What the L?
There seems to be a lot of confusion about Canon's L series lenses, and classification of lenses as such. Comments along the lines of "Optically this is as good as an L series, why isn't it one?" or "Why is this lens an L?" or "It's only not an L because it's not built like one".
This hasn't been helped by canon at times. The following is about what the L series is and is not, and of course the exceptions to the "rules".
L series status is not a judgement of nor guarantee of optical performance
This is an important point, but I'll come back to it later.
Canon introduced the L series in the late 70s on FD lenses, and built upon it in the New FD lineup. Of course, it carried over into the EF lineup, even with some FD designs simply getting EMD and AF motors with the same optical design (the 500 F4.5L being a great example).
The FD lineup contained many lenses of similar specifications but where different technologies were employed by canon. Ultimately this lead to some rather unwieldy names and many variants - for example, there were 5 different 55mm F1.2 lenses introduced within two years in the early 70s. The difference between a 55mm F1.2 AL and 55 F1.2 SSC AL and 55 F1.2 SSC Aspherical isn't necessarily immediately obvious. That, and 300mm F2.8 SSC Fluorite was a bit of a mouthful.
In a marketing move, canon created the L series as a catch-all for their top technologies - UD glass, fluorite and ground/polished Aspherical elements. Thus any lens that contained any one (or more) of these three things was classified as an L series. All L lenses use SSC.
In the New FD lens series, there were several lenses that were available as L and non-L variants, eg 300mm F4 and 50 F1.2.
The L variants had almost exactly the same physical construction, but they had optics built with those specific technologies. The technologies resulted in improved optical performance, creating the reputation that the L series still have for excellent optical performance.
Where high-performance super-tele lenses require UD or Fluorite, the big telephotos became L series by default - you can't realistically design an 800mm F5.6 of any reasonable performance level without low-dispersion glass.
When the EOS system was introduced, new L series lenses came with it. Two of the earliest L series zooms had non-L counterparts, where the build quality, AF, etc were identical, but the L version had Fluorite or UD glass (they were the 100-300 F5.6L and 50-200 F3.5-4.5L). From these examples and the FD examples, it's clear that build quality was not a qualifying factor.
Optical quality is also not a judging factor. All three of the original TS-E lenses have similar build quality and all three offer excellent optical performance. However, only the 24mm F3.5 became an L series because it required a ground aspherical element, which the 45mm and 90mm do not.
The 50 F1.0L is perhaps the best example of an optically poor (relative term) L series lens. Canon set out to build an F1.0 lens because they could - the large lens throat of the EF system made it feasible. In order to build this lens to an acceptable performance standard, it required ground aspherical elements, but its performance at any given aperture is bettered by the 50 F1.4 which also has faster AF (the F1.0 has to move some huge, heavy chunks of glass).
As it is today, all L series lenses still contain one or more of Fluorite, UD/Super UD or a ground/polished Aspherical element. It is the use of the required technology to achieve good image quality that confers L status, not the image quality alone.
So why did I mention exceptions?
Exceptions
Several lenses contain UD or fluorite and yet are not L series lenses. They fall into two group exceptions and the odd one out.
Exception no 1 is EF-S lenses. So far no L-series EF-S have been produced although several qualify through the use of UD glass (10-22, 15-85, 17-55, 18-135 and 18-200).
Exception no 2 is DO lenses. The 400 F4 DO contains a Fluorite element, but it is not L series. I believe that canon intended to make many more DO lenses than they have, but the technology hasn't delivered what it promised (yet).
The odd one out is the 70-300 IS. This one is an EF lens with UD glass and yet not classified as an L series.
I think that the reason for ef-s and the 70-300 is fairly clear - UD glass is no longer a differentiating feature. The price has dropped significantly since the 70s, and digital cameras demand significantly better resolving power from even consumer zooms than film ever did. Third party lenses and nikon lenses at lower price points now offer ED, SLD, etc equivalent low-dispersion glass.
Perhaps that's arbitrary and it's more about branding now, and the build quality we now expect to go along with an L lens. It is worth noting that all of the exceptions have occured only in the last few years.
Hopefully this is of some interest to people.
There seems to be a lot of confusion about Canon's L series lenses, and classification of lenses as such. Comments along the lines of "Optically this is as good as an L series, why isn't it one?" or "Why is this lens an L?" or "It's only not an L because it's not built like one".
This hasn't been helped by canon at times. The following is about what the L series is and is not, and of course the exceptions to the "rules".
L series status is not a judgement of nor guarantee of optical performance
This is an important point, but I'll come back to it later.
Canon introduced the L series in the late 70s on FD lenses, and built upon it in the New FD lineup. Of course, it carried over into the EF lineup, even with some FD designs simply getting EMD and AF motors with the same optical design (the 500 F4.5L being a great example).
The FD lineup contained many lenses of similar specifications but where different technologies were employed by canon. Ultimately this lead to some rather unwieldy names and many variants - for example, there were 5 different 55mm F1.2 lenses introduced within two years in the early 70s. The difference between a 55mm F1.2 AL and 55 F1.2 SSC AL and 55 F1.2 SSC Aspherical isn't necessarily immediately obvious. That, and 300mm F2.8 SSC Fluorite was a bit of a mouthful.
In a marketing move, canon created the L series as a catch-all for their top technologies - UD glass, fluorite and ground/polished Aspherical elements. Thus any lens that contained any one (or more) of these three things was classified as an L series. All L lenses use SSC.
In the New FD lens series, there were several lenses that were available as L and non-L variants, eg 300mm F4 and 50 F1.2.
The L variants had almost exactly the same physical construction, but they had optics built with those specific technologies. The technologies resulted in improved optical performance, creating the reputation that the L series still have for excellent optical performance.
Where high-performance super-tele lenses require UD or Fluorite, the big telephotos became L series by default - you can't realistically design an 800mm F5.6 of any reasonable performance level without low-dispersion glass.
When the EOS system was introduced, new L series lenses came with it. Two of the earliest L series zooms had non-L counterparts, where the build quality, AF, etc were identical, but the L version had Fluorite or UD glass (they were the 100-300 F5.6L and 50-200 F3.5-4.5L). From these examples and the FD examples, it's clear that build quality was not a qualifying factor.
Optical quality is also not a judging factor. All three of the original TS-E lenses have similar build quality and all three offer excellent optical performance. However, only the 24mm F3.5 became an L series because it required a ground aspherical element, which the 45mm and 90mm do not.
The 50 F1.0L is perhaps the best example of an optically poor (relative term) L series lens. Canon set out to build an F1.0 lens because they could - the large lens throat of the EF system made it feasible. In order to build this lens to an acceptable performance standard, it required ground aspherical elements, but its performance at any given aperture is bettered by the 50 F1.4 which also has faster AF (the F1.0 has to move some huge, heavy chunks of glass).
As it is today, all L series lenses still contain one or more of Fluorite, UD/Super UD or a ground/polished Aspherical element. It is the use of the required technology to achieve good image quality that confers L status, not the image quality alone.
So why did I mention exceptions?
Exceptions
Several lenses contain UD or fluorite and yet are not L series lenses. They fall into two group exceptions and the odd one out.
Exception no 1 is EF-S lenses. So far no L-series EF-S have been produced although several qualify through the use of UD glass (10-22, 15-85, 17-55, 18-135 and 18-200).
Exception no 2 is DO lenses. The 400 F4 DO contains a Fluorite element, but it is not L series. I believe that canon intended to make many more DO lenses than they have, but the technology hasn't delivered what it promised (yet).
The odd one out is the 70-300 IS. This one is an EF lens with UD glass and yet not classified as an L series.
I think that the reason for ef-s and the 70-300 is fairly clear - UD glass is no longer a differentiating feature. The price has dropped significantly since the 70s, and digital cameras demand significantly better resolving power from even consumer zooms than film ever did. Third party lenses and nikon lenses at lower price points now offer ED, SLD, etc equivalent low-dispersion glass.
Perhaps that's arbitrary and it's more about branding now, and the build quality we now expect to go along with an L lens. It is worth noting that all of the exceptions have occured only in the last few years.
Hopefully this is of some interest to people.
L for Luxury or Legend, L for Lust and Love, a brand within a brand - it's the stuff marketeers dream of.