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05-02-2013, 02:10 AM   #1
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Pentax-A 50mm f1.4 sometimes sets wrong aperture when using "A"-setting

I have a Pentax-A 50mm f1.4 that has worked perfectly a couple of months now, but lately has been starting to underexpose pictures seemingly at random when using the "A"-setting on the lens.

At the time of writing, almost 50% of the pictures turn out underexposed even thought he setting are set to manual and taking shots of the same subject in the exact same position.

For example: say I take three images with f2.0, 1/125 sec and iso 100. Two of these images might turn out fine, but one - usually the first taken - turns out really dark, sometimes pitch black. This is reapeated regardless of the set aperture, suggesting that the camera fails to set the correct aperture because of a bad contact. It must be the contact because the lens works flawlessly when setting the aperture manually, aka using it in M-mode with the green button.

I've checked the aperture pin on the lens and it does pop out as is should when set to A-mode and the spring seems firm. It also retracts as it should when setting the aperture manually on the lens. I have tried cleaning the pin and the contact on the body using a microfiber cloth but it doesn't help. It also should be noted that I also own an A-series 28mm f2.8 which works just fine every time.

Anyone know what I could try next? I mean sure, I can use the 50mm as an M-series lens because every exposure works perfectly when setting the aperture manually, but the only reason I got this was the A-mode so it's kind of a bummer if it's screwed.

05-02-2013, 02:17 AM - 1 Like   #2
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Perhaps some oily, sticky aperture blades? Tech ARP - Camera Lens Buying Guide Rev. 4.2
05-02-2013, 02:34 AM   #3
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QuoteOriginally posted by altopiet Quote
Perhaps some oily, sticky aperture blades? Tech ARP - Camera Lens Buying Guide Rev. 4.2
Yeah that thought had occured to me, but if there were oil on the blades, then it shouldn't work any better in full manual aperture mode as it does now, right? Also, when pressing the aperture preview button at varying apertures the lens seems to work flawlessly and without delay.

Again, I might be wrong but it does seem that the error lies within the contact / pin.
05-02-2013, 02:55 AM - 1 Like   #4
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Will have to give this some thought, I stripped my M 1.4 last week, but haven't stripped any A lens so far. As you say, if it is fine using the aperture in manual mode, it should have to be something wrong with the contacts . Perhaps somebody who's stripped an A lens might chip in...

05-02-2013, 04:29 AM   #5
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Perhaps you might get some ideas here, https://www.pentaxforums.com/forums/maintenance-repair-articles/179884-pentax...-database.html. There is nothing yet on the 1.4, but some info on the 1.7, which should be the same...
05-02-2013, 07:22 AM   #6
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The aperture lever on the lens is probably bent a little towards the center of the lens, this can allow it to jump past the camera lever it sits on. When
this happens the aperture can fully close instead of stopping where it should. This drove me nuts for a long time with my Tamron macro until I
figured it out, it was also intermittent but mostly when pointed up ?
The lens is usually in the closed position and the camera holds it fully open until you take a shot then the camera lever moves to a predetermined spot
dependent on the f stop chosen.
05-02-2013, 09:30 AM   #7
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QuoteOriginally posted by bobpur Quote
The aperture lever on the lens is probably bent a little towards the center of the lens, this can allow it to jump past the camera lever it sits on. When
this happens the aperture can fully close instead of stopping where it should. This drove me nuts for a long time with my Tamron macro until I
figured it out, it was also intermittent but mostly when pointed up ?
The lens is usually in the closed position and the camera holds it fully open until you take a shot then the camera lever moves to a predetermined spot
dependent on the f stop chosen.
Hmm, that would make sense. How did you fix it? I just checked the lever and it look to be pretty straight. I even compared it to my 28mm f2.8 and they look pretty much identical.
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