Bought a faulty Pentax-DA 11-18?

Started 7 months ago | Questions
GoodD New Member • Posts: 7
Bought a faulty Pentax-DA 11-18?

Dears,

I've been reading this forum for years, thanks for all the photos, discussions, tips&trics, et cetera!

Now it's time to ask a first question, actually it's a call for help

Today I received a brand-new star -> Pentax-DA 11-18mmF2.8 and made a short trip to the downtown late afternoon. I played with the lens but the results were... incosistent (not to say terrible).

I took several photos not changing the settings nor the scenery but there was a huge difference in the exposure level of the photos.

The lens works well at 11mm but at 18mm it looks like the aperture sometimes is not closed enough (or at all).

I ended up doing a quick test. A series of photos with the same parameters, same scenery, You can see them below.

Taken with K-3II, firmware v1.10. No postprocessing applied, just resized.

Perhaps it is worth mentioning that I made the same test with DA 20-40 Ltd. and every picture had the same level of exposure. No problems with the body I assume.

Is that something wrong with the lens or did I use it improperly? Do I need to upgrade the firmware to the newest one to make the lens work?

ANSWER:
This question has not been answered yet.
MightyMike Forum Pro • Posts: 41,347
Re: Bought a faulty Pentax-DA 11-18?
1

First off, why not upgrade the firmware and run the test again, if it still exhibits the same behavior take it back for a replacement

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DougOB
DougOB Senior Member • Posts: 2,649
Re: Bought a faulty Pentax-DA 11-18?

What is the metering set to?  Matrix, Centre weiht or Spot?

Doug

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PentUp
PentUp Veteran Member • Posts: 3,412
Re: Bought a faulty Pentax-DA 11-18?

It looks to me like it was an overcast day with moving cloud cover and I note occasional patches of blue sky (last image). I suspect that the ambient light was changing between shots and the camera was adjusting to light changes between shots... However if you want a technical/scientific explanation as to why/how... I'm the wrong person to ask!

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Kobie M-C
Kobie M-C Contributing Member • Posts: 546
Re: Bought a faulty Pentax-DA 11-18?
2

Have you accidentally changed your metering to be biased towards the focus point? Were you shooting in Manual? TAv?

The combo of "exposure linked to AF point" and those items can have a pretty big change on your exposure.

Also, the setting of allowing the camera to compensate for out of exposure shots can also cause what would seem to be inconsistencies. I can't recall off hand what that setting is specifically called though.

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ozdean
ozdean Forum Pro • Posts: 28,092
Re: Bought a faulty Pentax-DA 11-18?
1

Did you have your lens hood attached?

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Carlos U Regular Member • Posts: 333
Re: Bought a faulty Pentax-DA 11-18?
2

Besides the changes in light levels, if you look carefully, the foreground is softer in the lighter pictures, or a bit sharper in the darker ones (look in to the granite block on the right and the grill on the left, specially between photos #4 and #5). I asume all pictures where focused at the same distance.

This could be a sign of a sticky aperture. According to EXIF all pictures where taken at f5.6. Looks like on the three that are brighter the aperture was wider.

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Saludos,
Carlos.

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doceyes Regular Member • Posts: 150
Re: Bought a faulty Pentax-DA 11-18?

take the lens off the camera, turn the lens over and slide the aperture lever back and forth, make sure it moves smoothly and the diaphragm opens and closes properly. Look at a light source at the same time to make it easier to observe. Start with the lens set at 11 mm and then try it in between, then at 18mm. If it is not opening and closing quickly then something is stuck or bent.

Had a lens with a similar problem and the lever was stuck at one end of the focal length.

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kypfer Contributing Member • Posts: 786
Re: Bought a faulty Pentax-DA 11-18?
1

Looking at what EXIF data is left attached to the images, the two I checked were both taken at 1/125 @ f/5.6 1000ASA, which suggests to me the camera was set on manual and there were passing clouds affecting the light levels, but without a full set of EXIF data, that's only a guess

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KPM2 Senior Member • Posts: 1,805
Re: Bought a faulty Pentax-DA 11-18?

Hello

Carlos U wrote:

Besides the changes in light levels, if you look carefully, the foreground is softer in the lighter pictures, or a bit sharper in the darker ones (look in to the granite block on the right and the grill on the left, specially between photos #4 and #5). I asume all pictures where focused at the same distance.

This could be a sign of a sticky aperture. According to EXIF all pictures where taken at f5.6. Looks like on the three that are brighter the aperture was wider.

well spotted ! On the other hand, why this happens only at 18mm I have no idea, because the apertures have nothing to do with the focal length.

To the OP: you can try out easier test pictures: a close up, where the change of the DOF get very quick visible. When the illumination of the scene do can not change (maybe done at home with room light) and you use always the same setting and the same focus and camera position (maybe use a tripod or put the camera on a desk or something), than your aperture blades do close not always equal, when the pictures have different brightness and different DOF's.

On the other hand, for to do such test's, I have a lens, where such sticky apertures can happens only, when the lens get cold. Therefore, when you send back the lens and they try it out in a warm room....they may find nothing and send it back to you. Therefore try it out if a cold temperature of the lens cause it and mention this to them.

best regards KPM2

kriztian Senior Member • Posts: 1,425
Re: Bought a faulty Pentax-DA 11-18?
2

have you tried to change the metering method in camera? like single point metering and average metering? I think you should, just to eliminate the camera itself from the equation

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kriztian

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?

DougOB wrote:

What is the metering set to? Matrix, Centre weiht or Spot?

Doug

In this case Matrix, but I did test with Centre weight and basically got similar inconsistency.

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?

Kobie M-C wrote:

Have you accidentally changed your metering to be biased towards the focus point? Were you shooting in Manual? TAv?

The combo of "exposure linked to AF point" and those items can have a pretty big change on your exposure.

Also, the setting of allowing the camera to compensate for out of exposure shots can also cause what would seem to be inconsistencies. I can't recall off hand what that setting is specifically called though.

That was TAv with the following settings: 1/125, f5.6, EV 0.0, the body firmly kept ISO 1000.

Both custom functions (Link AE and AF point + Auto EV compensation) set to off.

This series was done with Live View, without changing the AF point.

All shots made within lest than a 30 seconds -> no noticeable change in the light of the scene.

Other tests with Center Weight and with viewfinder shooting (without Live View) resulted in similar inconsistent exposure.

I repeated the test with Pentax-DA 20-40mm Limited and all shots were accurately exposured. I've never had any problems with other lenses.

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?

ozdean wrote:

Did you have your lens hood attached?

Yes, all the time.

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?

Carlos U wrote:

Besides the changes in light levels, if you look carefully, the foreground is softer in the lighter pictures, or a bit sharper in the darker ones (look in to the granite block on the right and the grill on the left, specially between photos #4 and #5). I asume all pictures where focused at the same distance.

This could be a sign of a sticky aperture. According to EXIF all pictures where taken at f5.6. Looks like on the three that are brighter the aperture was wider.

Correct -> focus point wasn't changed.

I also suspect that even though the setting was f5.6, some of the shots were taken wide-open.

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?

doceyes wrote:

take the lens off the camera, turn the lens over and slide the aperture lever back and forth, make sure it moves smoothly and the diaphragm opens and closes properly. Look at a light source at the same time to make it easier to observe. Start with the lens set at 11 mm and then try it in between, then at 18mm. If it is not opening and closing quickly then something is stuck or bent.

Had a lens with a similar problem and the lever was stuck at one end of the focal length.

I played with the aperture lever as you suggested and it seems it works fine.

Swedish Viking New Member • Posts: 5
Re: Bought a faulty Pentax-DA 11-18?

Hi,

I noticed that the first picture in a sequence is probably taken with the aperture "lagging", the rest is fine. (If you look at the persons moving you can see which one is the first).

Its not likely due to clouds movin etc, since its a very short time between the exposures. Probably a sticky aperture, BUT: It could be your camera. I would try to take a couple of test shots with another camera if you have that possibility.

So: Probably sticky aperture, but could be "sticky camera aperture control".

OP GoodD New Member • Posts: 7
Re: Bought a faulty Pentax-DA 11-18?
3

I've got a replacement and it works perfectly consistent!

Thanks folks for your assistance!

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