Hi,

I have 2 Pentacon auto 1.8/50mm lenses (for p-thread camera's like my zenith-e's and Praktika's)
These 2 have exactly the same problem. The diaphragm sticks.
With these lenses a pin that sticks into the body of the camera pushes a lever with a spring (or at least spring characteristics) that pushes the pin that opens or closes the diaphragm blades.
When the the photo is taken, everything springs/falls back to the starting position and the diaphragm opens up completely. And that's the point where it becomes sort of stuck.
The pin is pushed in by the lever in the body, but then the lever/spring construction isn't strong enough to open the diaphragm blades again.
Once past this 'wide open point' everything works smoothly until everything falls back to the starting position.
What can be done about this ? is it a familiar problem ?

also.. when i tried to unscrew 1 of the lenses from the body, the outer focus ring became loose.
It no longer 'drives' the focus mechanism. Can that be fixed ? or can that only happen if something breaks off?

thanks in advance.

Mathew
http://www.webalistic.com/cams/

By Stuart Willis on Friday, March 29, 2002 - 10:56 am:

Mathew .....

The Praktica pin-type auto-iris mechanism is one of the simplest. The rear ring of the Pentacon ƒ1.8 and Meyer Orestor (same thing) is held by crosspoint screws and slips off easily to expose the mechanism. You can then activate the mechanism very carefully and slowly so as to pinpoint any binding. Clean and lube pivots and interfaces. (just a microdot of lube. No more).

The activator pins on the mentioned lenses are not very robust and seem to bend more easily than most other lenses of the same configuration. There is a known pin-bend problem associated with use of auto-iris lenses on the Zenit model which is not designed for auto-lenses. Indeed - some Pentax lenses when screwed on a Zenit become irremoveable.

The first thing to check is that your lens activator pins are perfectly straight and perpendicular. If the hole in the lens aluminium backplate has become enlarged - possibly due to a bent pin over a long period - you may need to install a marginally thicker pin so as to stop the side-play and resultant jamming/binding.

Of course - there are other possible causes of the described problem - not least of which is the iris activator ring integrity. But that's for another time.

By Mathew D on Tuesday, April 16, 2002 - 03:38 pm:

Thanx for your reply. I'll have a go at one of the lenses to see if bent parts are the cause or maybe the solution.