Converting an Auto-Aperture M42 Lens to Manual Aperture
Here is a typical M42 lens aperture mechanism inside the lens. This is an auto-aperture lens with no selectable manual mode. Such lenses are problematic with most adapters since the aperture control won't work unless the aperture pin is pressed. While most “native” M42 mount cameras can press the pin down, no modern DSLR system has this capability. Some special-purpose adapters will permanently press the pin in when the lens is mounted, but they are incompatible with some M42 lenses having protruding parts on the back.
I have converted this lens to a manual aperture lens by permanently pressing down the aperture pin with a piece of plastic cut from a cotton swab. The hollow piece of plastic is about 2 mm long and fits precisely over the aperture pin inside the lens, effectively holding it down.
When re-assembling the lens, take care to align the hook on the aperture lever with its counterpart, otherwise the aperture ring won't work. Typically this is best done with the widest aperture selected. Anyhow, first make sure that that part is aligned, then rotate the rear as required to align the screws with their holes.
After modification, the lens works with all apertures, stopping down to the selected aperture immediately when the aperture ring is turned.
On some other lenses it may also suffice to disable a mechanism (usually a single spring) holding the aperture open until released by the pin. This approach should only be attempted when certain of how the mechanism of that particular lens works; some lenses need the springs to close the aperture. Personally I've only done it when it would have required considerable disassembly to reach the pin for the plastic tube -modification. With some lenses (notably the MC Zenitar 16mm fish-eye) the simplest solution is to disassemble and remove the auto-aperture mechanism altogether.
(The pictured lens is a Fujinon, I have modified several different kinds of them thus.)