Q:

How weather-resistant are Sigma Art lenses?

I have a Sigma 50mm EX lens and am looking at an 18-35mm ART lens. I also put high quality protective clear filters on all my lenses. They will be affixed to a Canon 6D.
I am planning a trip to Colorado, and enjoy shooting on days with poor weather. I know that the Canon L lenses are built to be pretty well sealed, but I was wondering how water/dust resistant the Sigma lenses are when fitted with clear filters?

A:

There is only one Sigma Art lens with is weatheproof and that is the Sigma A 12-24mm F/4 DG HSM. Many of the Sport series are weatherproof though. A clear filter will not help as there is no sealing on the mount.
The one you are looking for is probably the 24-35mm F/2 DG HSM as it will work on your Canon 6D. The Art 18-35mm you mention only works for cameras with APS-C sensors.

