No attempt was made to equalise the framing of the Leica 19mm and Olympus 18mm primes in this instance.

Olympus 18mm Distortion
Olympus 18mm f3.5 at f8 (full frame)
10 points

The Zuiko 18mm has an excellent reputation for distortion control – users commonly report experience ‘no distortion’ for architectural work. As you see, Zones A and B are fairly neutral for a lens this wide, but there is an observable kink of positive distortion in Zone C.

Leica 19mm Distortion
Leica 19mm Elmarit f2.8 at f8 (full frame)
8 points

The Leica has a similar patten of distortion to the Olympus but it’s more severe in scale.

Canon 16-35mm Distortion
Canon 16-35mm f2.8 Mark II at 18mm / f8 (full frame)
10 points

The Canon 16-35mm II behaves like a less unruly version of the Mark I: the barrel distortion is uncomplicated and located only in Zones A and B. A simple Photoshop Spherize (-6%) does the trick without any degradation. In typical ultrawide zoom style, the 16-35 II progresses from barrel distortion at the wide end (at 16mm, it has the exactly same distortion as at 18mm), through to neutral (like the Nikon 17-35mm f2.8, it is dead neutral in the 20-22mm range), and onto pincushion from 24mm up, peaking at 28mm and flattening out somewhat by 35mm . . . like this:

Canon 16-35mm: 16mm Distortion<
Canon 16-35mm f2.8 L at 16mm (full frame)
Canon 16-35mm: 20mm Distortion
Canon 16-35mm f2.8 L at 20mm
Canon 16-35mm: 24mm Distortion
Canon 16-35mm f2.8 L at 24mm<
Canon 16-35mm: 28mm Distortion
Canon 16-35mm f2.8 L at 28mm
Canon 16-35mm: 35mm Distortion<
Canon 16-35mm f2.8 L at 35mm