Really just getting into astro on a limited budget and have made the first step by purchasing a used Sony a7s. I'll primarily be looking to create time lapse and even hyper lapse 4k video of the milky way, so basically short exposure wide field of view night shots with the lens likely being used at wide open aperture. Given the a7s is only 12MP there is not a great amount of head room for down sampling to 4k video, as such I think the 20mm focal length will give me the best balance between field of view, image compression and light gathering capability. I am looking to get a one lens fits all for the foreseeable future until my bank balance recovers, otherwise I'd probably just go with the 14mm f/2.8 & 24 f/1.4 combo. Also, given I will not be doing a whole lot of stills, I think at this stage my preference is light gathering capability as being more important than putting up with moderate undesirable lens characteristics.
Anyway, given the above use case I would really appreciate any feedback on the following (I have been using a GH5 with Laowa 7.5mm f/2 for reference);
- Sigma 20mm f/1.4 - I've noticed there are quite a few used options in my area of this lens (usually Nikon mount) which are selling for as little as a hundred dollars more than new copies of the other two lenses I'm looking at. I've looked at astro centric reviews for this lens and people seem to either trash it or love it with the caveat they acknowledge it has some coma issues. Given 95% of the content I'll be producing is 4k video, would this low res (compared to stills at least) requirement lessen the visual impact of coma in favor of the Sigma's increased light gathering capability? This lens alone would give me nearly a 3 ⅓ stop advantage over what I am using now (using NPF rule for exposure time). Given this lens costs close to double the other two when new, I'd only really be considering this option if I could source a good used copy fairly cheaply.
- Rokinon / Samyang / etc 20mm f/1.8 - This seems to be the gold standard for this focal length. On a good copy the coma is supposed to be best in class, however wondering as to the build quality / quality control as I've read a few claims that you may need to return this lens a few times to get a good one. Used ones do come up, but never seem to go for a huge amount less than new, in which case I'd likely look to purchase new so I have the option to return if it is a dud.
- Viltrox 20mm f/1.8 - A newer mirrorless only lens, Dustin Abbot and a few other reviewers seem to give it a solid rating. The 14 aperture blades vs 7 of the Rokinon should give stars a nice round bokeh when throwing the sky out to focus on a near object. Vignetting is supposed to be less than the Rokinon, but the coma is not as well controlled. Both these f/1.8 options would give me a 2 ⅓ stop advantage from the lens alone over what I am using now. Being newly released I've not scene any used options, so this would need to be a purchased new also.
Really appreciate any feedback comments or advise. Thanks.