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05-28-2010, 04:02 AM   #1
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Unable to control aperture

I recently purchase a Sigma 20mm F1.8 EX DG Aspherical second hand.

Everything is going fine until today.

I was using the aperture ring on the lens to control aperture. I moved the ring back in to A for automatic mode and my camera (Pentax K200D) still has the flashing "F--". I am unable to control the lens via the camera body. I have searched google to no avail.

Given the lens is no longer under warranty does anyone have any cost effective solutions?

Thank you

05-28-2010, 05:21 AM   #2
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Have you tried unmounting then remounting the lens? Moving the aperture ring back and forth a few times? Powering down the camera, then powring it back up?

It sounds like nothing more than a faulty contact to me, if you did nothing else than using the aperture ring.

Does your camera work ok with other lenses?
05-28-2010, 05:26 AM   #3
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Thanks for your help.

I have tried taking the lens on and off several times.

The camera works perfectly with three other lenses.

I have just realised that the aperture ring does not work. Every exposure is taken wide open regardless of what setting I use on the lenses aperture ring. [edit] Correction: the aperture ring works in manual (M) mode.

When I take the lens of and move the aperture I am able to see the hole getting larger and smaller.

Last edited by heasley; 05-28-2010 at 05:47 AM.
05-28-2010, 05:30 AM   #4
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the symptoms you describe indicate that the automatic aperture contact is not working on the lens.

This contact is the 3rd from the lens locking pin.

You can check it with an ohm meter between this contact and the lens mount, and it should be a very low resistance in the A position and open circuit in any other position.

since this function is very seldom used today, it could be that the internal switch contacts are oxidised or contaminated, and when you took it out of A position, you lost the contact.

It should be possible to restore the function by moving the lens in and out of the A position several times, as the contacts will be self cleaning to some extent.

If not, it would require removal of the lens mount and partial disassembly of the back of the lens to clean the contacts.

Unless you are using this lens on an old film body, there is no real reason to use the aperture ring, and once you get it working you should leave it in one place

05-28-2010, 06:50 AM   #5
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Thank you Lowell Goudge.

I first tried moving the aperture ring in out of the A position several times. Unfortunately it had no effect.

I then removed the lens mount and cleaned the contacts using an old lens cleaner (is there a better way to clean the contacts?). Still no luck.

I guess my next stop is to pay someone to fix it. What sort of cost do you think I'm looking at?
05-28-2010, 07:04 AM   #6
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QuoteOriginally posted by heasley Quote
I then removed the lens mount and cleaned the contacts using an old lens cleaner (is there a better way to clean the contacts?).
Triple or quadruple filtered vodka. It's a great solvent, and it leaves no residue behind like rubbing alcohol.
05-28-2010, 07:06 AM   #7
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Thank you cgoudie. I'll give that a try.
05-28-2010, 07:15 AM   #8
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you need to get to the internal (assumed sliding) contact of the switch itself it will be close to the aperture ring, or under it depending on the lens construction.

alternately, you could simply short the 3rd contact to the mount and have it always in the A position.

05-28-2010, 06:45 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
you need to get to the internal (assumed sliding) contact of the switch itself it will be close to the aperture ring, or under it depending on the lens construction.

alternately, you could simply short the 3rd contact to the mount and have it always in the A position.
I have carefully cleaned all of the contacts. They all appear to be clean.

Here are two photographs of the contacts:

Sigma f1.8 20 EX lens contacts - a set on Flickr

Thank you again for everyone's help.
05-28-2010, 07:13 PM   #10
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The contact I am referring to will be a sliding contact on the printed circuit board and a spring contact connected to the aperture ring
05-28-2010, 07:17 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
The contact I am referring to will be a sliding contact on the printed circuit board and a spring contact connected to the aperture ring
I cannot see that in the pictures, but maybe the Sigma design is different? BTW, I have noticed the 2 screws on the flexible circuits have tool marks which indicate the lens was worked on b4.
05-28-2010, 08:37 PM   #12
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QuoteOriginally posted by Lowell Goudge Quote
The contact I am referring to will be a sliding contact on the printed circuit board and a spring contact connected to the aperture ring
I have taken three more photographs which show under the plastic ring. Do you think it is worth unscrewing the flexible circuits and making sure everything is fine underneath?

http://www.flickr.com/photos/11460552@N08/sets/72157624032826895/detail/

QuoteOriginally posted by wlachan Quote
I cannot see that in the pictures, but maybe the Sigma design is different? BTW, I have noticed the 2 screws on the flexible circuits have tool marks which indicate the lens was worked on b4.
Interesting observation! Maybe I should contact the seller and ask if the lens has been repaired before?

To clarify the issue.

When I shoot in any mode other than manual (M) it only allows me to shoot wide open (exif shows f-stop as wide open). When I shoot in manual I am able to adjust the f-stop via the aperture ring however the exif shows "F---". Rather annoying.

Last edited by heasley; 05-28-2010 at 08:43 PM.
05-28-2010, 10:37 PM   #13
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QuoteOriginally posted by heasley Quote
When I shoot in any mode other than manual (M) it only allows me to shoot wide open (exif shows f-stop as wide open). When I shoot in manual I am able to adjust the f-stop via the aperture ring however the exif shows "F---". Rather annoying.
According to this, the issue does not seem to be the aperture ring contact problem because if it was, the camera should display "F---" too when the aperture ring is set to 'A'. I am only guessing but the circuit might be faulty, or the 7 contact pins have poor contact. You might remove those pins and clean them with pure isopropyl alcohol and see if it helps. If it doesn't, you might have to inspect the flexible circuit and soldering under a 10x loupe for any broken lines.
05-28-2010, 10:47 PM   #14
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QuoteOriginally posted by wlachan Quote
According to this, the issue does not seem to be the aperture ring contact problem because if it was, the camera should display "F---" too when the aperture ring is set to 'A'. I am only guessing but the circuit might be faulty, or the 7 contact pins have poor contact. You might remove those pins and clean them with pure isopropyl alcohol and see if it helps. If it doesn't, you might have to inspect the flexible circuit and soldering under a 10x loupe for any broken lines.
Sorry I am probably not communicating as clearly as I could. The camera's lcd always displays "A---" regardless of the mode. However the exif file displays F1.7 for photographs taken when the camera is set to a mode other than M. However there is no way of controlling the aperture when in a mode other than M (it is always wide open and accurately recording the exif as wide open).
05-28-2010, 11:12 PM   #15
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I just tried what you described on my K-m and I am convinced the aperture ring is not making contact with the lens circuit when set to 'A'. Judging from your pictures, that 'A' contact might be hidden inside the aperture ring module that you have already removed. I could be wrong however as this is not the typical Pentax design, but worth looking into since you have gone this far.
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