[quedecree]

Hi guys,

Just got a s/h Tasman 135mm 2.8 lens - looks like it's the M42 screw mount with the bayonet adapter. Will this work with the K50? First attempt I didn't seem to be able to get the aperture to close (even with the green button that works with an older pentax smc 50mm lens). If I cannot automatically trigger the aperture mechanism, can I take the screw mounting plate off and somehow jam or lock the pin down to make it a fully manual lens? Cheers - J

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[boriscleto]

Does the lens have a switch that is marked A/M or Auto/Manual?

[quedecree]

No switches....

[VisualDarkness]

That little pin probably need to be pushed down for the lens to stop down.

---------- Post added 01-08-15 at 08:59 AM ----------

A M42 lens won't stay open when changing the aperture like a K-mount lens, so the green button won't have to stop the lens down. The bad part is that the viewfinder gets dark when stopping down.

[Giklab]

The K mount works differently than m42, so there is no way for your camera to control the aperture. You seem to have got one of those m42 lenses with no A/M switch, which needs either glue or surgery to work "properly".

[quedecree]

OK, so I need to permanently keep that little pin pushed down. Doesn't look like enough metal to the side of the pin to put in a grub screw.... perhaps I could pop the pin out, shorten it, then tap the top part of the hole for a small grub screw to hold pin down....a little fiddly but possibly doable. Any suggestions?

[Al_Kahollick]

I'm not entirely sure about this, so maybe someone else can weigh in........... perhaps you need the adapter with the flange???

[Giklab]

One way to do it: shorten the pin, then take a small screw and round out the +. Put the screw in from the inside and rest the pin in the indentation.

[Not a Number]

They do make focus to infinity adapters that has an internal flange to push the pin in. Another poster said he had a problem with the one that B&H Photo is currently selling until he ground off the flange. Your Mileage May Vary.

If you search the forum there are numerous posts with various solutions.

[sterretje]

OK, so I need to permanently keep that little pin pushed down. Doesn't look like enough metal to the side of the pin to put in a grub screw.... perhaps I could pop the pin out, shorten it, then tap the top part of the hole for a small grub screw to hold pin down....a little fiddly but possibly doable. Any suggestions?

Open it up and look how the internals work. Although https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/121415-fyi-...-aperture.html is not for your lens, I posted an article about the modification I did; might be of help.

[Sliver-Surfer]

Hi guys,

Just got a s/h Tasman 135mm 2.8 lens - looks like it's the M42 screw mount with the bayonet adapter. Will this work with the K50? First attempt I didn't seem to be able to get the aperture to close (even with the green button that works with an older pentax smc 50mm lens). If I cannot automatically trigger the aperture mechanism, can I take the screw mounting plate off and somehow jam or lock the pin down to make it a fully manual lens? Cheers - J

bend the pin a bit with pliers then push it in on a tabletop it should jam it open.

[quedecree]

I ended up tapping in a small grub screw and also shortening the pin by the same amount. She's fully manual now and seems to work well. Not the most elegant solution, next time I'll try the bent pin option first. The overall quality of the pictures are not a patch on the Tokina mentioned above but I actually bought it for some astro work - of course it's been cloudy since! Many thanks for all your replies.

[Sliver-Surfer]

Yeah bending the pin slightly is reversible but it's not likely to be used on a film camera again so whatever works is a good solution.