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11-15-2008, 05:55 PM   #1
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No autofocus with a F 28mm

I have a SMC Pentax-F 28mm F2.8 lens that I got as part of a package deal. The glass is clean and it works very well in AV mode with my K10D as a manual focus lens. And that is the problem. The K10D thinks it is a manual focus lens; it does not recognize it as an autofocus lens. When I switch the MF lever on the camera to AF (either S or C) the drive screw engages, but it still reads in the viewfinder as MF. The camera has no problem recognizing or focusing with other AF lenses. I have cleaned the contacts on the lens with rough cloth and paper but with no improvement. My guess is that the AF circuit within the lens is somehow broken or shorted. I thought I would check here first to see if someone had any ideas before I tried taking the rear plate off the lens. Of course, I could have someone try to fix it. Any ideas there would also be appreciated.

11-15-2008, 06:20 PM   #2
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check the furthest pin away from the locking pin slot.

you can clean it with a soft pencil eraser, nothing abrasive.

This is the pin that sends AF data to the camera body, the remaining pins are apature contacts.

you might also want to see if there is somewhere an MF/AF switch on the lens.

Also, make sure this is a pentax KAF mount not a KF mount. Pentax came out with a little known non compatible to the present electrcial contact mount for AF in about 1980. post a photo of the back of the lens showing the lens contacts and the lens locking pin slot.
11-16-2008, 09:57 AM   #3
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Lowell,
Thanks for your help. Just to be clear, is the locking pin slot you refer to nearest to 3 o'clock in #2, making the AF contact nearest to 6 o'clock in the same photo? There is no MF/AF switch that I can find.
Mike

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11-16-2008, 03:36 PM   #4
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that is correct, the pin at the 6:00 position is the AF pin.

it may not be contacting the pin on the body, or it is dirty.

The other possibility, and I have this with my sigma 70-200 F2.8 is occasionally the screw drive does not engage, theck it's rotation and allignment with the body

11-16-2008, 04:31 PM   #5
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Well, this is definitely an AF lens.

Given what you're reporting, the problem does seem to be electrical. If cleaning the contacts on both lens and camera doesn't work, maybe you could try getting some aluminium foil in between the two contacts and see what happens.
11-17-2008, 03:47 AM   #6
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hope you'll get it working, good luck...
btw: this is one of my top of the shopping list lenses at the moment. So if you'll decide to get rid of it, send it my way please
11-17-2008, 06:20 AM   #7
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QuoteOriginally posted by Miserere Quote
Well, this is definitely an AF lens.

Given what you're reporting, the problem does seem to be electrical. If cleaning the contacts on both lens and camera doesn't work, maybe you could try getting some aluminium foil in between the two contacts and see what happens.
Alimuminum or other metallic "shim" you place between body and lens, will only short whatever contact is covering, cancelling all information. If you look very closely to mount contacts (body or lens side) you will notice this contacts are isolated and they need to be like that to work properly. Shimming the mount at contacts will only worsen the case and possibly mess up the contacts.
11-17-2008, 07:29 AM   #8
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I've tried all that has been suggested, but to no avail. I did notice, however, that the lens contacts did not seem to be as "stiff" as other lenses. My current guess is that there is a spring or tensioning device that pushes the contacts up and out of the mount. This device is probably either weakened or loose. So, time to pop the hood and take a look. I'll let you know what I find, although it may be a day or two before I get to it. Thanks again for your help.

11-17-2008, 12:26 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
Also, make sure this is a pentax KAF mount not a KF mount. Pentax came out with a little known non compatible to the present electrcial contact mount for AF in about 1980. post a photo of the back of the lens showing the lens contacts and the lens locking pin slot.
There was only one KF mount lens ever produced, a 35-70. It is quite noticeable because the focus power was from in lens batteries, and the battery housing is very noticeable. Some, including me, think that the weight of the lens was a negative sales point and contributed to the lack of enthusiasm for the system. As far as I have heard, the lens was good optically.
11-18-2008, 07:55 PM   #10
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Well, I went under the hood and came back in one piece. So did the lens, but lens surgery isn't for the faint of heart.

The lens works as well as before - and absolutely no better. For those of you who are interested, I should have taken pictures but perhaps I can describe what is under the back plate. First the five screws holding it down come out, then you carefully lift off the back plate. Experience now tells me you should be holding the lens pointing up and to the left (I'm right handed) with the contacts at 12 o'clock. The plate should be removed downward. Each of the five contacts consists of a solid pin in a plastic insulating ring which is connected to its own separate, teeny, tiny spring. The spring goes into a hole, under which is the circuit continuation for the AF, etc. contacts. Also in there is, of course, the aperture ring - which wants to jump off. I cleaned everything with solvent and put it back together. The aperture adjustment is very smooth now, but the camera still doesn't recognize AF with this lens. My guess is that the circuit board which is in the lens has probably gone bad, meaning that the lens as an AF is a goner from a practical standpoint. Or perhaps I forgot to adjust the dilithium crystal resonating matrix?
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