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11-16-2008, 01:15 PM   #1
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Pentax M50 1.7 rebuild

Last week I got a dismantled Pentax M50 1.7 lens to add to my Takumar collection. The previous owner dismantled it for cleaning and did not assembly it back. Luckily he did not dismantled the focusing, only the glass. I checked the lens and it is complete; the glass is in very good shape.

After rebuilding it, I have a bit of a problem that I do not manage to change the aperture while mounted on my K100D. On the camera the aperture diaphragm remain constant at 1.7 regardless of my action on the aperture ring. The lens do not close.

Out of camera I can change the aperture by rotating the aperture ring and work very nice. I looked in the camera that there is lever that keep the aperture wide open. I wonder how this is supposed to work?

With the converter and the M42 lenses I have no problems. I set the camera to allow aperture priority and put it in M or Av mode. As this is my first M lens, can you please help me with some suggestions on what I could do wrong?

Thank you.

11-16-2008, 01:27 PM   #2
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If I got you description right this is the normal behavior. You have to pull the DOF function on the on/off switch to close the aperture. If this does not close it your lens has a problem.

To take a picture you will have to meter before pressing the shutter. For this set the camera in M mode and press the green button (or AE-L?) after setting the aperture. The camera will stop down take the reading and will open again.
You also have to set the custom setting "using aperture ring" to "allow".
I hope I remembered everything- otherwise there is a chapter in your handbook which describes this.
11-16-2008, 02:35 PM   #3
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Your brave, I would never dream of taking lens apart or putting them together, it's all a bit like brain surgery for me.
11-17-2008, 03:58 AM   #4
axl
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let us know how did it end,
good luck

11-17-2008, 10:37 AM   #5
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Original Poster
Thank you!

Is working I have to clean it better but on minimal number of screws it worked very well.

This way I learned something interesting. If the lens is little over exposing this can be adjusted by slightly rotating the glass assembly in the metal body before fastening the three retaining screws. The disadvantage is that the highest apperture number is affected.

I will post some picture taken with it after I finish cleaning and assembly
11-17-2008, 04:54 PM   #6
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QuoteOriginally posted by dragnb Quote
Thank you!

Is working I have to clean it better but on minimal number of screws it worked very well.
Try scotch magic tape for the cleaning of the lenses. Just put it on and peel it off. For me this is the only method to get all the dust off. Use a fresh batch which was not exposed to sun or fatty fingers, otherwise there might be some residues left behind at the edges of the tape. This is only to remove the dust which is left behind from cleaning not to clean itself.
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