I’m sparingly using Fujinon XF 100-400mm lens for about a year. It’s great tele zoom lens for wildlife shooting, and I never had any issues with it. This zoom delivers sharp detailed shots at whole focus range and is durably designed and built. In this article I’m disassembling front area to remove dust located under the front glass element.
Fujinon XF 100-400mm has 77mm filter thread. Zooming from 100mm to 400mm is moving a lot of air in and out of front chamber. Lens has numerous sealing points preventing the moisture and dust to go inside. However intensive use will still let some dust to appear inside over time.
I purchased my lens sample used and don’t know it’s previous use history. I already had some dust amount in the front area, too little to cause any concern. But some dust was obviously located between the front glass elements, and that area is only exposed to the air only of factory assembly or during repair process. So look like my lens sample got repaired or cleaned before.
I decided to run non-destructive discovery disassembly approaching to also clean some dust there. Working with such lens requires extra caution steps to prevent optical misalignment. Otherwise complex calibration process will be required that can only be done properly with quite expensive lens servicing equipment.
First step of the disassembly is to accurately unscrew three black bolts around the front ring, and pull the ring out. It’s made of strong durable plastic.
There is a foam ring around the front glass metal frame. I’m removing it as well.
Three rectangular black film pieces are covering areas which you should not touch – eccentric mounts of the front glass frame. I’m just removing one film piece to show how that mounts look. Each of three eccentric mount position is precisely calibrated at the Fuji factory. Additionally large amount of cementing glue is applied around front glass frame to hold that position even in case of mechanical hits.
I don’t have equipment to recalibrate lens, so I can’t move forward disassembling this area to clean the dust under the second glass element. But I still can access the area under the front glass element by unscrewing the front metal ring. It has four slots for special factory key (which I don’t have). Though there is more safe way to unscrew the ring without damaging it. It’s the approach commonly used to unscrew too tightly secured lens filters. All you need is just long strip of durable leather or rubber material.
It should be tightly wrapped around front metal ring edges in the CCW direction while looking from top. Moderate pressure efort will be evently distributed to unscrew and remove the front metal ring.
It’s critically important to mark the radial position of the front lens glass element. I’m using red marker and add few marks in different edge areas.
First glass element is quite large and heavy. Fuji has high radial precision and it takes some effort to accurately pull the glass out. One of possible approach is to use some strong but removable foam sticky tape. First check that glass surface is clean, then apply few foam squares like shown below.
I’m using few plastic cylinders as a handles sticked to the foam top surface. Then I’m gently pulling the glass out. You’ll also hearing sound of the air getting sucked inside while pulling out massive glass cylinder.
Fter pulling out first glass element I can easily clean out all the dust using the air blower. It’s a pity to realize at this step that removing second element is only possible from the other side of the front lens frame. So I cant reach other surface of second front glass from this point.
Well, I’m still satisfied with being able to clean up dust located between the glass, so assembling everything back in the reverse order.
I’m matching radial position of the front glass, and screwing the front metal ring back. Then I’m removing sticky foam.
Then I’m attaching foam ring and front plastic ring with letters. Just make sure hood mounting mark is matching same line as focal length indicator. Gently center screw holes and screw in all three bolts.
It’s great to see significantly less amount of dust, that is only located under the second glass element which is typical area for the dust to appear during normal lens use.
If you ever see Fujinon XF 100-400mm lens sample with dust under the front glass area (not the second) – it clearly indicates that lens god previously disassembled after it leaved the factory. Looks like whoever serviced this lens before they did great job because it works flawlessly and deliver exact pictures as any brand new XF 100-400mm.
I’ve also tested the lens after re-assembly to make sure quality is not affected, and picture results are same : )