Canon FL 50mm f1.4 - the ignored radioactive fast 50?

There seems to be some confusion regarding how many versions of this lens were produced - maybe someone more knowledgable can clarify?

I have an early version (serial # 144xx) which I believe is a 6/4 design from 1965 as per this info from the Canon Museum. It is thoriated with characteristic yellowed glass element(s), as tested here.

It's a compact lens and has an Auto/Manual ring at the front, close to the aperture ring, but there also seem to be some later lenses (marketed from September 1966) which aren't radioactive and apparently use a 6/5 optical design. These lenses look identical to my version and are also featured on the Canon Museum website, referred to as Version I.

Then there is Version II which seems to be a 7/6 optical design, the body is longer/taller and has the auto/manual ring at the rear, close to the mount. The name ring actally says 'II' on it, although the latest copies of this lens don't have 'II' on the name ring.

So, there are 4 versions of this lens, if you count the latest lenses with the amended name ring as a separate version?

It seems strange that there's almost no mention anywhere online of the first version (as opposed to Version I ;-) ) being a radioactive lens... like that detail has been mostly forgotten.

Some info from other forums suggest that the Canon Museum info is wrong and that there never was a 6/4 design...

Any thoughts?

No takers? Shucks.

I'm not sure exactly which oddball Canon 50/1.4 I have and I don't recall where I put it yet (a more apt username for myself would be Too Many Fifties), but I believe I've got an older one somewhere with a much smaller front element than the nFD, SSC, and chrome-nose versions, and I'm at least fairly certain it's an FL lens. If/when I can find it I'll see if it is any of the lenses you're discussing. The FL II looks to have an oversized front element much like the chrome-nose and SSC 50/1.4s, which makes sense from a lineage standpoint.

EDIT: Found. Mine is serial 52656, has an A/M ring after the aperture ring, and I confirmed it is a 6e/5g. Is this a "I"? The Canon Museum entry doesn't show the A/M ring, at least the ring is not labeled as such... and after playing with the focus a bit I see why I tucked it away in a projects bin. ;-) No visual indication that this one is radioactive.

I would imagine the Canon Museum entries are correct. Regarding whether your lens is 6/4 or otherwise it should be fairly straightforward to count the reflections to determine the elements/groups count. If you're already familiar with the method, ignore the following :-D :

In a dark room, turn on a single light source. I prefer a plain diffused white bulb, and it may be polarized. With the light bulb at your back, look through the lens and count the reflections of the bulb you can see in the optic as you move it around every which way.

It may help to get counts from both the front and the back, and/or with the lens stopped down to split up the work and thus make the work more manageable.

A 6/4 lens would have 12 (6ele*2) surfaces but with only 10 visible reflections where two of them would be dim/color-neutral(uncoated)/shimmering (colorful wavy interference patterns) as you move the glass around which implies those are each reflections made at two cemented surfaces.

SimonOL said:

There seems to be some confusion regarding how many versions of this lens were produced - maybe someone more knowledgable can clarify?

I have an early version (serial # 144xx) which I believe is a 6/4 design from 1965 as per this info from the Canon Museum. It is thoriated with characteristic yellowed glass element(s), as tested here.

It's a compact lens and has an Auto/Manual ring at the front, close to the aperture ring, but there also seem to be some later lenses (marketed from September 1966) which aren't radioactive and apparently use a 6/5 optical design. These lenses look identical to my version and are also featured on the Canon Museum website, referred to as Version I.

Then there is Version II which seems to be a 7/6 optical design, the body is longer/taller and has the auto/manual ring at the rear, close to the mount. The name ring actally says 'II' on it, although the latest copies of this lens don't have 'II' on the name ring.

So, there are 4 versions of this lens, if you count the latest lenses with the amended name ring as a separate version?

It seems strange that there's almost no mention anywhere online of the first version (as opposed to Version I ;-) ) being a radioactive lens... like that detail has been mostly forgotten.

Some info from other forums suggest that the Canon Museum info is wrong and that there never was a 6/4 design...

Any thoughts?

Click to expand...

Here is the oldest one that I have. Super-Takumar with a serial number of 1386800. Looks like I need to do the UV treatment. Is it better to shine UV light through the front or through the rear of the lens?



The location of the IR to the right of the 4 is usually the we are told to look for.



Now this is a later version. But don't know which later version. Super-Multi-Coated Takumar SN: 5375402



This time, the IR mark is to the left of the 4.



Both of these lenses show have the rear element protruding out the back end when focused to infinity.

I also have a 55mm f1.4 SN: 4708433 Don't know if this is considered another version, or an entirely different lens? The rear element does not protrude out the back when at infinity,



And the IR mark is now the letter 'R'

My opinion (only an opinion mind) is in agreement with that found on other forums, that there maybe a mistake on the Canon site. Nobody seems to be able to produce the lens version in question, and surely plenty of these have been disassembled.

I also find it strange that Canon would mark the third version of an FL 50mm F/1.4 as version II if it was the case that it was not the second version of an optical arrangment.

But you never know, it may be the case after all, and information may eventually surface one way or another.

JimH123 said:

SimonOL said:

There seems to be some confusion regarding how many versions of this lens were produced - maybe someone more knowledgable can clarify?

I have an early version (serial # 144xx) which I believe is a 6/4 design from 1965 as per this info from the Canon Museum. It is thoriated with characteristic yellowed glass element(s), as tested here.

It's a compact lens and has an Auto/Manual ring at the front, close to the aperture ring, but there also seem to be some later lenses (marketed from September 1966) which aren't radioactive and apparently use a 6/5 optical design. These lenses look identical to my version and are also featured on the Canon Museum website, referred to as Version I.

Then there is Version II which seems to be a 7/6 optical design, the body is longer/taller and has the auto/manual ring at the rear, close to the mount. The name ring actally says 'II' on it, although the latest copies of this lens don't have 'II' on the name ring.

So, there are 4 versions of this lens, if you count the latest lenses with the amended name ring as a separate version?

It seems strange that there's almost no mention anywhere online of the first version (as opposed to Version I ;-) ) being a radioactive lens... like that detail has been mostly forgotten.

Some info from other forums suggest that the Canon Museum info is wrong and that there never was a 6/4 design...

Any thoughts?

Click to expand...

Here is the oldest one that I have. Super-Takumar with a serial number of 1386800. Looks like I need to do the UV treatment. Is it better to shine UV light through the front or through the rear of the lens?

Click to expand...

The yellow you see here is from coating reflections. Aged Thorium-doped glass will transmit yellow-tinted light.

JimH123 said:

The location of the IR to the right of the 4 is usually the we are told to look for.

Click to expand...

This means your Super-Takumar is the early 8-element variant, right? Congrats! Hold on to it, these are appreciating. Though Simon was asking specifically about a particuar Canon that may or may not exist. This one is not a Canon, nor is it radioactive. At least it shouldn't be. Again it's about the tint of the light transmitted by the glass - if images have a yellow or brown tint to them, your lens could be a bit of an oddball. Folks on the Pentaxforums entries for these lenses surmise that there are some transition-period lenses that use old external parts and radioactive glass.

JimH123 said:

Now this is a later version. But don't know which later version. Super-Multi-Coated Takumar SN: 5375402



This time, the IR mark is to the left of the 4.



Both of these lenses show have the rear element protruding out the back end when focused to infinity.

Click to expand...

The element curvature and housing will look a bit different, though. Guaranteed!

JimH123 said:

I also have a 55mm f1.4 SN: 4708433 Don't know if this is considered another version, or an entirely different lens? The rear element does not protrude out the back when at infinity,



And the IR mark is now the letter 'R'

Click to expand...

Nice Taks, Jim!

Tons o Glass said:

JimH123 said:

SimonOL said:

There seems to be some confusion regarding how many versions of this lens were produced - maybe someone more knowledgable can clarify?

I have an early version (serial # 144xx) which I believe is a 6/4 design from 1965 as per this info from the Canon Museum. It is thoriated with characteristic yellowed glass element(s), as tested here.

It's a compact lens and has an Auto/Manual ring at the front, close to the aperture ring, but there also seem to be some later lenses (marketed from September 1966) which aren't radioactive and apparently use a 6/5 optical design. These lenses look identical to my version and are also featured on the Canon Museum website, referred to as Version I.

Then there is Version II which seems to be a 7/6 optical design, the body is longer/taller and has the auto/manual ring at the rear, close to the mount. The name ring actally says 'II' on it, although the latest copies of this lens don't have 'II' on the name ring.

So, there are 4 versions of this lens, if you count the latest lenses with the amended name ring as a separate version?

It seems strange that there's almost no mention anywhere online of the first version (as opposed to Version I ;-) ) being a radioactive lens... like that detail has been mostly forgotten.

Some info from other forums suggest that the Canon Museum info is wrong and that there never was a 6/4 design...

Any thoughts?

Click to expand...

Here is the oldest one that I have. Super-Takumar with a serial number of 1386800. Looks like I need to do the UV treatment. Is it better to shine UV light through the front or through the rear of the lens?

Click to expand...

The yellow you see here is from coating reflections. Aged Thorium-doped glass will transmit yellow-tinted light.

Tons o Glass said:

The location of the IR to the right of the 4 is usually the we are told to look for.

Click to expand...

This means your Super-Takumar is the early 8-element variant, right? Congrats! Hold on to it, these are appreciating. Though Simon was asking specifically about a particuar Canon that may or may not exist. This one is not a Canon, nor is it radioactive. At least it shouldn't be. Again it's about the tint of the light transmitted by the glass - if images have a yellow or brown tint to them, your lens could be a bit of an oddball. Folks on the Pentaxforums entries for these lenses surmise that there are some transition-period lenses that use old external parts and radioactive glass.

Tons o Glass said:

Now this is a later version. But don't know which later version. Super-Multi-Coated Takumar SN: 5375402



This time, the IR mark is to the left of the 4.



Both of these lenses show have the rear element protruding out the back end when focused to infinity.

Click to expand...

The element curvature and housing will look a bit different, though. Guaranteed!

Tons o Glass said:

I also have a 55mm f1.4 SN: 4708433 Don't know if this is considered another version, or an entirely different lens? The rear element does not protrude out the back when at infinity,



And the IR mark is now the letter 'R'

Click to expand...

Nice Taks, Jim!

Click to expand...

Just hit me that we were talking Canon's and not Tak's. Once I started reading radioactive Thorium lenses, I was focused only on that.

JimH123 said:

Just hit me that we were talking Canon's and not Tak's. Once I started reading radioactive Thorium lenses, I was focused only on that.

Click to expand...

Gotcha, I can definitely empathize with you there as I've done similar things countless times. All good!

Just to re-iterate, nice Taks!

JimH123 said:

I also have a 55mm f1.4 SN: 4708433 Don't know if this is considered another version, or an entirely different lens? The rear element does not protrude out the back when at infinity,

Click to expand...

The 55mm (all variants) uses a 6-in-5 design. In my experience it renders differently to the 50/1.4s, with more even performance across the frame from f/2 on down.

-Dave-

Alun Thomas said:

My opinion (only an opinion mind) is in agreement with that found on other forums, that there maybe a mistake on the Canon site. Nobody seems to be able to produce the lens version in question, and surely plenty of these have been disassembled.

I also find it strange that Canon would mark the third version of an FL 50mm F/1.4 as version II if it was the case that it was not the second version of an optical arrangment.

But you never know, it may be the case after all, and information may eventually surface one way or another.

Click to expand...

Indeed - that seems to be the general consensus on other forums...

I'll try counting the reflections as suggested by Tons o Glass (thanks for the suggestion ).

The info regarding these lenses is very confusing. I have noticed that the earliest lenses appear to be radioactive and produce very warm tones, as you'd expect with thoriated glass. Version I lenses with a 5-digit serial number starting with around 3xxxx and upwards seem to have clear glass but the style of the body looks identical. Version II lenses are easy to identify.

My copy is currently under a particular Ikea LED lamp to see if that clears the yellowing. It's worked with other radioactive lenses so should be successful.

Tons o Glass said:

No takers? Shucks.

I'm not sure exactly which oddball Canon 50/1.4 I have and I don't recall where I put it yet (a more apt username for myself would be Too Many Fifties), but I believe I've got an older one somewhere with a much smaller front element than the nFD, SSC, and chrome-nose versions, and I'm at least fairly certain it's an FL lens. If/when I can find it I'll see if it is any of the lenses you're discussing. The FL II looks to have an oversized front element much like the chrome-nose and SSC 50/1.4s, which makes sense from a lineage standpoint.

Click to expand...

I can relate to the 'too many fifties' thing

Tons o Glass said:

EDIT: Found. Mine is serial 52656, has an A/M ring after the aperture ring, and I confirmed it is a 6e/5g. Is this a "I"? The Canon Museum entry doesn't show the A/M ring, at least the ring is not labeled as such... and after playing with the focus a bit I see why I tucked it away in a projects bin. ;-) No visual indication that this one is radioactive.

Click to expand...

I didn't notice that their photo of the V1 lens has a white dot instead of A/M... another strange anomaly.

Your lens sounds like a Version I lens but quite a late copy, well after the thorium had been phased-out according to my informal and totally inconclusive 'research'.

Tons o Glass said:

I would imagine the Canon Museum entries are correct. Regarding whether your lens is 6/4 or otherwise it should be fairly straightforward to count the reflections to determine the elements/groups count. If you're already familiar with the method, ignore the following :-D :

Click to expand...

You would think they'd know what they're talking about but I have noticed other errors on their pages so wouldn't be 100% confident.

Tons o Glass said:

In a dark room, turn on a single light source. I prefer a plain diffused white bulb, and it may be polarized. With the light bulb at your back, look through the lens and count the reflections of the bulb you can see in the optic as you move it around every which way.

It may help to get counts from both the front and the back, and/or with the lens stopped down to split up the work and thus make the work more manageable.

A 6/4 lens would have 12 (6ele*2) surfaces but with only 10 visible reflections where two of them would be dim/color-neutral(uncoated)/shimmering (colorful wavy interference patterns) as you move the glass around which implies those are each reflections made at two cemented surfaces.

Click to expand...

I'll give that a try. Thanks

Please let us know the results, as you've got me curious now hehe. It would be interesting if you have a v0 or whatever we're calling it and it does indeed turn out to be a 6/4.

The earliest serials I could dig up on eBay (a couple in the 20xxx) range looked thoriated, but a couple reflections visible in product pics of the front groups had me believing/assuming they were 6/5. At this point I wouldn't be surprised if there is a thoriated 6/4 v0, a thoriated 6/5, then the non-thoriated 6/5 v1s, with some slight revisions thrown in between for good measure.

This thread coaxed me into starting and completing a project via servicing the double helicoid of my v1 so thanks for starting it!

Tons o Glass said:

Please let us know the results, as you've got me curious now hehe. It would be interesting if you have a v0 or whatever we're calling it and it does indeed turn out to be a 6/4.

The earliest serials I could dig up on eBay (a couple in the 20xxx) range looked thoriated, but a couple reflections visible in product pics of the front groups had me believing/assuming they were 6/5. At this point I wouldn't be surprised if there is a thoriated 6/4 v0, a thoriated 6/5, then the non-thoriated 6/5 v1s, with some slight revisions thrown in between for good measure.

This thread coaxed me into starting and completing a project via servicing the double helicoid of my v1 so thanks for starting it!

Click to expand...

That project looks like a challenge - good luck

The yellowed element on my copy cleared very quickly with the Ikea lamp - about 18 hours to make it almost completely clear - so it must be just the rear element.

Not having much luck counting the reflections - I can only see 9 at max ;-)

Just taken out the front lens groups - looks like a single front element followed by a spacer and then what looks like it could be a cemented doublet, although I'm not certain about that. There is a retaining ring for this element or group underneath the spacer but my spacer seems to be stuck.

SimonOL said:

Tons o Glass said:

Please let us know the results, as you've got me curious now hehe. It would be interesting if you have a v0 or whatever we're calling it and it does indeed turn out to be a 6/4.

The earliest serials I could dig up on eBay (a couple in the 20xxx) range looked thoriated, but a couple reflections visible in product pics of the front groups had me believing/assuming they were 6/5. At this point I wouldn't be surprised if there is a thoriated 6/4 v0, a thoriated 6/5, then the non-thoriated 6/5 v1s, with some slight revisions thrown in between for good measure.

This thread coaxed me into starting and completing a project via servicing the double helicoid of my v1 so thanks for starting it!

Click to expand...

That project looks like a challenge - good luck

Click to expand...

Thank you, it went surprisingly well, actually! A retaining ring under the rear baffle secures the shimmed optical block into the helicoid assembly so I didn't need to mess with anything more than what was necessary - this is straight out of Zeiss' playbook. Another trick I associate with Zeiss lenses is that one of the helicoid alignment keys is conveniently split in two for some adjustability and springiness. In my case I was able to widen it a touch to practically completely eliminate any play.

It's as smooth as can be now, wee!

SimonOL said:

The yellowed element on my copy cleared very quickly with the Ikea lamp - about 18 hours to make it almost completely clear - so it must be just the rear element.

Not having much luck counting the reflections - I can only see 9 at max ;-)

Click to expand...

D'oh! So the reflection counting is so far inconclusive, drat. There are a couple of ways they can hide.

• Sometimes the reflections are quite large/magnified so it helps to vary the distance between the lens and your eye to see if you can catch it.
• Sometimes the cement is so well-matched to one of the joined glasses in a doublet such that the reflection is extremely dim to the point of being barely visible.
• Sometimes reflections can hide behind others, so you'll have to view the surface reflections at steep angles to see if any extras are revealed.

SimonOL said:

Just taken out the front lens groups - looks like the single front element followed by a cemented doublet... Is that the same as your copy?

Click to expand...

Well that's a sure-fire way of seeing for certain! Well done. Given that it may be somewhat of a rarity, I didn't want to suggest it.

No, my sample is not the same there - definitely 3/3 in the front (six reflections in front, with none looking like cemented surfaces. 5 in back, with one clearly [to me] belonging to cemented surfaces). Yours very well could be confirming the existence of the 6/4 v0. Did you go so far as to take the suspected doublet out of its housing and see/feel the seam? An air-spaced doublet would be housed similarly enough to a cemented one, but the reflections wouldn't lie in either case.

I wonder how safe it would be to assume that the yellowed ones are 6/4... Should I be pouncing on one of the eBay listings I watch-listed? Hehe.

Tons o Glass said:

Tons o Glass said:

Just taken out the front lens groups - looks like the single front element followed by a cemented doublet... Is that the same as your copy?

Click to expand...

Well that's a sure-fire way of seeing for certain! Well done. Given that it may be somewhat of a rarity, I didn't want to suggest it.

No, my sample is not the same there - definitely 3/3 in the front (six reflections in front, with none looking like cemented surfaces. 5 in back, with one clearly [to me] belonging to cemented surfaces). Yours very well could be confirming the existence of the 6/4 v0. Did you go so far as to take the suspected doublet out of its housing and see/feel the seam? An air-spaced doublet would be housed similarly enough to a cemented one, but the reflections wouldn't lie in either case.

Click to expand...

I can see a retaining ring underneath the front element spacer but that spacer seems to be stuck or glued. Those spacers usually just fall out... ? Don't really want to force it.

There's another retaining ring at the rear of the front element housing (the part that unscrews from the body as a unit) but that doesn't have any notches for a wrench.

Tons o Glass said:

I wonder how safe it would be to assume that the yellowed ones are 6/4... Should I be pouncing on one of the eBay listings I watch-listed? Hehe.

Click to expand...

I'm not convinced yet.

SimonOL said:

SimonOL said:

SimonOL said:

Just taken out the front lens groups - looks like the single front element followed by a cemented doublet... Is that the same as your copy?

Click to expand...

Well that's a sure-fire way of seeing for certain! Well done. Given that it may be somewhat of a rarity, I didn't want to suggest it.

No, my sample is not the same there - definitely 3/3 in the front (six reflections in front, with none looking like cemented surfaces. 5 in back, with one clearly [to me] belonging to cemented surfaces). Yours very well could be confirming the existence of the 6/4 v0. Did you go so far as to take the suspected doublet out of its housing and see/feel the seam? An air-spaced doublet would be housed similarly enough to a cemented one, but the reflections wouldn't lie in either case.

Click to expand...

I can see a retaining ring underneath the front element spacer but that spacer seems to be stuck or glued. Those spacers usually just fall out... ? Don't really want to force it.

Click to expand...

Gotcha, understandable. I didn't have a need to dig into the front grouping on my sample but it sounds very similar judging by what I saw. Canon did use glue in a couple other locations.

SimonOL said:

There's another retaining ring at the rear of the front element housing (the part that unscrews from the body as a unit) but that doesn't have any notches for a wrench.

Click to expand...

That ring in particular is not only un-notched, but is also glued in place on my sample. I'd only mess with these rings if the lens was looking tilted or de-centered in some way through the viewfinder.

SimonOL said:

SimonOL said:

I wonder how safe it would be to assume that the yellowed ones are 6/4... Should I be pouncing on one of the eBay listings I watch-listed? Hehe.

Click to expand...

I'm not convinced yet.

Click to expand...

Fair enough.

Tons o Glass said:

SimonOL said:

I can see a retaining ring underneath the front element spacer but that spacer seems to be stuck or glued. Those spacers usually just fall out... ? Don't really want to force it.

Click to expand...

Gotcha, understandable. I didn't have a need to dig into the front grouping on my sample but it sounds very similar judging by what I saw. Canon did use glue in a couple other locations.

Tons o Glass said:

There's another retaining ring at the rear of the front element housing (the part that unscrews from the body as a unit) but that doesn't have any notches for a wrench.

Click to expand...

That ring in particular is not only un-notched, but is also glued in place on my sample. I'd only mess with these rings if the lens was looking tilted or de-centered in some way through the viewfinder.

Tons o Glass said:

Tons o Glass said:

I wonder how safe it would be to assume that the yellowed ones are 6/4... Should I be pouncing on one of the eBay listings I watch-listed? Hehe.

Click to expand...

I'm not convinced yet.

Click to expand...

Fair enough.

Click to expand...

Seeing as I'm totally useless at counting reflections, thought I'd do a search and came up with this page

The OP uploaded a couple of photos that show what I see when holding the 'maybe a doublet' from the FL lens in my hand. Not sure quite what I'd see if there were two separate elements though!?

Would take my own photo but it's been a long day and it's too late for that...

SimonOL said:

Tons o Glass said:

SimonOL said:

I can see a retaining ring underneath the front element spacer but that spacer seems to be stuck or glued. Those spacers usually just fall out... ? Don't really want to force it.

Click to expand...

Gotcha, understandable. I didn't have a need to dig into the front grouping on my sample but it sounds very similar judging by what I saw. Canon did use glue in a couple other locations.

SimonOL said:

There's another retaining ring at the rear of the front element housing (the part that unscrews from the body as a unit) but that doesn't have any notches for a wrench.

Click to expand...

That ring in particular is not only un-notched, but is also glued in place on my sample. I'd only mess with these rings if the lens was looking tilted or de-centered in some way through the viewfinder.

SimonOL said:

SimonOL said:

I wonder how safe it would be to assume that the yellowed ones are 6/4... Should I be pouncing on one of the eBay listings I watch-listed? Hehe.

Click to expand...

I'm not convinced yet.

Click to expand...

Fair enough.

Click to expand...

Seeing as I'm totally useless at counting reflections, thought I'd do a search and came up with this page

The OP uploaded a couple of photos that show what I see when holding the 'maybe a doublet' from the FL lens in my hand. Not sure quite what I'd see if there were two separate elements though!?

Click to expand...

Okay, yeah, that is a cemented doublet in the link. Regarding the reflections in that image, note how there are three reflections where one of them is colorless overall but has some interference patterns on it - that's a great example of a cemented surfaces reflection. Regardless of the reflection count itself, if you see a reflection in the front half of the lens that has those characteristics, especially if it's dimmer than the others you can spot, then it would confirm a different layout than the v1.

An air-spaced set of two would have four visible reflections, and they'd be coated. For these FL lenses in particular, the [single?] coatings' reflections are predominantly amber or magenta. As the air-spacing gets thin two reflections would appear as one except at extreme angles.

SimonOL said:

Would take my own photo but it's been a long day and it's too late for that...

Click to expand...

Not a problem.

FWIW I took a gamble on one of the 20xxx-serial ones I had watch-listed. I'm the proud owner of yet another fast fifty!

As expected it's definitely radioactive (fairly yellow in the rear half of the lens), and this one is also a 6e/5g, which is bittersweet. Comparing the size and positions of reflections of my two samples the formulas appear to be slightly different at least, which makes sense if different glass types were used.

There is one other notable build difference I came across. When investigating a lot of play in the focus ring, I noticed it has one helicoid alignment key/tab instead of two.

I still have to wonder about your even earlier sample as to whether the 6/4 exists hehe.

You're not the only one: this one drove (and in some ways, still drives) me nuts as well. I happened to come across this great thread just after I bought what I thought was my "version 1" of this lens, got confused, then in doubt en wound up make an inventory of what I could find on the internet.

I think this lens "Canon FL 50mm f/1.4" comes in 2 shapes, 3 versions and no less than 5 variants:

• short shape, recognisable by the narrow focus ring, the "A M" on the ring below the aperture ring
  • version 1, variant 1, April 1965
    • no special name from Canon, picture missing from Lens Museum
    • optical design 6/4, thoriated glass i.e. radioactive
    • nameplate: "Canon Camera Co., Inc. LENS MADE IN JAPAN CANON LENS FL 50mm 1:1.4 No.12345"
    • this serial number runs (reportedly, couldn't find the OP) through 3xxxx, which is in line with SimonOL and Tons o Glass's copy. the highest I could find was 2869x
    • Tons o Glass reports that this version has 1 focus cam internally
  • version 2, variant 2, Sept. 1966
    • called "I", "(I)" or "mkI", visually indistinguishable from version 1. That is already very confusing in itself.
      • picture on the Canon Lens Museum has serial# 6408x (can't read the last digit), the mysterious white dot on the A/M ring below the aperture ring (instead of the "A M" letters), the infrared R is missing and, as opposed to version 1, has a DOF mark for f2.8.
      • I have looked at a dozen specimen on the internet, serial numbers ranging from 121xx through 83xxx and have NOT find any copy like the one displayed on the Canon Lens Museum's photo
    • optical design 6/5
      • the 6/5 formula was introduced in 1966 as an improvement over the 6/4 and (assumption probably negated the need to use thoriated glass. The form factor was kept the same and the serial numbering continued. Now it is possible that there was a non-radioactive version 1 or a radioactive version 2, but it is more likely that the use of thoriated glass was tied to the optical formula itself: 6/4 vs. 6/5.
    • serial numbering continues (assumption)
    • Mine has #3795x and is NOT radioactive. The glass is not yellowed (but that's just an indication) and proven by my geiger counter that only showed background radiation.
    • nameplate same as version 1: "Canon Camera Co., Inc. LENS MADE IN JAPAN CANON LENS FL 50mm 1:1.4 No.12345"
    • serial numbers could indeed run from 3xxxx. That includes my copy (#3795x which is not radioactive and has two focus cams internally) and the Lens Museum one (#6408x). We'll probably need a couple more votes here to define the split.
• tall shape, recognizable by a wider focus ring and the A/M switch ring just above the breech lock ring. the front element is also a bit larger in diameter than the version 1 and 2. this is the most common form (and if you look a the serial numbers and production periods, that's not a surprise - there were a lot more produced for a much longer period).
  • version 3, May 1968
    • version 3 actually comes in 3 variants which is very confusing
    • called "II", "(II)" or "mkII"
    • optical design 7/6. I don't know if any of these are radioactive but haven't found any report that they were (but that's no proof). it is also reported that the design was reused in the FD 50/1.4 (chrome nose and SSC versions) and in the FDn 50/1.4, neither of which are reported radioactive.
    • serial numbering is reset, but continues through the variants below (assumption)
    • variant 3
      • this is the original version 3 variant with "II" in the name plate (makes sense, eh?)
      • nameplate "Canon Camera Co., Inc. LENS MADE IN JAPAN CANON LENS FL 50mm 1:1.4 II No.123456"
      • serial numbers run through around 1536xx
    • variant 4
      • at some point, between serial number 1536xx and 1549xx (I found photos of both), the "II" was dropped in the nameplate
      • nameplate "Canon Camera Co., Inc. LENS MADE IN JAPAN CANON LENS FL 50mm 1:1.4 No.123456"
      • this is a source of confusion because some might think that, without the "II", this version is earlier than the first version 3 1968 mkII (variant 3), or the version 2 (instead of the version 3).
    • variant 5
      • to add to the confusion, after another ~10k lenses produced, between serial number 1576xx and 1624xx Canon decided to drop the "Canon Camera Co., Inc." and go for the shorter "CANON" - there are other lenses around where this change was made.
      • nameplate "CANON LENS MADE IN JAPAN CANON LENS FL 50mm 1:1.4 123456"
      • serial numbers (that I could find) ranging from 1624xx through 2757xx, but there may be higher numbers.
      • so this is the most common form

The most common error (what I now think) is that the variant 4 was seen as the original version 3 and some thought that the variant 3 was a later variant/version with serial number counting reset. Also, the Canon Lens Museum has anomalies ("surprise!") because, in addition to the mystical photograph on the variant/version 2, it states in the description of the version 3 that "A newly designed high performance standard lens to replace the FL50mm f/1.4 (marketed in April, 1965. 6 elements in 5 groups, Gauss type).", which is clearly wrong because the April 1965 first version was a 6/4. That's what started this whole discussion, right?

Of all of the above I'm pretty certain after looking at dozens of specimen on the internet and all of it makes a lot of sense now, but only if the serial numbers are contiguous. I'm also 100% certain that my copy #3795x has the version 1 form factor, has two focus cams and is not radioactive, which led me to believe it is actually a 1966 version 2 6/5. However, it could still be a version 1 6/4 that is not radioactive, if these ever existed... It is also very strange that I have found NO examples in the real world that have the dot instead of the "A M" text on the ring below the aperture - it seems a ghost photo on the Lens Museum* site. It is of course also possible there is more crossover between the 6/4 and 6/5 designs (particularly as there doesn't seem to be a mechanical difference) and thoriated/non-thoriated glass. We'd need more measuring points, people who are good at counting reflections (not me) or taking them apart - which, for a copy with thoriated glass, can be tricky.

*) if you have one, I'd love to see it. The white dot itself is not so strange because the FL 58/1.2 and FL 85/1.8 also have it.

--
Photography is all about looking.