I have this lens (8mm Rokinon) and a Samyang 14mm full-frame (Samyang Korea makes both of these lenses). Both have the same style cup-shaped lens cap, though different sizes. I don’t know what’s wrong with this fellow’s sample of the lens cap, but both of mine snap into place and hold securely until the two tabs are squeezed to release them. I have never had either release unexpectedly. I can’t even muscle the cap of the lens. Even pushing one tab isn’t enough to release the cap. In the video, it looks just how one of my examples of this lens cap behaves if it was placed on the lens without aligning the tabs to snap them in place. Since I’m sure this isn’t what the reviewer did, Samyang must be guilty part to part variation in their lens caps.

i just bought this lens for Astrophotography , i think i can get away with a 40 second shot without star trailing although i will use a vixen star tracker later……cheers

I bought the 6.5mm version , I agree awesome lens for the price but same problem with lens cap that is the only thing that lets it down ,

Samyang (also branded as Rokinon etc) makes some great manual focus lenses (for the price). Lens hood mounts are usually rather flimsy and lens caps are not too great (even the “normal” ones).

Was the video clips of the sky a time lapse done with an intervalometer/timer?
What settings were you using?