Some of the Rokinon lenses are excellent optical quality.
Their 16mm is sharper than the Zeiss equivalent and their 135mm f2.0 is sharper than Canon's L version.
I love Indian cinema. I wish we had more Indian films in USA theaters. :/
This camera seems really cool. It's seems a bit pricey for most indie shooters, but with 10 bit 4:22 I could see it as a good camera for small feature films.
I'm interested in what the image looks like.
I still love the image from this camera. It looks amazing.
Due to the engineering of the most lenses, they are usually sharpest between F/5.6 and F/8.0. Stopping down lower than F/8.0 will usually increase softness and stopping higher than F/5.6 will usually increase softness. (It's a good rule of thumb, as very lens will always be sharp within the F/5.6 and F/8.0 "sweet spot.")
DXO Mark is a website that scientifically analyzes lenses. It is a great reference tool for individual lenses and how they are engineered.
Also, according to DXOmark, the Sigma Art 18-35mm F1.8 is actually quite a lot sharper than the Canon 50mm F1.8 prime, so unless your lens is defective, than it is not the problem.
Between those two cameras, (despite the budget) I would still choose the Canon C100.
If the GH5 sensor is anywhere similar to the GH4, the dynamic range is heavily tilted in the direction of highlights rather than shadows. This makes it an amazing camera to shoot outdoors with, but it struggles indoors with shadow detail. The C100's 12-stop dynamic range is well-rounded in the middle of both highlights and shadows. It can handle them both.
It's higher ISO capabilities of the C100 can come in very handy as well. I don't think you'll be disappointed with this camera.