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07-04-2014, 02:54 AM   #1
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k-30 underexposure (all manual settings)

Hello there. I have a k-30 for about a year and a half now. i recently bought a sigma 17-50 2.8 and shortly after getting that i noticed that every now and then shots would be underexposed. At first just put it down to using a different brand lens on another brand camera. It only happened every 50 shots or so anyway so no big deal. However its now gotten worse but also seems to happen to my other(pentax 55-300 and 15 limited) lenses, but not as often on those lenses.

Basically what happens is, i set everything in manual, so say iso 100, f2.8, 1/50th and take 10 shots, out of the 10 maybe 2 will be very very underexposed (as if it took it at f22 with the same iso and shutter speed). if i set it to f10 the same thing happens but its not as underexposed (smaller jump from f10 to f22 i think).

I have noticed however if i shoot landscape(horizontal) i don't really have an issue, maybe 1 in 50-100 shots this happens, but in portrait(vertical) it can happen 1 in 5 shots. This is with the sigma. with the pentax lenses its only the 55-300 that i get a problem with and thats usually only 1 in 20 shots vertical, horizontal very very rarely gives a problem.

When testing this out to try to find the problem i've noticed if i take shots with the camera upside down then the sigma underexposes 8 out of 10 shots, the 55-300 about 4 out of 10 and the 15 limited about 2 out of 10.

I have already sent back the sigma for repair (before i noticed the other lenses had a similar problem, and also for focusing issues). It came back with a repair document that just said adjust front/back focus, reset check and test. so not sure if they actually looked at this problem that i had mentioned in the warranty claim form thing.

Has anyone any idea what the problem might be, and how much it could cost to fix (think i'm out of warranty). It seems a bit odd that which way you have the camera turned makes a difference.

07-04-2014, 04:05 AM   #2
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Hm, i would suspect the aperture lever in the camera getting stucked sometimes and not communicating properly with the lens. Since Sigma is not 100 percent to Pentax specifications in the lens mount, it can be more affected by this than with Pentax own lenses, but since it also happens with Pentax own lenses - if by a much lower degree - then it do points to the aperture mechanism in the camera. Pentax has a mechanical linkage in the body that controls the aperture mechanism in the lens. I suspect the problem is with this mechanism in the body.
07-04-2014, 04:19 AM   #3
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Have a look at this thread, you might need a mirror box replacement, as I did...https://www.pentaxforums.com/forums/120-general-technical-troubleshooting/229...ml#post2848161
07-04-2014, 05:30 AM   #4
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thanks for the quick replies. thought it might be something like that. between postage and paying for the fix, it might not be worth it. might just upgrade to a k-3. ill email them first and see if they might fix it for free as it seems to be some sort of known issue.

Quick question tho. i know i probably should search the forum, and i will, but the focus on the k30 is terrible. ive got the 55-300 and ive tried taking a few shots of birds, and i go to alot of cycling races and bring the camera along there. but the percentage of shots that come out of birds or cycling is very very low. I usually just prefocus manually for the cycling now and that works out better i think.

is the focus on the k-3 much better for this sort of stuff. or is part of that problem the lens? if its the lens this would be faster to lock focus one of the pentax * lenses (50-135 or 300f4) with the sdm motor or a sigma with the hsm motors?

07-04-2014, 05:46 AM   #5
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Upgrading from my Kx to my K-30, was a big improvement in focus with the 55-300. While I waited 3 months for my repair on the K-30, I decided to go for the K-5IIs, and again much improved focus on the 55-300. I don't shoot much BIF, but use it mainly for sports like rugby and hockey. I understand that the K-3 is even better at focusing....

I use my K5IIs mainly with the 55-300, and my K-30 as backup, with WA and short tele lenses, and with manual lenses and focus peaking.

It must be said that the 55-300 is notoriously slow in focusing, so just another lens like Sigma 50-500 or such might be better, or the DA300* with the new 1.4 converter, but unfortunately I don't have them, so can't really comment.
07-04-2014, 06:39 AM   #6
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check the aperture lever.it might not be locking in place causing the random underexposure. i had a similar issue with my k-50 and i sent it in for repair. the lever should come down and then lock in place when the shutter is pressed.try it without a lens and see how it acts. pic below shows the lever :


07-04-2014, 06:48 AM   #7
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QuoteOriginally posted by schnitzer79 Quote
check the aperture lever.it might not be locking in place causing the random underexposure. i had a similar issue with my k-50 and i sent it in for repair. the lever should come down and then lock in place when the shutter is pressed.try it without a lens and see how it acts. pic below shows the lever :

In the thread I mentioned above, it was also mentioned that the aperture motor might be failing, and that is what I thought would be fixed, but in the end they "replaced the mirror box"....in the end it's not a minor problem, and may probably only be fixed at an official repair facility, IMO...
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