A lot of folks seem to be wondering whether they can use Lee or Cokin filter systems (adapter rings, holders, rectangular filters) on their UWA lenses (viz., Canon 1022, Sigma 1020, Tamron 1118, Tokina 1224). I have tested out different combinations on the Sigma and Canon only, but I assume if it works at 10mm for these lenses then it will work for the Tamron and Tokina also at their widest FL. IIRC, all 4 are 77mm threads.
Anyway, I don't know what folks know or don't know, so I am going to assume little here. On the 10mm lenses, you can use a normal (screw-in type) 77mm CP filter with no problems (i.e., no vignetting). You don't need a "slim" CP filter. Note that depending on direction of lighting and other factors, you can still have an uneven polarizing effect across the sky.
Now, here is where it gets a bit more tricky. Most landscapers prefer the "rectangular" filter systems because they are just, flat-out better for landscapes, especially with regards to graduated NDs. Being able to line up your horizon is important, something those circular versions can't do. But the problem with these UWA lenses is that you'll get severe interference due to "the holder" at 10mm (or probably also 11mm or 12mm for the Tamron and Tokie).
Now, Cokin's "P" system is very popular because it offers reasonable pricing and good performance. [Their new "Z" system is very expensive.] The problem is this: with their "P" system, at 10mm you get interference problems because the holder has "3-slots" and the outer 2 slots cause the problems.
Because I have a ton of "P" style (Lee and Cokin) filters, adapter rings, and holders, I decided to try to "modify" one of my "P" holders and see if "cutting off" the outer 2 (it only takes 3 minutes to cut off) slots would allow me to use my entire system. Guess what? IT WORKS EXCELLENT! 
Now, the Cokin (or Lee) CP filter will not work even with the modified, 1-slot "P" size holder. As I mentioned already, the normal screw-in 77mm CP filter works fine. But for ND grads (which are the most important filters for landscapers to even out the lighting), for your UWA lens only need to get:
---> Cokin's "P" 77mm adapter ring, holder (it comes as a 3-slot for stacking filters), and whatever rectangular filters you want (most importantly a 2-stop ND Grad). The catch is that you must saw off, cut off, remove the outer 2 slots. It's very easy!
I'm heading to Yosemite in April, the Grand Canyon in September, and Monterey in November. The most important filter, again, is going to be the 2 stop ND grad. I need really ONLY this filter (although I have other ND filters) because my main objective is to balance out the lighting in those landscapes, then on my PC use Photoshop and Nik Color Efex Pro 2.0 (Complete edition) and all the built-in filters to go from there. But if I blow out my highlights, even PS can't do much then. So this is why I did this "test" with my current system, and behold, it works great.
I have images and test shots to show for it, but PBASE IS DOWN AGAIN. As soon as it's up, I'll load everything and you'll get to see better what I mean by all this. Hope this is useful to those of you with UWAs.
[A very useful technique if you're ever without your ND grad or just don't use one is this http://www.luminous-landscape.com …ls/digital-blending.shtml
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Oh well, I saved some $.
