I recently discovered a large flake of dust behind the front element of my Canon FDn 50mm f/1.4 lens that I purchased off eBay awhile back. While I understand it's normal to find some speck of dust inside the lens of this age, this particular dust is just too hard to ignore. I set out to Google some information on how to remove the front glass and quickly learned from various forum postings that front ring on the "New" FD lens series is not screwed-on but snapped-in instead. Since I couldn't find any pictures of the procedure, I thought it might be interesting to document my attempt here.
|Canon FDn 50mm f/1.4|
Disclaimer: Please do not attempt to disassemble your lens if you are not confident about your ability. Always use common sense and take your time. Do not hold me responsible for your own mistake.
|Flat blade, Weatherstrip adhesive, Precision Phillip screwdriver.|
The front ring is snapped into the groove on the filter thread barrel. There are six tabs behind and along the circumference of the ring. Three of them are snap type and the rest are simply straight. Every tab is glued with some sort of adhesive. To dislodge the tabs and break the adhesive bond, carefully insert the flat blade between the front glass and the plastic ring. Slowly wiggle the blade to work it into the gap. Make sure your blade is not scratching the glass.
|If done properly, the blade should not scratch the glass.|
Once the blade is fully inserted, gently pry the blade away from the glass and you should hear a small popping sound from the dislodging of the snap tab.
Next, pull the blade back out and repeat on another adjacent location.
Work your way to the next adjacent location.
Continue on to another location.
As you progress, each successive pry should lift the ring out more.
Eventually the entire ring will fully dislodge.
To remove the front glass, the threaded barrel needs to be removed. Use the precision screwdriver to remove the three screws.
Remove the threaded barrel and the front elements should drop right out. If I'm not mistaken, the first three glasses are glued together as a group. In my case I was able to clean my problem speck here. If you open up the aperture, you can also access the glass behind it for cleaning from this stage of dis-assembly. There are three more screws to remove from this point if you want to remove the focusing ring. You will be on your own if you choose to continue. I stopped here as I accomplish what I set out to do.
|Use a micro fiber to catch your lens|
As you reassemble the lens, pay attention to the alignment of the three screw holes. It was designed to align one way only.
|Green arrows indicate the holes are properly aligned.|
If aligned improperly, two of the the holes will not match.
|Notice the lower two holes are not aligned.|
Do not over tighten the screw. Before reattaching the front ring, apply a small amount of weatherstrip adhesive on every tabs. Don't use too much, otherwise it gets very messy when they ooze out as you snap the ring back on to the threaded barrel.
Use a toothpick to rub off any glue that ooze out.
|Make sure the filter thread is free of any glue.|
The weatherstrip adhesive doesn't permanently bond the ring. If you ever need to remove the ring again, you'll find it break apart pretty much like the factory adhesive.