Sigma fans have multiple options at the 24mm focal length, both offering the traditional Sigma blend of image quality and prices that undercut first-party options. The 24mm f/2 DG DN Contemporary and the 24mm f/3.5 DG DN Contemporary offer different user experiences depending on your priorities, and this excellent video comparison will help you choose which is right for you.
Coming to you from Tom Calton, this great video compares the Sigma 24mm f/2 DG DN Contemporary lens to the 24mm f/3.5 DG DN Contemporary. Both lenses are priced quite decently, though they serve different purposes. The f/2 version offers a decently wide maximum aperture for low-light work or relatively narrow depth of field, but on the other hand, the f/3.5 version is significantly more portable, with an impressively small footprint and very light weight (just 7.9 oz or 223 g). An aperture of f/3.5 is certainly a bit narrow compared to the majority of prime lenses, but with the high-ISO capabilities of modern cameras, it is not as limiting as it might have been in the past, and when combined with a lightweight mirrorless body like the a7C, you have a very portable setup. Check out the video above for the full rundown from Calton.
Samyang 24/1.8 : especially if you do Astro where it excels. Cracking lens.