Lens advice…

I think you’ll do fine with either f/2.0 or f/2.8 for astrophotography.  On the other hand, there’s a big difference between 12mm and 19mm focal length.  I’d suggest 12mm for astro, but 19mm is probably better for a walkaround lens.  I wouldn’t worry about autofocus – with a wide-angle lens on a cropped sensor you will probably be able to shoot with a hyperfocal focus and not have to adjust much during the day.  Look up “hyperfocal distances”.

I’ve done a fair bit of research on legacy lenses, and unfortunately old wide angle lenses generally don’t play well with digital sensors – digital sensors require light to hit them at a 90-degree angle, or close to that.  Film has no such requirement, so using older wide-angle lenses on digital sensors often results in color casts near the edge of the frame.

That might not be as big of a deal with a cropped sensor, but newer wide-angle designs are still better and lighter than ones of yesteryear.  The best value for using old legacy lenses on the Sony models is in the normal to telephoto range, unfortunately.

Another question is whether you’re interested in star trail photography, milky way photography, or both.  For star trails I think you’d be much happier with the 12mm, but for milky way photography you can always shoot stitched panoramas to get a wider angle (and effectively get a super high resolution sensor out of the deal too).  This is why I ended up with an ultra-wide, but slow lens (15mm f/4.5) combined with a normal-wide faster lens (28mm f/2) for my A7 – I can shoot all of my milky way shots as panoramas and that’s worth the weight savings for me.  Bear in mind that the A7 is a full frame camera, so the equivalent focal lengths for your camera would be 10mm and 18.5mm – basically pretty similar to the two choices you’re looking at.

The Rokinon is also sold under the manufacturer’s brand, Samyiang, just in case you find it for less under that brand.

Franco I’m looking all over the place – but it seems to be in the $320-400 range no matter the seller or the name.

The Sigma can be had for $199…which for my current financial goals this year is a much more palatable sum.  But I’m just afraid I’ll end up getting the Rokinon anyway, which means it cost $200 more than I would have paid.  those are lessons I’ve learned the hard way….DOH!

Nick thanks for the info about the legacy lenses – I was afraid I was missing a huge opportunity by not knowing how to use adapters (and the sony lenses are just so damned expensive!).

Jennifer,

Yeah, the Sony lenses are a bit pricey, unfortunately.  I ended up buying my ultrawide and normal-wide lenses new, and then I got old 50mm (Minolta f/2) and 100mm (Canon f/2.8) lenses used.  I don’t expect to take them on many backpacking trips, except maybe some overnights – but they’re great cheap lenses for other uses.  The 50mm is about 8oz w/adapter, and the 100mm is about a pound.

This website has a lot of good advice on cheap lenses.

The adapter thing isn’t too hard, really – I got cheap adapters for both of mine and they work ok.  One from Amazon and one from Ebay.  You just attach the adapter to the old lens and then it basically becomes an E-mount lens.