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01-21-2023, 10:09 PM   #1
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D-FA* 50mm f/1.4 sticky blades? Anyone seen this before?

Hi folks.

I just got a used Pentax-D Fa* 50mm f/1.4 from KEH for an absolute steal.
However, after using it to shoot a family dinner tonight, I noticed some pretty erratic exposures when reviewing the images on my K-1 (running firmware 2.10, the latest). From one image to the next, the exposure was hopping all over the place. Given that I was shooting in manual mode and in continuous drive mode (NOT bracketing), this was odd. The exposure ought to have been identical from frame to frame, since the camera's settings and the scene I was shooting weren't changing between shots. One image would be over exposed and the next would be under exposed, and the next after that would be right on the money or, just as likely, two stops too dark.


When I got back home I began running a few tests and took this video of the issue as it happened.
It looks like the aperture is sticking, but there is not evidence of oil (or anything else) on the blades. The strangest bit of all is that when the aperture gets stuck in the stopped down position, it can be reset by cycling the power switch on the camera body.

This made me start wondering that perhaps this stuck aperture was a firmware issue rather than a sticky blade issue. To test this theory, I repeated my tests on my only other KAF4 lens, a D-FA 70-210mm; but, the 70-210mm stops down and opens up again just peachy-keen.

If you have had any experience with this, please let me know! I'm thinking that I'll be sending this lens back to KEH for a refund. I would love to keep it if I could get it working properly, it absolutely SLAPS!
Am I missing something obvious, or is this 50mm lens defective?

01-22-2023, 01:20 AM - 1 Like   #2
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I would just return the lens. There is most likely an electrical fault either in the connections or the diaphragm actuator. The cost of repair and what you paid for the lens is likely to exceed what the lens new would cost.
01-22-2023, 04:28 AM - 1 Like   #3
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I agree with Not a Number.

It is defective and not worth your time as it might become unreliable again if you can clear the fault.
01-22-2023, 07:38 AM   #4
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QuoteOriginally posted by Gerbermiester Quote
I just got a used Pentax-D Fa* 50mm f/1.4 from KEH for an absolute steal.
I presume the fault was not mentioned prior to purchase ?

01-22-2023, 08:55 AM   #5
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Doesn't KEH still include a 6 month warranty?
01-22-2023, 10:03 AM - 1 Like   #6
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Maybe unrelated and probably a faulty lens but it looks like the battery was very low. I don't know if that may have an impact on the lens performance.
01-22-2023, 10:51 AM - 1 Like   #7
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I'd take them on on their warranty.
They have to solve it.

01-22-2023, 03:18 PM   #8
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Thanks everyone. That seems like the best way forward. Sometimes it's just nice to have a sanity check!

QuoteOriginally posted by pschlute Quote
I presume the fault was not mentioned prior to purchase ?
That is correct. No mention was made.
01-22-2023, 03:38 PM   #9
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Send it back. No question.
01-22-2023, 06:46 PM   #10
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It is troubling to hear this story. I had redefined the word "perfection" in terms of this lens.
01-23-2023, 08:19 PM   #11
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Who buying used, one never knows what that particular item might have been through- usually only the previous owner. Luckily, KEH is a very fair and reputable outfit, from what I have seen, so you should be able to make the return without any problem.
01-29-2023, 09:21 AM   #12
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Let us know how it works out. Since it is under warranty they should be able to return a fully functional lens to you. The DFA 50 1.4 is worth a little hassle.
01-29-2023, 09:48 AM - 1 Like   #13
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QuoteOriginally posted by Paul the Sunman Quote
It is troubling to hear this story. I had redefined the word "perfection" in terms of this lens.
My copy is being held together with electrical tape.....
01-29-2023, 10:02 AM   #14
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QuoteOriginally posted by Paul the Sunman Quote
It is troubling to hear this story. I had redefined the word "perfection" in terms of this lens.
I have owned this lens from when it first came out without issue. It is a shame the OP has issues with his, but as has been pointed out, who knows what stresses a second hand lens has been through before purchase.

If there was a general problem with this lens I think we may have seen more than one post on the issue.
10-01-2023, 09:00 PM - 1 Like   #15
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Hi all, just giving this thread a long overdue resolution.

KEH took back the lens without any hassle. They even comped the shipping from Canada to their location in the US. I'll definately do business with these folks again.

As for a DFA 50mm replacement, I've not been able to track down another copy at a similar price. I've been enjoying my FA 50mm f/1.4, in the meantime (but at DFA, it ain't).

Thanks again to all who participated in this thread and for your helpful input.

All the best!
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