A little while ago, I wrote about fixing the zoom ring on the Tokina SD 28-70mm lens that I got with a parts camera. Fixing the focus ring took a little more effort, and it needed to be done during the day so I could test infinity focus with a distant object, but it followed roughly the same principles.
As with the zoom ring, the focus ring was held together by scotch tape as suggested in this MFlenses forum post. The scotch tape’s adhesive had degraded to the point it was a slimy mess. However, the focus ring’s function is a little different. Below the focus ring rubber is a join between two parts: the rearmost is a metal ring that bears the distance markers and the focus stops, and the foremost is a metal ring that forms part of the front lens group’s mounting (on this lens, the front group rotates when focusing). The ring with the markings can come away from the front ring and move a considerable way down the lens barrel, which lets the front group move freely. Calibration of focus depends on sticking the two rings together in just the right alignment, ideally aligning the infinity marking with the correct focus stop when the lens is perfectly focused at infinity.
Because the ring with the markings can move a long way down the lens barrel, the scotch tape adhesive had a lot more scope to get into the wrong places. It soon became evident that I needed to remove the front element to clean it all up. And it’s just as well I did — it seems that someone had attempted this repair in the past, as there was a great big fingerprint on the rear lens element of the front group. I hadn’t noticed this when inspecting the lens optics, but I was grateful for the opportunity to clean it up.
Once cleaned up, I reattached the front element (I don’t think I got the alignment correct, but as the front element rotates on this lens I don’t think it matters greatly), then put it on a camera and went outside. I focused the lens on an apartment block about 10km away by rotating the front element directly, using the camera’s split image to get a decent focus. Then, I aligned the markings ring with the infinity focus stop and used a small piece of scotch tape to fix it in place. Then I checked by focusing on closer things then back to the apartment block, and also by focusing at 28mm zoom instead of 70mm. I got it as close as I could, erring beyond infinity slightly if anything.
I fixed the ring in place more securely with two more pieces of scotch tape, then put the focus ring rubber back on. This one hadn’t stretched like the zoom one had, so it didn’t need any padding.
With both rings repaired, the lens is now basically back to normal. It’s not the most amazing of lenses, and it has a strange rotational feeling when taking a photo (possibly the aperture mechanism is a bit out of alignment), so it might need some further work. However, it’s much more usable than when I got it, so I count it as a win so far.