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06-05-2023, 02:50 PM   #1
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Anyone else get erratic exposure settings on long exposures with IRIX lenses?

Here's a problem that's occurring with My K-1 ll and both mu IRIX wideangle lenses, the 11 and 15mm Blackstone models. I use these for real estate photos, mostly shooting HDR brackets of five frames each. I use a constant f9.5 and hyperfocal focusing. When those brackets reach one second or longer, in dim rooms, I'm getting inconsistent exposure behavior that messes up the smooth sequence of brackets. Both in M mode and with autobracketing, the problem shows up. The longer exposures seem to have a mind of their own, sometimes jumping two or more stops of exposure. I've even seen the aperture opening up wider, though I'm only manipulating the shutter speed dial. So I wind up shooting extra exposures and sorting out a suitable sequence of exposures in post.

Exposure brackets work fine at shorter outdoor exposure times.

The camera was just sent in to Precision's repair shop, but they couldn't duplicate the problem and returned it unrepaired. Perhaps I should have sent in an IRIX lens with it, but I didn't. As many have found out, communication with Precision is slow and bureaucratic, and I've lost patience with them. At this point, my choice seems to be to live with the problem, or buy a new K-1 ll and see how it behaves with my IRIX lenses. I love these lenses. The quality/cost equation is unbeatable, and the focus lock makes focusing errors a thing of the past. The K-1 ll has a killer feature that I won't be without, too- Composition Adjustment in camera, giving me significant perspective adjustments without the expense and bulk of a shift lens.

This is one of those times I wish I could visit the local camera shop stocked with Pentax products and see if "They all do that." Since that's not possible, I'll consult the collective wisdom of the Pentaxian community. Has anyone else ever seen this problem?

06-05-2023, 04:09 PM   #2
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Can you please post a few photos that show the problem, and with their EXIF data intact?
06-05-2023, 05:04 PM - 2 Likes   #3
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Here's a list of common problems that could create erratic exposures:

1) light entering the viewfinder(causes under-exposure): If your eye isn't against the camera, light from behind the camera can enter the viewfinder and fool the meter into under-exposing the image.

2) dirty lens mount contacts (both directions possible): intermittent connection between the lens and camera may provide erroneous data on the minimum and maximum aperture of the lens leading the camera to misinterpret the meter readings and to incorrectly set the aperture.

3) sticky aperture blades, weak lens spring, friction in the lens (over-exposure): in the Pentax system, the camera body "lets" the aperture close to the set value which implies that friction in the lens can prevent the proper closing of the aperture.

4) spot meter mode (both directions possible): it's not hard to end up in this metering mode and then happen to have the spot on dark or bright feature of the scene.

5) loose lightmeter sensor inside the camera (erratic over-exposure): If the sensor is knocked loose it will typically see much less light off the focusing screen and handling of the camera between portrait orientation, landscape, point up, down, or inverted for a battery change can jiggle the sensor to change the exposure.
06-05-2023, 08:39 PM - 1 Like   #4
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Any chance the low light noise compensation is involved. You might make sure it's off and if it isn't, try switching it off and see if it makes a difference.

06-07-2023, 04:17 PM   #5
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More details on the problem:
- I'm shooting these brackets in RAW, in a tripod
- the problem only happens with my IRIX 11, not my IRIX 15mm
- in M mode, I'm doing a five-stop bracket either 1.0 or 1.5 EV apart

The problem comes when I select exposures above +1 EV, usually beyond 1 second. Switching between 1 and 1.5 second resulted in several stops of overexposure. In one scene, while set on 1 second, the rear preview screen switched itself rapidly between +1 and +4 exposure! This always happens with this lens, and I've also seen it with my K-3 lll.

Having wasted time and money checking out the K-1 ll, and buying and returning another IRIZ 11mm lens which did the same thing, I guess the only step left is to try another camera. So I've ordered a new K-1 ll from B&H (now with a free battery grip) to test that with the IRIX 11mm. If it works fine, I'll know that the old K-1 is faulty. Then comes another round with Precision repair. This is NOT the best part of my Pentax ownership experience.

Thanks for your ideas and suggestions.

---------- Post added 06-07-23 at 05:26 PM ----------

QuoteOriginally posted by photoptimist Quote
Here's a list of common problems that could create erratic exposures:

1) light entering the viewfinder(causes under-exposure): If your eye isn't against the camera, light from behind the camera can enter the viewfinder and fool the meter into under-exposing the image.

2) dirty lens mount contacts (both directions possible): intermittent connection between the lens and camera may provide erroneous data on the minimum and maximum aperture of the lens leading the camera to misinterpret the meter readings and to incorrectly set the aperture.

3) sticky aperture blades, weak lens spring, friction in the lens (over-exposure): in the Pentax system, the camera body "lets" the aperture close to the set value which implies that friction in the lens can prevent the proper closing of the aperture.

4) spot meter mode (both directions possible): it's not hard to end up in this metering mode and then happen to have the spot on dark or bright feature of the scene.

5) loose lightmeter sensor inside the camera (erratic over-exposure): If the sensor is knocked loose it will typically see much less light off the focusing screen and handling of the camera between portrait orientation, landscape, point up, down, or inverted for a battery change can jiggle the sensor to change the exposure.

1) I'm indoors when this happens, away from strong lighting.
2) I've sprayed the lens contacts with electronics cleaner, with no change.
3) No way for me to check on this, but a new 11mm IRIX lens also showed the problem
4) I'm in M mode, ignoring the metering and judging exposure from the rear preview.
5) I'm shooting from a tripod, so no jiggles. Other lenses and brighter scenes show no problems.
06-07-2023, 10:32 PM   #6
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We can't help diagnose your problem blind. Please post some image examples showing the issue with the EXIF data intact and members are more likely to be able to help.
06-08-2023, 02:24 AM   #7
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There have been multiple instances with exposure issues with the Irix Blackstone in the past. My son with his D500 experienced this type of issue with the Irix 15mm (not on long exposure though) and returned the lens for reimbursement.
Not sure it is your K1 which is the culprit.


Last edited by scc; 02-05-2024 at 11:05 AM.
06-09-2023, 04:42 PM   #8
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Well, the new K-1 ll arrived. On initial testing, it shows the same problem with my 11 Blackstone. The IRIX 15mm seems to be working normally on both. My 11mm is less than a year old, so I'll pursue a warranty claim on it, and return the K-1.

I'd prefer to stay with the IRIX brand for my real estate wideangle duties. The sharpness and distortion control is quite good. Laowa would be my other alternative, but I was unhappy with the Laowa 15mm shift lens I tested. Sharpness was poor away from center. The ergonomics were worse, and the lack of a focus lock adds greater chance of focus error on the job.
06-10-2023, 07:55 AM - 1 Like   #9
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Given that it seems that two different copies of the IRIX 11mm had this problem and that at least one IRIX 11mm had this problem on two different K-1 II, it seems that the problem is a design incompatibility between the K-1 II and the IRIS 11mm.

It might be useful to look deeper into the EXIF data (Pentax Tags), especially the tags related to LensType, LensInfo, LensData, and AEInfo. A bunch of those tags should show identical values across all your shots (assuming you bracket the shutter speed, not aperture). Perhaps the lens is occasionally sending garbled or incorrect values for the max or min aperture causing the camera to mis-configure the lens' aperture.

This may be a design defect in the IRIX 11mm -- getting another copy of the lens might not solve the problem. If so, you might try to pin down the exact circumstances that cause the problem so you can find a way to avoid problematic settings or add a consistent step to your workflow to easily capture all the images you want without fear of having a problem.

Good luck!
02-02-2024, 11:04 AM   #10
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Bringing this back from the dead for a good reason.

I purchased a "used" IRIX Blackstone 11mm f/4 lens and noticed that the aperture was not closing down properly.

The lens was cold as it was outside for a while after it was delivered before I returned home to bring it inside.

Once the lens got up to room temperature, the aperture stopped sticking.

My guess is that the tolerances are too tight from the factory on at least some copies of this lens which is causing the aperture to stick part way or all the way open.

I'm also guessing that this will self-resolve as the lens is used for a while but if not, the mount will most likely need to be taken off to see what exactly is causing the aperture lever to not return properly.
02-05-2024, 10:12 AM   #11
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I have not seen any bracket issues with my 11mm Irix on the K-3 mk 3...yet. I will have to try it out more.
What I have noted about the 11mm Irix is that my photos are most often looking somewhat underexposed...for some reason.
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