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03-06-2015, 05:22 PM   #1
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DA*55mm aperture depending focusing issues

Hi there,
I'm new to this forum and english isn't my native language, so please excuse my bad english.

I've come to find a rather unpleasant behavior of my DA*55mm, which is:
It seems to have two sets of focal-lengths settings written somewhere in it's lensrom (or wherever this information is stored).
If I use the focus-correction to be perfectly true for apertures 1:1.4 - 1:2.5 then it will have a significant backfocus (~30cm for 4m distance) for 1:2.8 and smaller.
When I set the camera to focus on 1:2.5, then disable the AF and change to 1:2.8, the focus will be 100% true. Should I change the aperture from 1:2.5 to 1:2.8 and then activate the AF again, the camera will change the focusing and i will get a blurry picture as a result.

I've already tried out 5 different Pentax cameras (3 of my own - K-r, K-30 and K-3 - as well as 2 of a friend - K-5iis and K-3) and got the same result on each camera.

Has anyone else noticed this kind of behavior?
I've already tried sending the lens in for fitting, but they were unable to fix it. :/

I've attached 2 photos from my test-runs. The camera was not moved in between shots and both times I turned the focus to infinity before pressing the shutter for focusing, so it had to focus fresh.

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PENTAX K-3  Photo 
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PENTAX K-3  Photo 
03-06-2015, 06:58 PM   #2
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The camera doesn't stop down while focusing, so the selected aperture shouldn't affect the focus.

You also should use a focus test chart to calibrate your lens, rather than an optical test chart. In this case it's more likely that the target threw the camera off, since the 55mm is known for occasional AF accuracy issues. I have a friend who can't get his to focus consistently either.

Adam
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03-06-2015, 09:38 PM - 1 Like   #3
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Not sure about the K-5 II, but on at least some earlier models (K-5, K-01) if you focus with CDAF (live-view) in bright light, it WILL stop down as it is focusing, which is absurdly stupid. So if you intend to shoot at f/2.8, and it is stopped to f/4 or f/5.6ish while it focuses (which is does automatically when it deems it too bright), it will then open up again to shoot at f/2.8 and it will be severely out-of-focus every time. Drives me nuts and makes CDAF rather useless outdoors with any fast lens. You can get around it by:

A) Not using CDAF, but only through-the-viewfinder PDAF

B) Manually focusing with CDAF *while* using the zoom/magnify assist function (this seems to disable this "feature" and keeps the aperture wide-open)

So if you are using CDAF for your tests, listen for the aperture opening and closing in response to scene brightness. Some people have said their cameras don't do this, if you can get it not to, I'm all ears. (I'm usually in Av mode.)
03-07-2015, 02:59 AM   #4
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@Adam: I know that the camera does not stop down during focusing and that it "should" not affect the focus. But for some reason it does with me.


@vonBaloney
I am talking only about PDAF!
If i use CDAK all is fine.

Not having a Focus test chart at hand I tested it with my tilt-able PC-screen.
The distance between camera and target is approx. 70cm.
The BF you see on the photos relates to roughly 1-2cm.

I'm not having problems with the consistency of my focus with the DA*55. Of all my lenses it is for sure the one with the most exposures taken, so i got quite some experience with it .
As long as i keep above 1:2.5 or below 1:2.8 all the time, my shots will be consistent throughout the whole set.

Attached Images
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PENTAX K-3  Photo 
View Picture EXIF
PENTAX K-3  Photo 
03-07-2015, 05:28 AM   #5
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All lenses have efect with name fosus-shift... ;-)
03-07-2015, 07:42 PM   #6
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For what its worth, I noticed something strange with my K-3 earlier this week. I have a properly calibrated KatzEye screen on my camera and was just fooling around with my FA 35/2 to see how good the match was between the AF and the split image. Wide open the match was very good However, when I chose a narrower aperture (f/8 as I remember), the match was off a little (split image did not line up). "Well!", I thought to myself, "That is certainly strange." It appears that the camera was using a different AF strategy at narrower apertures. This behavior was very repeatable. I didn't make a special effort to see, though, if it happened near-far and far-near both, nor did I try any other lenses.

Is this related to your problem? It is hard to say. I have not looked at it further and would like to see if the same is true without the screw drive.


Steve

Last edited by stevebrot; 03-07-2015 at 07:48 PM.
03-07-2015, 08:20 PM   #7
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QuoteOriginally posted by stevebrot Quote
For what its worth, I noticed something strange with my K-3 earlier this week. I have a properly calibrated KatzEye screen on my camera and was just fooling around with my FA 35/2 to see how good the match was between the AF and the split image. Wide open the match was very good However, when I chose a narrower aperture (f/8 as I remember), the match was off a little (split image did not line up). "Well!", I thought to myself, "That is certainly strange." It appears that the camera was using a different AF strategy at narrower apertures. This behavior was very repeatable. I didn't make a special effort to see, though, if it happened near-far and far-near both, nor did I try any other lenses.

Is this related to your problem? It is hard to say. I have not looked at it further and would like to see if the same is true without the screw drive.

I discovered last week (and posted about it in some other thread) that when manual focusing, that the aperture you have set on the body (K-5 this was) makes a difference for how/when it will confirm focus with the green hexagon, i.e. if you have it set on f/2.8 it has to be pretty much spot-on, but if you have it set at f/6.3 you only need to be close. So it would appear it is calculating depth-of-field to some degree given the currently selected aperture when determining "in-focus" or not. Presumably this could also be in effect for AF so it is more lenient if it thinks it can get away with it given the aperture. Either that or it is really fancy and is trying to compensate for focus shift...
03-08-2015, 10:17 AM   #8
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QuoteOriginally posted by vonBaloney Quote
Presumably this could also be in effect for AF so it is more lenient if it thinks it can get away with it given the aperture. Either that or it is really fancy and is trying to compensate for focus shift...
Since most quality lenses do not suffer from focus shift*, I would vote for the former. I think that Ricoh shifts to the less precise and possibly faster f/5.6 functionality when the narrower apertures are in play. If true, this would have implications when doing AF fine adjustments.


Steve

*despite the current obsession with such on this site. I have one known culprit on my shelf...a vintage f/1.4 Rokkor.

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