Recently, I brought my medium format Bronica S2a camera and a 135mm f/3.5 Nikkor-Q lens with me to a lake to take some landscape photos. The waist-level viewfinder that I use with this camera requires as much light as possible to make focusing easier. So it doesn’t matter which aperture is set on the lens, the camera always keeps it wide open and only closes the blades when taking the photo or while the depth of field preview button is pressed.
At some point, a scene in the viewfinder appeared much darker than it was. I looked at the lens and saw that the blades were stuck at f/8. After turning the aperture ring to f/3.5 the blades opened. I set the ring to f/5.6, pressed the depth of field preview button, and the lens closed the aperture but didn’t open it back. This lens is quite new to me, but I didn’t recall it behaving that way last time. I thought maybe the grease has dried up, or it’s just due to cold weather. Until the end of the photowalk, I had to manually turn the aperture ring to f/3.5 after every shot to be able to focus.
At home, I detached the lens from the helicoid and started investigating how a fully mechanical camera communicates aperture settings to the lens. The way it is engineered is so brilliant and simple that I was able to fix the mechanism without disassembling the lens.
The rear end of the lens has a small lever that pulls the blades. Depending on the position of the aperture ring, the lever path becomes longer, so the blades can be pulled farther. When the aperture ring is set to f/22, the lever goes to the end and pulls the blades to the maximum, leaving only a small aperture opening. It seems that inside there is a spring that pushes the lever back and opens the blades. When the shutter or DoF preview buttons are pressed, the camera moves the lever to the maximum path allowed by the aperture ring to make the aperture narrower.
The lever is connected to the aperture mechanism via the screw marked with the red square. In my case, the screw was too tight, and it was preventing the lever from being pushed back. I used an SL2.5 flathead screwdriver to slightly loosen the screw. After that, the blades started to jump back when the lever was released.
Now my lens is as good as new! The film from that photowalk isn’t developed yet, so there are no photos to share this time.