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Fixing the Stuck/Jammed Zoom on a Pentax/Tamron 18-250 Lens

I suspect this is a somewhat common problem, potentially affecting more lens models and brands in a similar way. I have found a few online posts reporting the same symptoms with this lens. Fortunately, the fix is quite easy, and requires very little mechanical skill to accomplish.


The zoom mechanism on our SMC Pentax-DA 18-250mm F3.5-6.3 ED AL [IF] (apparently made by Japanese lens manufacturer Tamron, who sells it as the A18) lens recently stuck in its widest position. Experimentally working the zoom ring while orienting the lens in different directions would eventually result in the lens releasing. Sometimes, the freed zoom would work normally for many cycles. Sometimes, it would immediately jam after one trip back to full wide position. Sometimes, the freed zoom moved with some abnormally high drag. But the lens would eventually jam again at the full-wide end of the zoom range. Working the zoom lock switch at the circumference of the base of the lens made no difference in the symptom (and the lock still worked as a lock).

THE SOLUTION (the short version)

Tightening a single screw which had worked itself loose solved the problem. Accessing the screw requires only removal of the rubber grip around the zoom barrel. The whole procedure can be accomplished with a single small Phillips screwdriver in less than 5 minutes.


Unfortunately, my failure occurred while attending an air show, where I had little need for the lens at 18mm, rather shooting flying aircraft most of the day at the full 250mm.

If I fiddled with the zoom assembly constantly, it would eventually, but not predictably release. But my habit when carrying the camera is to immediately zoom out to “park” position and lock the zoom as I’m dropping the camera to my side. So as hard as I tried, I accidentally got the lens stuck in wide several times during the remainder of the event.

My initial reaction was that it might be easier to just replace the lens than have it repaired. My research revealed several more interesting “superzoom” lenses which might provide better performance (it’s slowish, not that sharp at longer focal lengths, and autofocus speed on a K-3 is so-so).

I found a low-resolution PDF of an exploded assembly diagram of a Tamron A18 lens in this Pentax Forums post (see post #11). It was too crude to get real information, but gave me some hints as to the location of some internal components.

Instead of going to sleep at 3am, I started to disassemble the lens to see if I could find an obvious problem. As it turned out, I did a LOT more work than necessary. The actual fix is very simple, and can be accomplished in a matter of minutes.

After disassembling not quite half of the lens from the mounting plate end forward (but before ever moving an optical element), I realized that there would have to be fasteners accessed through the side of the moving zoom barrel.

Removing the rubber hand grip from the zoom barrel revealed a pair of apertures for accessing internal fasteners. Turning the barrel through the focal length range revealed a total of three screw heads accessible through the two openings at different barrel positions.

When I attempted an exploratory loosening of one of the screws, I found it loose. Immediately, I realized I’d found the culprit. The screw passes through a small collar, at it was obvious that the collar and/or screw would interfere with the access hole in the zoom barrel if they extended out from their designed location. Even if the screw never moved further, the collar could sometimes drift outward to lock itself into the access hole.

I was relieved. There was nothing else to do but tighten the screw and put the lens back together. I hope I’ve saved you the trouble of unecessarily disassembling the lens as I did.