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09-06-2009, 04:55 AM   #1
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Need help with a DA*50-135 focus test.

As i bought my 50-135 used, i wanted to test it as soon a i got a tripod.
I've been unsure about its performance wide open, so i thought that maybe some of you could just take a look at the samples below.

Pictures taken in DNG, converted to JPEG in PSE without any editing and loaded into Photobucket. Exif is in the pictures.

135mm 2,8


95mm 2,8, cropped to almost fit the above picture


My own thoughts is that the lens is softer to the left side, maybe no biggie, you tell me.

FF or BF? Or good?

One more thing. The lens does not hold focus when zooming, should it? Or not?

Hope to get some feedback!

Edit: About the "hold focus": Maybe its hard to determine that in such large aperture as 2,8. I find using the tripod, it holds focus if i work the zoom several times fom 50 to 135 and half press, but it doesnt behave consistently. Always a mixed behavior regardless of aperture.


Last edited by the swede; 09-06-2009 at 06:30 AM.
09-06-2009, 09:04 AM   #2
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My advice... tear up the focus chart and go take some photos! It's like a beautiful woman, stop trying to find out if she has a freckle on her face and just go out to dinner!
09-06-2009, 09:38 AM   #3
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For what it's worth, the chart looks slightly rotated clockwise which might throw your results; otherwise, looks pretty balanced to me!
09-06-2009, 09:49 AM   #4
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Although it does look a little softer on the left to me there's nothing saying that you had everything set up perfectly.

I agree, go take some real life photographs, and see what they look like.

09-06-2009, 10:21 AM   #5
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Thanks for looking. I have been using it for a month taking pictures. Bought a tripod and thats when the idea about focus testing came up.

I asure you that im not obsessing over testing focus, because i do get good results. Just wanted to check. Its a 2,8 so i think its important to at least test it once.

I think we can say that there isnt any issues with FF or BF. But if its only a little FF or BF in this distance, will it result on a miss at longer distances?

Beeing a litle soft on the left side: I was very thoural about it. Even if it is slightly turned, you can still se the plane in focus and where it begins and ends. I know that this doesnt matter much in real world photo and im not shoting bricks so i'll let that pass

About "holding focus" i cant find ny info of how the lens "should" behave.
09-06-2009, 03:24 PM   #6
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Try changing the focus point to one on the far left, then take a real life photo wide open and see if the subject's sharp there.

It's challenging to get these focus charts at exactly the right orientation to give you correct results. Thus it's very easy to be just a little off and give you results that mislead you into thinking your lens is faulty.
09-06-2009, 04:44 PM   #7
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Swede, it's good to hear that you were simply testing out the focus chart. I would be more concerned about checking front- or back-focus issues and then once I saw that everything looked okay, I'd smile and just go out and shoot. The only time I've ever had to use a focus chart was for a Vivitar 28mm f/2.8 lens that had a terrible front focus issue. I confirmed this with the chart just to ensure I wasn't crazy and took it back to the camera shop. Other than that, I try to avoid them like the plague.
09-06-2009, 10:20 PM   #8
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QuoteOriginally posted by Ash Quote
Try changing the focus point to one on the far left, then take a real life photo wide open and see if the subject's sharp there.

It's challenging to get these focus charts at exactly the right orientation to give you correct results. Thus it's very easy to be just a little off and give you results that mislead you into thinking your lens is faulty.

Thanks, i will try that after work.

Another thing: Has anybody any ideas about the parfocal issue?

I made a search on PF and DPR, and found that a few have asked about it. Most of them has been talked down by various ideas about what parfocal realy means.

Is the lens stated as parfocal or not? If it is, my copy is NOT.

I will however test this more thouraly because i have an idea about the size of the object one should aim at to do a test.

I think if you focus on 135 mm on a small object, large enough to fill the af point, then when you zoom out to 50mm, the af point will detect other contrast surrounding the object. Thus: The lens does not know what you intend.

So testing that, one should aim from 50mm on an object large enough for af point, then zooming in and just halfpress to see if the lens refocuses.

Thats my idea, will test tonight.

09-06-2009, 10:31 PM   #9
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I've found my DA* 50-135mm to be my most parfocal lens (just learned that word from you - thanks). I seems to hold focus fairly well while zooming, but maybe I'm just lucky in that area with this copy. My DA* 16-50mm is quite the opposite - any zooming requires refocusing.
09-07-2009, 02:59 AM   #10
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QuoteOriginally posted by heliphoto Quote
I've found my DA* 50-135mm to be my most parfocal lens (just learned that word from you - thanks). I seems to hold focus fairly well while zooming, but maybe I'm just lucky in that area with this copy. My DA* 16-50mm is quite the opposite - any zooming requires refocusing.
ok.

Fairly well, isnt good enough for me though. When one pays alot of money for a lens named "star", at least i expect it to deliver the best. Or deliver what people say it does.

Im bothered by the fact that no one on the forums seem to agree of how the lens should behave regarding "parfocal". But i understand that its importand to test i correctly. User error is the most common issue, just like with computers.

I'll try to test it more thouraly and if nothing else i will contact Pentax so maybe we all can get past the issue Or at least i.

Last edited by the swede; 09-07-2009 at 03:27 AM.
09-07-2009, 07:33 AM   #11
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I will say that my 50-135mm is more parfocal than the seven other zooms I've used and can compare it too (including the Tamron SP 70-200mm, DA* 16-50, Sigma 24-60mm). I just assumed that this quality was sacrificed in the design of modern zooms since it seems to be so rare.
09-07-2009, 09:40 AM   #12
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Ok, so i've done some more testing. I used a white flat bakground, in this case a white painted door. I made sure it was brightly lit. I the took a black tape and basically made an big version of the box area in the focus test chart. So, big white background and a big black line

I got the lens to hold focus two or three of times at 2,8 moving. But i tried it for 10 minutes. So very rarely my lens holds focus, and i remeber noticing it in the field to. The times it held focus i was surprised, because it almost never does. It was on the forums i saw people mentioning "parfocal".

If you think im overly obsessed with this.....im not. Just want to share my experience with the ones that maybe wondering about this, searching the net if they maybe think somethings wrong with their lens.

The lens is great and gives me great pictures. But i still suspect i have a back focus issue easier spotted in longer distances, real world shooting that is. I will however send an mail to my Pentax repair and ask them to have a look.
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