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I have to admit that this set of filters has served me very well over the years, but my switch to full frame a long with my acquisition of my first UWA (love my 17-35mm f/2. has led me to finally decide it is time to move on.....lets not mention that the Cokin filters are starting to look a bit worse for wear.

Now the decision comes down to a few factors.....(in no particular order)
-Price - yep being a poor student sucks (yes I am silly enough to go back to being student)
-Quality - I want something that will last the distance and stand up to a bit of abuse, and not give me colour cast issues when stacked
-Coverage - I NEED something that will let me stack 2 filters at 16-17mm full frame (looking at you Rodney to see how your experiences at 16mm has been so far)

I am not going in completely blind and am currently looking at the 4x4, 4x5, 4x6, 100mm filter size (ie. Lee, Cokin Z, HiTech). My understanding is that the actual filters are somewhat interchangeable but there are differences in the holder. I would really like a hard edge ND8, a reverse ND and maybe a hard edge ND4. I currently have a Hard ND8 and a Soft ND4 but have found myself filter stacking a lot.

Thoughts, advice, recommendations etc

OK...

With my 16-35 I find if I'm using my normal filter setup (Lee holder with 77mm wide angle adaptor ring with it setup for two slots (not three) but also with the 105mm filter ring on the front to allow my 105mm CPL) then...
- it vignettes the extreme corners at 16 & 17mm and even 18mm (but not at 19mm and up)
- from memory it did this even when I didn't have the 105mm ring on the front so I don't think that is altering things - I think it is the holder itself which gets in the frame at extreme wide
- In practice I've not really found this to be a problem (I'm often just shooting at 18 or 20mm and up) - note also that if I use the ACR/LR lens correction profile for this lens the corners get both brightened and pushed out (so you loose them) during the correcting warp so the vignette almost totally goes at 16mm and is totally gone for 18mm and up (just tried in LR5 just now with some library images)

I've not tried with just one slot but then I couldn't stack filters which would be a pain (usually a grad + the bigstopper say)

I've only got 0.6 Hard and 0.9 Hard grads which I find pretty much covers most of my uses of grads - I've only once from memory needed to stack them both (to get five stops worth).

I don't have a reverse grad but would like to try one (and potentially own one if it works well for me) - I see they generally come as a 1.2 (4 stops) - I think Suren might have a HiTech one?

So my advice is basically the old chestnut... "buy once...".
I have the Lee kit and whilst it is the most expense option, it's the buy once option so I expect cheaper in the long run.
I do have a different brand for the CPL (same as Rodney's) as the Lee option is quite thick (although the lee is easier to use as a result).
Down the track it is likely that filters become redundant and it can all be done in-cam... but possibly not for sometime.
Even the cheaper options are not that cheap which is why I closed my eyes and handed over the cash.
Mixing filter brands... especially when you start using ND10 etc can become really tricky.

I already have and use the B+W ND110 (10-stop) and use a mix of Rodenstock and B+W CPL's this is really for grads....Lee certainly sounds like the way to go...although depending on price I may be enticed to go with the Cokin X-Pro system (130x170mm filters) to get that extra wide angel coverage without going to the 165mm filter from Lee.

As you well know, I have the Lee setup, including the UW adapter (77mm) and three hard grads (which are actually pretty soft), on the tokina 11-16mm (DX) i get vignetting with two stacked filters at 11mm and it disappears around 13mm with a screw on filter its pretty much the same but a bit worse. I think most UWA will have some degree of vignetting when you stack two or more filters (100*150 sized ones)

The cokin X-pro system actually looks pretty good, there appears to be plenty of filters for it 130*170mm and from what i see they are not that overly priced, ie the filters are around the 110 US mark

They also have some good adapter ring options ie the one that fits up to 118mm (~200+) and the standard 77mm adapter ring (~70US). There basic kit includes the holder and 3 grads this is around 270US, the prices are pretty good and comparable to the LEE system