I don't normally compare my lenses side by side but in this case, between the 135L and EF-S 18-135mm STM I had to make sure I wasn't imagining things.
I bought the EF-S 18-135mm STM for shooting videos on my EOS M, where it performs well. I tried the lens on both my 50D and 20D and came back very surprised. This lens performs and produces some amazing images. This is coming from a fast prime lover and someone who isn't all that keen on kit lenses.
Enter my trusty 135L. As we all know, the 135L is one of the sharpest lenses made by Canon, so I decided to compare the two side by side.
The EF-S 18-135mm STM having a maximum aperture of f/5.6 at 135mm, I decided to shoot the 135L at f/5.6 also. At f/5.6, the 135L should be somewhere in its sweet spot. The EF-S 18-135mm STM wide open. Should be interesting. All shots were tripod mounted at 100 ISO. All pic were shot in RAW, no sharpening added, then converted to jpeg and resized.
First pic, the 135L at f/5.6
Second pic EF-S 18-135mm STM
Please don't ask for 100% crops. I'm trying to keep this in the realm of 'real' photography.