Another option perhaps is to go for this, with the older 12-24 f4 for Canon EF. It's an EF to L-mount adapter with a built-in variable ND.
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You'll get more vignetting using an adapter, especially with UWA. I noticed this last week with my Samyang 14mm f2.8 and no in-camera lens corrections, that Samyang doesn't even have a built-in lens profile and I was using a dumb adapter anyhow. I can however correct in Lightroom.
Also does that Fotodiox adapter transmit any 3rd party profiles and can Lumix use a Canon profile if it has one? I doubt it so you'd need to in post.
TBH I wouldn't bother spending that money on an adapter for old DSLR lenses. I'd also reckon you wouldn't like variable NDs either.
As rigger said the newest Sigma Art 14-24 f2.8 has the rear filter holder, this is the best method for UWA lenses that need a bulbous front element for sharp corner to corner performance with minimal coma/astigmatism. This is usually for astro i.e stars as why else would you need that, so brings me to the question why do you want such a lens seeing you use f8?
BTW the Samyang 14mm f2.8 is actually very good for this and I already had it, I paid about £229 or something for it about 10 years ago. That's what I used for the aurora last week so that's a bargain option for you if the aurora has sparked something in you, pun intended
The new Sigma is a whopping £1319 but with 18 elements including FLD it is worth it over buying old DSLR lenses and adapters.
Personally I'd love the Sigma 14mm f1.4 for UW field astro and it's basically a specialist for this, it has one more element than the 14-24 and 3x FLDs.
BTW Sigma claim their FLD is equivalent to flourite elements which can be found in APO astronomy telescopes and Nikon has them in their uber expensive £15k 600mm f4 etc. I doubt it matches flourite but you are getting value for money in that £1319 and the rear filter holder can't be found in anything else besides the mentioned 14 f1.4.
Just get it
