The Carenar 50mm f/2.8 with M42 screw mount was a cheap Japanese prime lens made by Cosina Co., Ltd. with a two blades iris.
Unlike other M42 lenses with automatic aperture control pin, this Carenar can’t be adapted by a ring that keeps the control pin pressed because the pin can be full pressed only if the aperture ring was set to f/16.
On the other hands, if the aperture ring was set to f/2.8, the pin cannot be pressed at all.
Some M42 lenses have an AUTO/MANUAL switch that leaves the pins free to move, but the Carenar 50mm f/2.8 doesn’t have that feature.
The mod consists of replacing the spring between the pin and the lever and unhooking another little spring.
This is the original mechanism. We need to unhook the spring “1” and replace the spring “2” with a larger one.
In the original mechanism the small spring hits the washer of the pin. The new spring needs to be larger than this washer so it can be fitted between the bottom of the barrel and the little lever.
We can get the new springs in a retractable pen.
We need to cut it.
And fitting between the bottom of the barrel and the lever using a little washer on the top of the spring.
If the mod works, the lever that moves the aperture blade is kept against the barrel walls by the new spring and not in the center of the barrel.