just giving up before focus has been achieved.
That is an excellent point. AF-C is an evil thing if the system gives up. While probably not the case here, there is a long history of user's on this site complaining of poor AF performance when it turned out they were exposing before attaining focus.


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02-26-2015, 01:26 PM   #11
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If I focus on an object at a short distance, let say , the AF ring is in the middle of it's course, then it works fine. If I focus on an object (like the tree) that is such that the AF ring rotates up to the hard stop and tries to rotate a bit backward, but because of the play , the position of the ring slightly moves back (like a mechanical hysteresis) closer to infinity after the AF is confirmed and the AF motor drive is stopped. So, after the AF confirm, le focus ring come back slightly as soon as the focus motor stop exerting a force. So the camera "think" that the object is in focus but it is not: Since near infinity a rotation by only 0.2 mm (I can see when I zoom in on the display in CDAF). Le problem is that the repair center said that there is quit a lot of play on my lens but they said it is normal... but this lens did not have any play on the AF ring when I bought it. I used the DA 40 Limited when I did a 860 Km trip by bike through the alps (from Austria to France), the camera was directly in my backpack to save weight and space, so the lens probably had some hard times (like me. :-) ).
02-26-2015, 01:31 PM   #12
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I was just playing with some AF adjustments with my K-5 with an f/2.8 AF lens I just received. I've never heard this mentioned before, and I never noticed this before, probably because all my fast lenses up to now have been manual focus, but the focus confirmation is actually affected by what shooting aperture you have set. In other words if I am in Av mode and have something almost but not in perfect focus, the green hexagon will not be lit at f/2.8 but if I raise the aperture it will come on at say f/6.3. (Keep in mind that that K-5 only has AF sensors rated at f/5.6 I think.) But if it is in perfect focus the green hexagon will come on at f/2.8. Does this presumably also affect the AF itself? So if you have it set to shoot on f/8 and then engage the autofocus, it will lock on easier because it is giving itself a bigger window to work with (depth-of-field wise) than if you set it to shoot wide-open?
02-26-2015, 03:14 PM   #13
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QuoteOriginally posted by vonBaloney Quote
I was just playing with some AF adjustments with my K-5 with an f/2.8 AF lens I just received. I've never heard this mentioned before, and I never noticed this before, probably because all my fast lenses up to now have been manual focus, but the focus confirmation is actually affected by what shooting aperture you have set. In other words if I am in Av mode and have something almost but not in perfect focus, the green hexagon will not be lit at f/2.8 but if I raise the aperture it will come on at say f/6.3. (Keep in mind that that K-5 only has AF sensors rated at f/5.6 I think.) But if it is in perfect focus the green hexagon will come on at f/2.8. Does this presumably also affect the AF itself? So if you have it set to shoot on f/8 and then engage the autofocus, it will lock on easier because it is giving itself a bigger window to work with (depth-of-field wise) than if you set it to shoot wide-open?
Not on the A lenses, the aperture is always fully open during the AF and light metering. With manual lenses that mention, you refer to stop metering. It is not the case for the DA limited lenses.
02-26-2015, 04:46 PM   #14
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QuoteOriginally posted by biz-engineer Quote
Not on the A lenses, the aperture is always fully open during the AF and light metering. With manual lenses that mention, you refer to stop metering. It is not the case for the DA limited lenses.
I'm talking about an autofocus fully automatic 2.8 lens. I mean just changing the aperture setting on the body -- it is not stopping down but it is compensating for the depth of field as you change the setting (apparently), i.e. it is saying "ok NOW it will be in focus if you shoot at that aperture." See what I mean?
02-26-2015, 06:43 PM   #15
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QuoteOriginally posted by vonBaloney Quote
I'm talking about an autofocus fully automatic 2.8 lens. I mean just changing the aperture setting on the body -- it is not stopping down but it is compensating for the depth of field as you change the setting (apparently), i.e. it is saying "ok NOW it will be in focus if you shoot at that aperture." See what I mean?
I understand the behavior you describe. I'm surprised any company wasted time engineering that feature, which is more difficult to implement than a consistent AF method that doesn't need to know the aperture setting.
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