Canon 300mm FD - Stiff focusing ring

[gary_tagg]

Hi folks,

I've just bought a Canon FD 300mm F4 and the focusing ring is very

stiff at slow speed. I'm looking for help/instructions on how to

take the lense apart so I can clean and lubricate the Helical.

 

Thanks,

 

Gary

[alan_swartz]

Gary,

 

I've never been that far into one, but you'll find tiny set screws under the rear of the hood and under the focusing grip. The lens threads apart in sections from the front. At the rear, the mount is removed like most other New FD's.

 

I would offer NO GUARANTEE that the lens will function properly after the front groups are reinstalled. The distance between assemblies must be correct. It's not a matter of just screwing them down tight.

[mark_rossano]

First, if I recall correctly, all of the Canon 300/4 lenses were internal focus designs. That being the case, there is no "helical." The focus is driven by a cam slot (or slots) in the focusing ring mechanism that brass bushings attached to the rear (moving) group ride within. The lens body has straight, axial slots that the bushings pass through to engage the focusing ring cam slots. You may be able to correct the sticky focusing without major disassembly, but you have to at least get to the point where you can inspect the various moving parts to see why it's stiff. I've had in past times several Canon lenses with this type of focusing system and stickiness is usually an indication of physical damage or derangement, not lubrication problems. There just aren't the kind of large bearing areas or close tolerances that a standard helical has. In fact, most of these IF lenses have clearances that are quite large; almost sloppy in my opinion!

 

Having said all that, don't go removing the lens mount mechanism. That's not necessary for focusing mount service in most cases, and on these longer lenses in particular, it can be a royal PITA to put it back together, particularly on the New FD (bayonet-style) mounts.

 

As for front group reassembly, I would dispute the previous poster's assertion. Unless you actually disassemble the front cell for cleaning (not recommended, and never required, in my experience) rather than simply removing it as a unit, it *IS* simply a matter of screwing it back in the way it came out. There is no provision for adjusting the position of these components in relation to the rest of the lens body or the element in front of the focusing group.