Basically, lenses don't transmit 100% of the light hitting the front element. Some light can be lost at each interface between the optical elements in the lens, and although this loss can be mitigated with modern coatings, it still happens.
Having said that, the loss is typically only a small fraction of 1 stop, and can pretty much be ignored for most photography.
In film-making, the transmission is more important, to avoid changes in apparent brightness during a scene. Motion picture camera lenses have a "t/stop" number rather than (or as well as) the "f/stop" to indicate how much light is lost. For example, an f/2.8 lens might have a t/stop of t/3.0.
Recently Roger Cicala over at LensRentals.com referred to the Samyang (and Bower and Rokinon, since they all come from the same place) as "disposable" as they are both relatively fragile and nearly impossible to repair. Apparently they are good optical designs built at seemingly an impossibly low price point.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!
It also has a large front element sheltered (only) by a tulip-shaped in-built hood, but still, for all what I read about the Nikon zoom, I would say much less than that, but considerably more than my good old 18mm Zeiss - which I can even utilize for exposures directly into the afternoon sun.
A peculiarity of the lens is the very long focussing thread which needs, for me, some getting used to. Quality seems fine - well, I will see.
>........ It >seems to be supersharp, but otherwise not so well corrected. >Well, for that price...
I had a chance to toy around with one. It had tremendous compound distortion. By the time I corrected it, I had to crop it to the same FOV as a 16mm. Having a 17-35 already, I couldn't justify the 1mm gain.
I never did look into other optical issues, such as CA & vignetting. The straight lines that were rendered ocean waves almost made me vomit.
I suppose if you're not shooting anything with straight lines it would be a great lens, but that's not what I'm shooting.
I hate when people ask me what I see myself doing in 5 years...... I don't have 2020 vision!