The Sigma 50-500mm f4-6.3 EX APO RF HSM is a pretty decent lens for the money. It provides a reach that is generally inexpressible without a huge investment of 5 to 10 times it’s cost in equipment. Recently, it was necessary to repair one. The issue was a malfunctioning focus. Manual and auto both were being impeded at their far range. It was rather maddening because, first it worked, then it didn’t, then it did, then it didn’t. I wasn’t about to invest any money in this lens, so, I decided to fix it myself. At least, I was going to give it the old college try.

I think I spent more time looking at this lens, trying to figure which screw to pull first and next than actual repair. It was a tricky little devil to start out. It’s also maddening how little information is available for doing such work. Nothing on the web for free or for sale. You would think some industrious souls would be selling schematics or manuals but I guess maybe the manufacturers probably don’t want that sort of thing getting around.

So here’s a couple photos of the process. Fortunately, I didn’t have to go too far into this puppy to figure out the problem and fix it.

First up is a shot of the mounting plate. It’s straight forward enough to remove the obvious screws, but that’s not the way. In the sides of the vertically oriented sections of the mounting flange are itty (and I do mean itty) bitty screws. You can probably see the the black piece in the center which is obviously disjointed. There are four screws. Two hold the round piece and two hold the contact plate in place. Remove them all.

Once that’s done then remove the mounting flange retaining screws. That will allow the aperture selection ring to come loose. Remove it and note that there are some (5 in my case) shims underneath it. I’m not sure if sorting order of the shims is important, but keep them in order just in case. They are flimsy and will interweave with each other if you don’t keep them tidy and in order. There will also be a long metal arm which works the iris of the aperture. Be sure to note that it has to be reinstalled into a slot down inside the barrel.

Now you have exposed the entire circuit board and you will notice there are three screw heads visible in the outer most diameter. These are the next to be removed and will facilitate the removal of the upper most part of the lens barrell.

A note about screws in general, and specifically, about screws within a lens. You MUST BE EXTREMELY DILIGENT when attempting to loosen and tighten these screws. It it more than likely that they have had a thread locking compound applied and are GLUED in place. Further, it is imperative that you use the appropriate driver bit. There are many different sizes, and use of an inappropriate driver bit will result in a stripped head. Another source of a stripped head will be allowing the driver to slip within the head while applying heavy pressure.  Make sure you have good contact and good bearing.  Don’t ever go at a screw in any manner other than square to the head.  I don’t want to think what sort of grief that might cause. So, BE SURE and apply a good amount of pressure.

Here is the lens with the upper portion of the barrell housing removed. Once removed it will reveal the focus gearing mechanism. It consists of several (an inner and outer) ring gears and several smaller drive gears.

The manual focus ring is just below the arrows and is still operable at this stage. I worked it around a few time to see if I could find a problem, but didn’t notice anything obviously out of whack. I didn’t really want to tear into this thing any further unless I had to, so I went and got an halogen lamp and looked very carefully around and, low and behold, I found the problem. There was a loose screw floating around in the mechanism. Next was to find out where it had come from.

It didn’t take long.  It was pretty obvious, though I had to rotate the focus mechanism to reveal the spot.  There are holes in part of the workings and there is no other reason for their existence than to provide access to a screw.  Here’s the empty threads.  That’s the same hole in the photo above.

Now I can head to the mountains and give this baby a workout at the elk rut, where it failed last year.