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04-05-2013, 09:04 PM   #31
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If the lens is new, why not replace it?

btw.... r u a graduate student? saw the phdcomics on the wall...

04-05-2013, 09:15 PM   #32
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@ Just1MoreDave

1 more question.
How about the FA, or F lens that have the aperture ring. The question is if I adjust the lever position, then can I still change the aperture manually via aperture ring correctly? Won't the size of diaphragm be wrong when I change aperture?


@Ikarus

Thanks Ikarus for your sharing!
04-05-2013, 09:18 PM   #33
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QuoteOriginally posted by LFLee Quote
If the lens is new, why not replace it?

btw.... r u a graduate student? saw the phdcomics on the wall...

I brought it used in like new condition. SO....
And, Yes, I'm a grad student.
04-06-2013, 12:41 AM   #34
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My copy of 40/2.8 XS, serial# 5024743 behaves the same way; no difference in exposure until f4.0. After which, stopping down each third stop shows the effect on the exposure by progressively getting darker.

04-06-2013, 01:01 AM   #35
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There's a much easier way to test this, you know.

Our eyes aren't terribly sensitive to a two thirds stop increment in brightness (particularly since the iris / brain compensates automatically) but the camera's metering system is.

So: point your camera at the wall, set mode to Av and set aperture to F2.8. The over/underexposure meter on your screen should be at the 0 position. Make a note of the shutter speed. Now turn the dial to "M". Set shutter speed to what it was on Av and change aperture to F3.5. The meter should now show that you're two thirds of a stop underexposed. If so, your theory is incorrect. If not, your theory is correct. Job done.
04-06-2013, 07:50 AM   #36
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QuoteOriginally posted by top-quark Quote
There's a much easier way to test this, you know.

Our eyes aren't terribly sensitive to a two thirds stop increment in brightness (particularly since the iris / brain compensates automatically) but the camera's metering system is.

So: point your camera at the wall, set mode to Av and set aperture to F2.8. The over/underexposure meter on your screen should be at the 0 position. Make a note of the shutter speed. Now turn the dial to "M". Set shutter speed to what it was on Av and change aperture to F3.5. The meter should now show that you're two thirds of a stop underexposed. If so, your theory is incorrect. If not, your theory is correct. Job done.

That's not the right way to test cause the camera will show the metering value from the programmed standard value. It's a value that is calculated from wide open. Every times you change the aperture the camera will think automatically and show the metering value that already calculated.

Your way will be right if using stopping down metering though.
04-06-2013, 09:33 AM   #37
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QuoteOriginally posted by zany225 Quote
@ 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.
You're only moving the lever enough for a half stop. I don't think it needs to move a lot.

I just got these: "JIS Type S Driver Set, Pollicis 4Pc by Moody Tools" through Amazon, sold by MicroTools. If you pull those up on Amazon, they'll also show you sets from Hozan, which I also got (version 2). The Hozan set is mostly for larger screws. The smallest size (labeled 00) will work OK but lenses often have smaller screws which concentrate too much pressure on the 00 size and ruin it. The Moody set has a 000 size. In the US, these drivers seem to always use Phillips designations for sizes, and it's really annoying sorting out whether the seller is trying to pawn off a Phillips or not.

QuoteOriginally posted by zany225 Quote
@ Just1MoreDave

1 more question.
How about the FA, or F lens that have the aperture ring. The question is if I adjust the lever position, then can I still change the aperture manually via aperture ring correctly? Won't the size of diaphragm be wrong when I change aperture?
Once you move off the A position and use the aperture ring's settings, the lens uses a separate mechanism to set the aperture blades. The camera relies on the lens to get that right and just shoves the lever fully up in M mode.
04-06-2013, 11:40 AM   #38
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Thanks a lot for your info Dave!

Now there are 3 copies in this thread that have poor aperture calibration. I think there will be more.

However, let me conclude this. Now I know that this tiny XS lens is still f/2.8 wide open NOT f/3.5 as I suspected! I apologize for misunderstood. (Do I need to change the thread topic?)
But the problem of the lens is an improper calibration of the aperture in the lens. That leads to over-exposure when the lens has been used with any automatic modes, such as Av or Tv,
The camera will get 1/3EV over-exposure at f/3.2 and 2/3EV at f/3.5. Beyond f/3.5 the over exposure will remain 2/3EV. This confirmation can be done by taking 2 pictures with f/2.8 and f/4 in Av mode. The latter picture will be brighter about 2/3EV.

Is it a big problem? I cannot answer this. It depends on how you use and what you want. In some situation, such as marriage, if you want to change aperture from f/2.8 to whatever back and forth frequently then this might cause your problem with details of bride costume. Raw files could help this.

I suggest you better check your XS lens before serious using.

04-06-2013, 11:55 AM   #39
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Before you all start screwing with your lenses, make sure there are some that do work correctly, or as I thought before, it may be a firmware glitch. Just a thought.
04-06-2013, 12:18 PM   #40
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Nothing to do with firmware at all.

The aperture stop down mechanism is adjusted at 2 points - the rear lever as Dave said, or dig deeper adjusting the blades themselves. The difference is that the latter dictates the actual max wide open position which affects both the 'A' setting and manual aperture (if the lens has aperture ring). For the OP, altering the rear lever will be a quick solution which will work just fine since the 40XS has no aperture ring. Still, you want to be careful adjusting the lever on some lenses because the tiny screws can drop inside the lens and you will curse yourself to strip everything down just to get them back. Also, those screws should be threadlocked (light duty ones will do) afterward or they will loose themselves for certain. But really, once you start digging, you will find many Pentax lenses have less than precise apertures and at some point, you will just have to live with it unless you plan to find tune them all. For low bearing screws like those on the levers, philips 000 screwdriver will do (Wiha makes quality tools, Moody not, IMHO). For tight screws on the lens mount, you should look for some proper fit JIS drivers. Canon USA used to sell some high quality drivers, I don't know if they still do.

Last edited by wlachan; 04-06-2013 at 12:26 PM.
04-06-2013, 02:25 PM   #41
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QuoteOriginally posted by wlachan Quote
Nothing to do with firmware at all.

The aperture stop down mechanism is adjusted at 2 points - the rear lever as Dave said, or dig deeper adjusting the blades themselves. The difference is that the latter dictates the actual max wide open position which affects both the 'A' setting and manual aperture (if the lens has aperture ring). For the OP, altering the rear lever will be a quick solution which will work just fine since the 40XS has no aperture ring. Still, you want to be careful adjusting the lever on some lenses because the tiny screws can drop inside the lens and you will curse yourself to strip everything down just to get them back. Also, those screws should be threadlocked (light duty ones will do) afterward or they will loose themselves for certain. But really, once you start digging, you will find many Pentax lenses have less than precise apertures and at some point, you will just have to live with it unless you plan to find tune them all. For low bearing screws like those on the levers, philips 000 screwdriver will do (Wiha makes quality tools, Moody not, IMHO). For tight screws on the lens mount, you should look for some proper fit JIS drivers. Canon USA used to sell some high quality drivers, I don't know if they still do.
Yes, I know about the adjustment as I needed to do it on a tamron PK-A adapter. I thought of firmware because of the behavior of the sigma 100-300mm f4 I had when coupled with the 1.7 afa. the body would show the max aperture right and the display would tell you it was stopped down some, but it actually wasn't. I found out by looking into the lens and using DOF preview. It had something to do with the lens communication and showed up on photome as a 70-200mm f2.8, so I think it was throwing things off. The lens worked fine without the 1.7 AFA so it wasn't the aperture adjustment. I wound up using the aperture ring when using that combo. Of course that isn't possible with a DA lens. I just thought it might be a possible glitch as there seem to be multiple users with the same thing. Thought I may save some body some grief (or not).

I do know these cameras can lie to you, and not feel bad about it at all. : )

Last edited by borno; 04-06-2013 at 02:31 PM.
04-06-2013, 10:19 PM   #42
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Mine changes. I put it in manual, kept the shutter speed and iso the same, and changed the aperture to see if the exposure was different. There was a change in exposure going from 2.8 to 3.2 and going from 3.2 to 3.5. The amount of change was more when going from 3.2 to 3.5, than it was going from 2.8 to 3.2 though. I tried the bokeh test to. the bokeh did change a bit going from 2.8 to 3.2, and the circle size changed a bit, but it didn't really get flats. Going from 3.2 to 3.5 was a bit more significant. The size of the dots changed again, and it was still round, but you could kind of see a shadow of flats starting to form. That was all on the k-01 it came bundled with.

On my ist-ds (the only other camera I have to test it on), the change in exposure from 2.8-3.2 was more significant, and closer to the amount of change going from 3.2 to 3.5.

It kind of sounds like it might be a tolerances issue? One thing to keep in mind if doing the bokeh test. The xs has curved aperture blades and the limited does not.
04-06-2013, 10:54 PM   #43
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Testing aperture blades take a few separate tests:
1) Look at the aperture blades with the camera in A or M mode. The actual stop down can be different with different orientations.
2) Check histogram with camera in M mode.
3) Check histogram with camera in A mode.
4) Compare 2 & 3 results.

Since there is certain acceptable calibrating tolerance (even stated in their parts manuals), every camera (not just model) is different. The difference could be due to the body, the lens, or that particular combo. You can fine tune lens A to match body B perfectly, but could be off again with body C. Bokeh test is not very useful here.
04-06-2013, 10:56 PM   #44
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QuoteOriginally posted by ripit Quote
Mine changes. I put it in manual, kept the shutter speed and iso the same, and changed the aperture to see if the exposure was different. There was a change in exposure going from 2.8 to 3.2 and going from 3.2 to 3.5. The amount of change was more when going from 3.2 to 3.5, than it was going from 2.8 to 3.2 though. I tried the bokeh test to. the bokeh did change a bit going from 2.8 to 3.2, and the circle size changed a bit, but it didn't really get flats. Going from 3.2 to 3.5 was a bit more significant. The size of the dots changed again, and it was still round, but you could kind of see a shadow of flats starting to form. That was all on the k-01 it came bundled with.

On my ist-ds (the only other camera I have to test it on), the change in exposure from 2.8-3.2 was more significant, and closer to the amount of change going from 3.2 to 3.5.

It kind of sounds like it might be a tolerances issue? One thing to keep in mind if doing the bokeh test. The xs has curved aperture blades and the limited does not.
It's good to hear your positive results. I really think it might depend on the camera models as well. I will borrow my friend's K7 and let's see how it will be.

Still hope many of you will try and share your experiences here more and more. Thanks in advance!
04-06-2013, 11:53 PM   #45
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Unless I'm affected by bad luck, it doesn't appear to be a rare problem either. Out of the ten lenses I own that allow for aperture control via the camera body, four have the issue - apart from the DA-40 XS, it's the A-50 2.8 macro and two old Vivitar zooms.
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