My Pentax 15-30 arrived today, and--WOW!--so did my Fotodiox WonderPana66 filter adapter ring. I've used a WP66 filter system on three other UWA lenses and find it highly satisfactory. This adapter ring, listed by Fotodiox for the Tamron 15-30/2.8, fits the Pentax perfectly.
It's composed of two pieces.
The rear piece slides forward from the rear of the lens and ends with the large, inner-threaded (red) ring into which the front portion screws after sliding over the integral lenshood. The forward portion is threaded for 145mm screw-on filters; I have the multicoated CPL and 4-stop ND.
The adapter ring and 145mm cap retails for $170.
https://www.fotodioxpro.com/wonderpana-freearc-core-rotating-filter-system-h...unit-only.html
The system also accommodates mounting 'ears' for 6.6"/150mm slide-in filters.
https://www.fotodioxpro.com/wonderpana-66-upgrade-bracket-6-wide-holder-bracket-only.html
I have 3-stop grad-NDs that they call 'hard' but look pretty 'soft' to me.
I've used this system on a Canon 17mm TS-E, 24mm TS-E, and 16-35/4 and experienced NO corner darkening. I highly recommend it even if their site is difficult to find things in.
Here's everything I have but the slide-in grad-ND which lives in the black pouch under the WonderPana cap.
Last edited by jeffreybehr; 05-25-2016 at 08:54 PM.