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01-28-2011, 02:07 PM   #1
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How to stop down Tamron adaptmatic lenses with M42 mount

Recently I purchased a tamron 105/2.5 adaptamatic(I think) lens with an M42 mount, which is shown in the picture below. I could mount it on my k-x with the original pentax screw mount adapter and use it in M mode with the green button metering. But to stop down the lens after setting the aperture on the aperture ring I have to 'hold' the A/M switch at A position. If I leave the switch it comes back to the M position and the lens becomes wide open. The only other M42 lenses I have seen are Super/SMC takumars and there the A/M switch remains at whichever position I keep.

Unlike in the case of takumars, here the 'pin' that moves the aperture blades does not stay depressed when the lens is mounted. But once I press the pin the lens stops down properly even if the A/M switch is at M. So one possible fix could be to glue the pin at its depressed state.

I wonder if this is the expected behavior of this lens or if something is wrong with the A/M switch. Or do I have to use any other adapter to make it work?

Please help me out.

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01-28-2011, 02:26 PM - 1 Like   #2
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please see the following image I posted previously

although an adaptall II mount, the same idea applies



I used the end of a ball point pen refil, about 1/8 inch long, slit on one side to easily slip over the pin

this is the best way as it can be undone if needed
01-29-2011, 12:14 AM   #3
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QuoteOriginally posted by Lowell Goudge Quote
please see the following image I posted previously

although an adaptall II mount, the same idea applies



I used the end of a ball point pen refil, about 1/8 inch long, slit on one side to easily slip over the pin

this is the best way as it can be undone if needed
Thank you Lowell Goudge, So you mean that the lens functions as expected. I will right away start searching for a ball pen refill for the reversible fix.
Thanks,
Sunil
01-29-2011, 06:03 AM   #4
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It will work just fine stopping down as you turn the aperture ring

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