And how would I know which size adaptor I need? There are no markings on my lens. I'm looking into buying a square filter holder to try help with this. Thanks.
Back in 2014 Samyang introduced the SFH-14 filter holder specifically made for their 14mm lenses. Cokin made three special filters that fit the holder, which uses an odd size larger than even "standard" 150mm filters: 161 x 139 x 3 mm.
The filters were:
- ND8 (three stop) neutral density
- ND4 (two stop) neutral density
- Half Blue (two stop) blue graduated neutral density
Due to the limited availability of suitable filters, it seems to have never really caught on. Four years later in 2018 a few of the holders can still be found for sale, but good luck finding the 161mm filters to fit it! They're sometimes marketed as 'Samyang 160 filters'.
A number of third party holders made to fit the 14mm Samyang lenses for 150mm filters are available from makers such as Nisi, Haida, and Lee (three piece ring system + Lee 150mm holder). The slightly smaller 150mm filters require the holder to be closer to the lens than the 161mm filters that fit the Samyang holder. One must be careful to avoid scratching the filters with the front of the built-in lens hood on the lens. The advantage of using a 150mm holder is that there are many more options in terms of what kind of filters are available in that size.
You will probably have to go with something like this:
Lenses with no front threads might have a filter slot nearer the rear of the lens, but the Samyang you've got is not among those.
So, the semi-janky >_< filter above is probably your best bet.
You already have many choices of makers to get the filter holder for the new ultra-wide lenses and the correct filter size, if you like the Lee filters, they have an online tool to choose the right components for your lenses and camera make: Lee Sytem Match.