Vignetting with an ND filter

Here are a couple of photos I took earlier, both with the DFA 15-30mm at 15mm and a 3-stop soft grad filter. For the second I added a Lee 10-stop big stopper to extend exposure to 30 secs; I was quite pleased with the effect but there is serious vignetting which I guess is due to the light path at the edges having to pass obliquely through both filters. Also, more worryingly, there is serious loss of sharpness at the upper left and right edges. This is my first try with this filter but I am rather disappointed with the result, has anyone else experience with this set-up?




And now with the 10-stop filter:

Did you use the Lee wide-angle adapter, which keeps the holder closer to the front element ?

I don't have much experience yet as i am a new user of the Lee system, but I would expect vignetting at 15mm. Have a look on the Lee website where joe cornish gives some tips in a video and I think he mentions limitations on very wide angles.

Thanks pschlute, I will look at the video. Yes I used the SW150 adapter and had a go last night in Lightroom to reduce the vignetting, but it is the distortion that bothers me most.