I will chime in on wall charts.
I'm the first to admit, I'm cursed with an eye for detail. I shoot a lot of small birds and wildflowers which make a sharp lens very important. However, more importantly, the AF has to be very precise, not just on wall charts, but in the field. Otherwise, sharpness is worthless.
Having said this, I'm not a pixel peeper. When I download new images, I want one or more to stop me in my tracks for its subjective qualities. A soft lens can ruin an image, but sharpness alone doesn't make a compelling image.
I called the 135 2L my "magic" lens for the compelling images it has a knack for. It is an old lens and the AF is not as good as newer ones, so my keep rate wasn't particularly good. Mounted on a R5 or newer, the AF greatly improved, more so than the rest of my EF lenses (about 15.) When the R5 and R6 were launched, Canon's Rudy specifically mentioned they would give new life to the 135. I guess Canon saw what I did.
I read or watched every review of the Sigma 135 1.8 I could find. A lot of reviewers give glowing reviews to everything. They are worthless. The honest reviews found some AF issues, though it was better than some other Sigma offerings. They also found some funky colors. Overall, the images I saw didn't move me.
Some people have referred to 135 2L lovers as a cult. Count me in. When I think about my most memorable images, or my large prints on the wall, it is overrepresented. When I ordered the RF 1.8 version, I was worried it wouldn't have the same magic. I'm happy to report it is also overrepresented on my wall.
In short, if a lens is soft on a wall chart, I'm out. However, most modern lenses do very well on these tests. Wall charts won't tell me how AF performs in the field or anything subjective at all.
I very much agree, I've been a fan of the EF 135mm f2.0 for well over 15 years. my copy is beaten to heck and well used. It still puts in great shots and I've not seen optical sharpness issues with it on my R6ii or R5.
However....it's a very old lens and it's AF sometimes can be less consistent that newer designs. Its remarkable that Canon go so much right with this lens, when relatively newer lenses (like the ef 50mm f1.2L) had far worse AF in low light.
One of the more memorable uses of this lens was back from my "available light" Wedding photographer days and I had a wedding contract to cover a ceremony in Canterbury Cathedal in the crypt, lit only by candle light. No flashes allowed and the ambience was amazing. This was the youngest of two sisters and her older sister was married at the same venue two years previously and the other photographer used f2.8 zooms (with flashes on a crop camera) and couldn't get a usable shot because it was too dark and was using Nikon at the time. These days, it would be so much easier in these shooting conditions because the MIRC have better high iso abilities (both Canon and Nikon) and great AF as low as -2ev or better in many cameras. Back then, on Canon full frame, only the single central AF point (in one shot only) or the central 5 points (depending on which model) were usable at these light levels.
To be fair, this lens struggled in this context more than my ef 35mm f1.4 and ef 85mm f1.2, but that's the reality of combined darker aperture and the need to shoot at a higher shutter speed due tot he focal length. A 35mm f1.4 fains a whole stop AND it can get sharp images at 1/30th sec, where as the EF 135mm f2.0 looses a stop and needs to shoot at 1/125-150th to reduce hand wobble or camera shake. Tricks like bracing against a wall / pillar / column help a lot too.
Generally with weddings 1/50th is the slowest you can shoot because the subjects need to be clear, defined and without movement (unless it's clearly a creative shot).
The RF cameras (as we all know) have a far superior Af system than their older DSLR cousins. Continuous, reliable and accurrate tracking across the whole frame is something that we are all enjoying and it's easy to forget the AF deficiencies of the past and that this lens was originally designed to operate with. This lens was designed for film cameras way before digital was anything more that prototype and early concepts, it was launched as an enirely new design around 1997 with a new optical formula, improved over it's famous FD counterpart. Other memorable lenses launched the same year: EF 180mm f3.5 Macro and EF 400mm f2.8 LIS II....it was quite a year for Canon lens design, maybe one of Canon's finest. Here we are 20 years later seeing that some of these lenses from this particular generation are still very relevent. This was a year that legends were born.
One of the sweet things that I love about the ef 135L is it's tiny size and weight. It's truely a discrete telephoto lens. Often it frames similar to a 70-200 @ 200mm with tight close headshots. This is due to the zoom having more focal length breathing at MFD and that many 70-200's are actually quite short of the magic 200mm declaration on the barrel. I've had an older Sigma 70-200 EX DG that was really a 85-185mm (excluding focal breathing).
often, I found at weddings I could take a step closer and I'd get the same frame filling I would from my EF 70-200mm LIS II, but with a far les obtrusive lens that was lighter and way less heavier. Sure, no IS and the AF isn't quite so good, but I usually prefered the images from the 135/f2L.
The new RF is a stuning lens in every way except in the areas that made it's predessor so versatile. The new lens is one of Canon's sharpest primes...seriously it blows away every prime under 200mm by some margin....but you can't fit teleconverters to it. Duhhh.
It's gained 1/3 of a stop of brightness, a truely superior Af system and a new IS system. It's now got more focal breathing than before but can focus closer...all this adds up to a lens that is almost as heavy and the similar bag size as the RF70-200mm f2.8 LIS, which says a lot about the zoom lens!
For me, if I was still shooting weddings in the UK and I needed a newer set of lenses (warrenty and service life are a big factor here), I woudn't choose the EF 135L, I would definatly get the RF 70-200/2.8 and pick up a mint used EF 135L for my bag and review it's use over the season.
But it leaves the RF 135L out in the cold as a worthy sucessor. It superior in every lab tests except it's use case scenario that the EF 135L excelled in. To be honest, this could all change if Canon drops a RF 135mm f2 VCM lens on us, that basically takes the old EF lens and gives us a new lens that gives us an improvement of the features that made this then great. But if you already have a RF 70-200/2.8 then you probably won't ever see much use for the current RF135L or potentially a newer, smaller, lighter VCM option. This is because Canon did a packaging miracle with the RF70-200/2.8 LIS and made it so small and light. This is hard for me to write because the RF135mm f1.8 LIS is a superlative lens, one of Canon's finest...it's just lost 80% of it's photographer's appeal compared to the EF version.
My conclusion, there are a few EF primes that adapt well and are still relevent in the Canon Mirrorless ecosystem. The EF 85mm f1.4 LIS , the EF 35mm f1.4 II L and the EF 135mm f2.0 L are the three that stand out and still stand out as exceptional. Sure we could add the EF 200mm f2.0 LIS to this list, but that's one of the great whites and that's a slightly different topic. While the new RF 85mm 1.4 L VCM and RF 35mm f1.4 L VCM lenses are excellent, if you already have the previous EF version's there's not much to gain from the side grade cost. The Rf 135L is a vast improvement in every metric that is unfortunatly not very helpful to the photographer and their shooting needs and this leaves the EF version a curious better option for many.