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03-22-2020, 01:34 PM   #1
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Lens or camera issue?

Iím using a Tamron 100-300 on my K-70. Iíve noticed several times after taking a shot that the next exposure is underexposed. Adjusting the aperture has little effect. If I take the shot, most the time, but not always, the exposure will correct back. When it doesnít, I have to turn the camera off, take the lens off/on, and turn the camera back on. I have not noticed this with my other lenses and I donít think this sounds like the aperture block failure. Any thoughts?

03-22-2020, 03:33 PM   #2
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Are you setting the aperture manually via the aperture ring, or do you have it set to A? Is the issue repeatable if you try setting it to A or manually, respectively?


If this isn't happening to other lenses, then I suppose it's more likely a lens issue. Perhaps the aperture blades are sticky, but then I don't see why they would ever stop down more than is necessary. Could you check the aperture lever on the lens to see if it's bent, or maybe if the action is stiff or sticky?

Also, maybe this is a silly question, but are you shooting with manually set exposure values, and then zooming in after your first shot? If you perhaps forgot that it has a variable aperture, maybe you didn't notice that wide-open at high zoom is slower than the widest zoom.
03-22-2020, 04:38 PM   #3
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Sample photo?
03-22-2020, 05:12 PM   #4
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Take the lens off, move the aperture in manual mode if you have it, turn at a good speed between wide open and closed, and do the same in auto aperture setting. Is the aperture opening and closing quickly to the correct position? If no, sticky blades, and you have your answer. This is the first step in your diagnosis. I have a 17-28 that came broken, repaired by me, ok for 2 year, now only works correct at one focal length.

03-23-2020, 12:48 AM   #5
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Check also if the aperture lever is bent.
03-23-2020, 10:30 AM   #6
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QuoteOriginally posted by mikecnorthwest Quote
Iím using a Tamron 100-300 on my K-70. Iíve noticed several times after taking a shot that the next exposure is underexposed. Adjusting the aperture has little effect. If I take the shot, most the time, but not always, the exposure will correct back. When it doesnít, I have to turn the camera off, take the lens off/on, and turn the camera back on. I have not noticed this with my other lenses and I donít think this sounds like the aperture block failure. Any thoughts?
You may want to check out this thread! K-70 Underexposed Photo Reports - PentaxForums.com
03-24-2020, 05:26 PM   #7
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Thanks for everyone's replies. I haven't had time yet to explore the issue based on your responses. Hopefully soon.
03-25-2020, 06:27 PM   #8
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Do the 10 second test...lens off the camera...put focal length at one end...slide the aperture blade...does it spring back and forth when you let go..,go to the other end of the zoom...slide aperture blade...moves freely, springs back easily. If answer is no you have found the problem.

According to reports if the lens is the problem , aperture problem, then you may be putting a lot of stress on the camera or lens and possible future damage.

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