A few Rokinon/Samyang lenses are offered in chipped vs. non-chipped varieties. The non-chipped has absolutely no electronics (your camera doesn't even know a lens is attached). When using non-chipped, you are completely on your own *and* this can make metering a problem.
Ordinarily a camera "meters" a scene with the lens aperture at wide-open. Suppose it's an f/2.8 lens... the camera knows it's metering with an f/2.8 aperture. If you wanted to shoot at say... f/8... it would know that's 3 stops down from the metering level and would recommend a correct shutter speed to go with it. But in an un-chipped lens... the camera has no idea that there even is a lens. This means the metering system doesn't have enough information to work. (It has no idea if it's metering through a lens at f/5.6, f/3.5, f/1.8, etc. and that means it can't determine how much light is available.)
The "chipped" version of the lens has electronics to communicate with the camera. The lens is still completely manual (manual focus, manual aperture), but at least the camera now knows "Oh, I have a 14mm f/2.8 lens attached and I metered at f/2.8". So now it can determine how much light you're actually working with and make practical exposure recommendations.
What I'm less certain of (because I don't own one of these) is that "technically" this is enough information for the camera to work in Aperture priority mode (you set the aperture, it will take care of the shutter speed) but I don't know if that actually works on a chipped version of the lens.
I would think it's also enough for the focus confirmation system to work ... but I don't know that it does work (seems like it would).
Possibly if someone here owns a chipped version they may be able to confirm.
For astro-images, everything is completely manual and we don't rely on the metering system. But for other uses ... if you go with the un-chipped version of the lens, you might also want a basic hand-held light meter such as a Sekonic L-208 (about $125 - a simple basic meter) or L-308 (about $190 - still simple/basic but also knows how to meter for flash as well as ambient light).
Good luck!