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04-05-2013, 12:10 PM   #16
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It seems it's working correctly so far in every aperture. The brightness seems constant in the previous test.
--------------------------------------------------------------------------------
Second test is to keep shutter speed constant. Changing only aperture by 1/3EV step
The pictures follow by this order

f T
1. 2.8 100
2. 3.2 100
3. 3.5 100
4. 4.0 100
5. 4.5 100
6. 5.0 100
7. 5.6 100
8. 8.0 100
---------------------------------------------------------------------------------------------

Yep, it's working correctly. The pictures get gradually darker as I narrower the aperture.

So, I know it's not my camera problem.
It should be from lens, only my copy or every copies could be possible. Or camera compatibility, maybe? The 40mm XS might work correctly with the K01 because they came together at the first place.

I recommend you test by yourself and share your experience here!

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Last edited by zany225; 04-05-2013 at 12:15 PM.
04-05-2013, 12:21 PM - 1 Like   #17
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Third test from 40mm limited to compare to 40mm XS is just compare between f/2.8 and f/4

f T
1. 2.8 60
2. 4.0 50
3. 2.8 50


Not likely the 40mm XS, it looks like at f/4.0 the picture is darker.
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04-05-2013, 12:33 PM   #18
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Also, I took a look at the blades again to compare between them. I found the blades noticeably moved from f2.8 to f3.2 in the 40mm limited unlike in 40mm XS!!!

So, in case that the XS's really f2.8 wide open, I think it has poor calibration of aperture blades.
The picture from XS is brighter than the limited at f4 and same shutter speed.
They difference is shown in the previous pictures.
04-05-2013, 12:33 PM   #19
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Could be sticky aperture blades? I'll test mine as well and post if its interesting.

04-05-2013, 12:43 PM   #20
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It's a calibration issue. Pentax K mount aperture linkage requires very very precise calibration on both the body and the lens to produce accurate stop down. If either or both are slightly off (but will within spec because they accept certain tolerance in the factory), the stop down can be off by up to 1 stop (when the lens aperture is set to 'A', and 40XS is always at 'A') depend on which lens. The aperture blade should stop down at f3.5 even by slight amount (maybe 1mm). If not, the lens can be readjusted easily by any technician. If it's new, better exchange it if it bothers you. Also, mechanical aperture is not perfectly linear so the actual exposure will vary as you stop down, and the difference can be quite dramatic.
04-05-2013, 12:53 PM   #21
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It doesn't bother me that much. I just was surprised what's wrong with the lens and tried to find the answer.

Now I'm feeling that it might not be just only my copy. It could be the QC problem? And the poor calibration could be done for a whole lot of the XS!?! If that is what's happening then Pentax has to do or explain something.
04-05-2013, 01:50 PM   #22
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If it is not the lens calibration, you may have found a firmware glitch. Not likely,but I did have a similar problem with a sigma lens and the k20d. Someone with the same lens and body should check it out just in case.
04-05-2013, 02:05 PM   #23
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+1 for your persistence ,Zany
On the old SMC Pentax-M 1:2.8 40mm the clicks are f/2.8 then f/4
Looking through the camera end of the lens that can be seen as a big change (as expected) of the 5 segment diaphragm.
By the way, how many segments do the new pancakes have?

04-05-2013, 04:09 PM   #24
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QuoteOriginally posted by zany225 Quote
It doesn't bother me that much. I just was surprised what's wrong with the lens and tried to find the answer.

Now I'm feeling that it might not be just only my copy. It could be the QC problem? And the poor calibration could be done for a whole lot of the XS!?! If that is what's happening then Pentax has to do or explain something.
This can happen to any Pentax KA or later lens. It is not a defect for sure but rather loose QC IMHO, but doesn't seem to bother the majority of the people. When I came across lenses like that, I just fix them myself. Less pain than sending them for service which is a gamble in itself.
04-05-2013, 05:07 PM   #25
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QuoteOriginally posted by wlachan Quote
This can happen to any Pentax KA or later lens. It is not a defect for sure but rather loose QC IMHO, but doesn't seem to bother the majority of the people. When I came across lenses like that, I just fix them myself. Less pain than sending them for service which is a gamble in itself.

How do you fix them? Could you explain it for me, please? If it's a video clip or pictures step by step that would be awesome!

I think I might have another one with this problem but worse. Need to check on my lenses.
04-05-2013, 05:13 PM   #26
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That's a lot of information, but I have no idea why you're doing it. It seems like a lot of work to try and do a demonstration that you probably won't remember very long anyway. Histograms are wonderful things.
04-05-2013, 05:22 PM   #27
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I don't have any video myself but there should be some if you search hard enough. I'd rather not teach how to fix it as you have never done it before, and you will likely do more damage than good.
04-05-2013, 08:03 PM   #28
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I hate to say it, but you could take one more set of shots. Take a shot of a small point of light surrounded by a dark background, with the light out of focus. Something like a street light at night is good. When the aperture blades are fully retracted (lens wide open), the light should be a perfectly round circle. When the lens is stopped down by any amount, the circle should change to a polygon with one side for each aperture blade. So at f3.5 you should see something like this:



I don't know anything about the 40XS. It might be a lot different than typical lenses because it's so tiny. On a typical lens the fix is not that hard. Here is a photo of a lens (F70-210) with an adjustable aperture lever. Only the mount has been removed here:



The aperture lever is on the right. Two screws are at its base. The holes in the base are slotted, so you can simply loosen the screws and shift the lever a bit each way. Probably a good idea to mark it before unscrewing. It would be really easy if you already knew which way to move the lever and exactly how much. Otherwise you'd have to put the mount back on and test. The lens in the photo makes that process extremely tedious, with each contact having a spring, contact and insulator, all just about to fall or jump. Newer lenses are often easier. At least you won't need to worry about the aperture ring. You'll want a set of JIS screwdrivers for this.
04-05-2013, 08:53 PM   #29
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@ Just1MoreDave

Thanks! I think I understand how you would fix it. I took a look to my broken lens that sit on my desk. I saw the holes in the lever base underneath the screws. Aren't they too short for adjusting lever position? I think they are just about 3mm long. The screws size are about 1mm though.

Also, this might be silly questions, what brand of the set of screwdrivers should I have? Where can I get them? And what size should I buy?
I tried to take a part a broken lens with the "general jewelers screwdriver set" but they were too soft. The tiny screwdrivers tip were just damaged. And the screws just still sat there! lol.
04-05-2013, 08:57 PM   #30
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Mine does the same on both the K-x and the K-01. There is no difference between f/2.8 and f/3.5.The out of focus highlights look round to me, so it seems like 3.5 and 3.2 are actually still 2.8, and the lens doesn't stop down until the camera is set to f/4.
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