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LENS MOD: Canon FL 135mm f/2.5, Conversion to Nikon F Mount

Canon FL 135mm f/2.5, Convert to Nikon F Mount
Canon FL 135mm f/2.5, conversion to Nikon F mount

Another lens for my project -- the Canon FL 135mm f/2.5. Canon made their FL lenses with a short lifespan from 1964 up to 1971. Then followed the FD mount, which is like the FL but with extra mechanical features.

Canon FL 135mm f/2.5, Conversion to Nikon F Mount
Image from dpreview

Nowadays, there are Canon FD/Nikon F adapters with optical glass that allow the lens to focus to infinity. But despite the high quality multicoated glass, my experience says that it has obvious signs of image degrade, darkens the lens by 1-stop (if you're at f/2.5, it becomes f/4, at f/4 becomes f/5.6). So particularly for Nikon DSLR users like me, this mod is the only path to go if I want to mount this lens on my D5200...


Figuring Things Out...

Canon FL has 42mm (not the new EF/EF-S), while Nikon F mount has 46.50mm. Now, to figure how to convert the lens into a Nikon mount. First, we need to remove the breech-lock ring. Then take care of the metal protrusion by sawing it off so we can glue the new Nikon F mount to the lens. 

Canon FL 135mm f/2.5, Conversion to Nikon F Mount

I tried several ways to make the lens focus to infinity, but did not work. In the end, the only way to do it was to glue the entire rear group of elements on top of the ring that holds the lens into the focusing helicoid (see below). The lens now focuses to infinity at the 7 meters mark, then beyond infinity afterward. There is no other way for me to fix that. Beyond infinity is still better than no infinity.

This ring holds the focusing helicoid and the lens together.
Removing it will split the lens into 2 parts (below).
Now we can examine better what's inside.

Next thing to do is to disable the auto diaphragm feature. If we don't, the iris will always stay open. Once we figure which mechanism to remove, then the lens will become 'manual stop-down aperture'. In other words, the iris will close in real time together with the f-stop that you choose.

It would be nice if we can add an extra f-stop between f/2.5 and f/4,
because we want the lens to be wide open,
but to close down a bit for better sharpness.

The glass in this lens was clean, but the front/rear elements could benefit from better contrast and flare control if I would blacken the edges. I also HAD TO do this, because the rear element had permanent scratches and faded coatings, which may contribute to the glow at f/2.5 when shooting outdoor (I mean duh, who uses such wide apertures outdoor? This is just for test.)

The last section is figuring to install the new Nikon mount in place -- if I position too far, it would not focus to infinity, positioned too close, focuses too much beyond infinity. At first, the plan was to permanently remove the chrome breech-lock ring so a 55mm reverse ring would be glued in its place. I was having doubts whether it would hold the lens firmly in place.

On the good side of things, the 55mm reverse ring took forever to arrive. An alternative plan came to mind. There was a 49mm reverse ring lying around unused for a long time. So I imagined putting back the chrome breech-lock ring so it would retain the original look, then glue the 49mm reverse ring over its cavities. Just a try... but it was a perfect fit. Hence, this was the solution.

It's been a week afterward, and the glue is able to hold the weight of the lens even when I'm holding just the lens on my hand and the camera hanging on it, as my quality test. So there, I have peace of mind now that nothing is rattling or loose. It's ready for battle!

Canon FL 135mm f/2.5, Conversion to Nikon F Mount


The Results...

Very pleasant at f/2.5, but given the long focal length,
you can easily get razor thin DOF so you will
need to stop down to f/4 anyway!

Super razor sharp by f/4

At f/8, I can still easily get
blurry backgrounds for shooting portraiture.

UPDATE! Read my feature here:

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