ejenner wrote in post #18311000
I guess I could check, but is the 2.8 much sharper at f8?
Probably so close it doesn't really matter:
I usually take a bunch of shots on an ISO test chart when I get a new lens - mostly just to check there are no issues. My sample of the 24-70II seems good (it matched another one I tested, and did well against a Canon 50mm f/1.4 that was stopped down to f/2.8).
The 16-35 f/4 IS I have certainly isn't shamed by the 24-70II, so I was initially surprised to see that the f/2.8 III was even better still (hats off to Canon).
However, low-to-medium differences on a black and white test chart, zoomed in, are unlikely to be field relevant. I.e. for real world shooting the differences are far less obvious.
I'd definitely choose the f/2.8 III if I were an astro guy, but the image quality of the f/4 is good enough, and for shooting without a tripod (e.g. castle or stately home interiors where tripods aren't allowed) the IS makes a huge difference; as you can trade 2-3 stops of IS for a smaller aperture and lower ISO.
marsaz wrote in post #18311056
I usually shoot between f8 and f11 and I do bring a tripod. I'd like a 2.8 lense for star photography but I'd probably get a Samyang 14mm 2.8 at some point for that purpose. Unless I get a fast zoom close to that focal range which I could use for both purposes. A used 17-40mm seems to be going around for quite cheap but 16-35mm sounds very nice indeed. I'll have to check the prices on those. I don't want to spend over a 1000$ (preferably as little as possible) but I understand you have to spend some money if you want better glass.
I'd really recommend the new 16-35 models (f/4 or f/2.8 III) over the old 16-35s or 17-40. Whilst the two new lenses are both very good, there is daylight between them and the old models in terms of sharpness.
The Samyang 14mm is a great budget lens. Just make sure you buy it from somewhere that will take returns, as the quality control is a little iffy. Some are great, but many (like the first copy I had) are really bad. Once you get a good one it's a gem though.