I recently bought a Rokinon 14mm f/2.8 lens for Canon off Amazon (india) during a sale, but I received a Nikon version of the same lens. Now Amazon India has a policy of only replacing items fulfilled by Amazon, and this was from a third party seller. The lens now costs almost twice as much, and the only option I have of receiving a Canon lens is to get a refund on this item, and spend twice as much to buy the Canon version. I'm in a dilemma. I can either return the lens and get a refund, or buy a Nikon to Canon adapter and use the lens. (The lens right now is too expensive for me, considering import duties and shipping - it's imported from the US). I was wondering if there would be any degradation in image quality if I use the Nikon version with an adapter on a Canon body. I read online that since Nikon focuses shorter than Canon, there would be no trouble achieving infinity focus, and since this is a fully manual lens, I don't mind not having electronic contact with the camera body. I searched on stackexchange and elsewhere on the web, but couldn't find a good review of how a Nikon lens adapted to a Canon body would perform in the real world. What are your suggestions? Should I just get an adapter, or just return the lens, and save up more for a Canon version (which will, unfortunately, take me a long time). I'm interested in this lens because, although I have a EF-S 10-18mm, the aperture is too small, and I'm primarily into astro and nightscapes photography. I feel that the Rokinon would give me an advantage here. P.S. I'm using a EOS 1200D, a crop-sensor camera, but I'd like to upgrade to a full frame Canon some time in the future.
It will depend a lot on the quality of the adapter you get, but I'd say that it would probably work well enough that if it gives you a significant cost-savings, and you don't mind the possible image quality vagaries of adapting, it's worth trying. As you say, there's no electronic communication to be lost, anyway, and you were already set to manually focus, manually set the aperture, and use stop-down metering on your Canon.
The problem is that a cheap adapter ring is liable not to be machined with a great deal of precision, and the distance at which the lens is held away from the sensor is key in the lens being able to focus throughout its full range. If the ring is too thick, it will lose the infinity end of the range, and if it's too thin, it will lose some of the close end of the range, so if you do get a Nikon F-mount to EOS adapter ring, don't go for a super-cheap one. You may want to consider a chipped ring so you'll at least get some EXIF information off the lens as well as AF confirmation; but you can probably get along fine with an unchipped ring.
I would check, however, to see what version of the 14/2.8 you have. The 14/2.8 had two different versions, the earlier MC (Multi Coating) version, and the later UMC (Ultra Multi Coating) version, which fixed some ghosting and flare issues with updated coatings on the lens. That may account for the price difference; may not. If it's the older version, that may change your mind about returning/exchanging it.
One more thing. The lens is made by Samyang in Korea, but is essentially made as a generic and rebranded. I'd look for listings not just on the Rokinon, but also as Samyang, Phoenix, Walimex, Pro-Optic, Bower, etc. The vast number of names and suppliers and mounts that offer this lens means there can be a spread in prices for it. In addition, check that the double-the-price one is not the Cine version of this lens. Samyang also has cinematic versions of many of their lenses that are far more expensive, because they've got focus/aperture racks for camera rigs, stepless aperture control, and calibration in T-stops.
This is just my personal opinion, but purchasing a wide angle or ultrawide angle for full frame to use on crop until you get a full frame camera is something of a fool's game. Because of the crop factor, whatever you end up with is unlikely to be particularly wide on a crop camera. And the money you "save" in the scheme of things, given the cost of a full frame body, isn't really going to mean that much against all the shots you lost because you could've gone wider with a crop lens. A Samyang 10mm f/2.8 is probably better-suited to what you want to do than the 14/2.8, and when you do move to full frame, you can just sell it and swap for the 14/2.8 at that time, rather than use a compromise lens for years. Look at how you tend to use your 10-18 and really try to figure out if you'll be happy with 14mm.