A couple of days early, but I was bored last night:
Using the Cokin P-series system with the DA 15mm f/4.0 Limited Lens:
Step 1: Find a sacrificial 49mm filter which you no longer use (don't like or one that you just want to destroy for the fun of it)
Make sure of the following:
a) The retaining ring holding the glass of the filter in place is at the front of the filter and not in the rear
b) Brass filters would work best. While you can use aluminum, aluminum filter rings can bind to the Cokin 49mm adapter ring.
c) Tiffen filters are fine to use, but because they use a thick filter ring of 7mm or so, you will end up doing a LOT of sanding. A thinner ring would help things go quicker.
Step 2: Pop the glass out
Because the retaining ring holding the glass in place is quite tight, you'll likely need to break the glass to get it out.
As a tip, take an old rag or a tea-towel to wrap around the filter prior to breaking it. You may also want to put card-board directly above/below the filter to prevent glass dust from contaminating the rag.
Another tip: Unlike what I did, turn the filter up-side-down. This will prevent the device being used to break the glass from sliding to the side and will likely require less force to actually break the glass.
The 'Glass Extraction Kit':
Step 3: Clear the glass and remove the retaining ring
Carefully (over a small box) remove the glass. This will leave a gap between the retaining ring and the filter ring.
Use gloves for this!
With the glass gone, the retaining ring unscrews quite easily.
No purpose, other than to celebrate destruction:
Glass cleared:
Retaining Ring being removed:
And gone:
Step 4: Sand the filter ring
On a flat surface, turn the filter up-side-down and start sanding the filter ring down to size. It may help if you attach the filter to the lens first so you have a rough idea of how much material needs to be removed, then remove the filter ring from the lens (I know you assumed this, but I figured I'd be safe and state it) and start sanding it down.
I used 100 Grit Sandpaper for this and even still, it took a lot of sanding....(see Step 1 c - or, as I like to refer to it: do as I say, not as I do)
After sanding for what seems like ages, wipe the filter down and attach it to the lens. See how much further you have to go.
**Caution**
The inside edge of the filter takes on quite a sharp edge.
Don't say I didn't warn you
Sizing:
Sandpaper on a flat surface: NOTE: you will get dust on the surface, so make sure it's not your new dining-room table. Wipe the surface down when you're done....your spouse won't thank you, but he/she won't kill you for leaving fine metal dust laying around
When you're done, the filter ring should sit just a hair lower than the top edge of the lens-hood:
**I'm almost embarassed by the abuse my 15mm has been put through, and it's showing, but it's been/being used rather ruthlessly**
Step 5: Thoroughly wipe down/rinse/dry the filter ring. Attach the Cokin 49mm adapter ring to the filter and and you're good to go.
Cokin adapter ring attached to the filter ring after sanding is done. NOTE how deep the adapter ring threads into the filter. Also, don't over-tighten as you don't want to bind the adapter and the filter ring:
I've been through this process twice now and I haven't had any problems attaching anything. The sanding doesn't seem to clog the inside threads of the filter ring...only sharpens the inside edge.
Here's two photographs taken with the filter ring/49mm adapter ring attached to the 15mm. These shots are taken wide open (f/4.0). The first shot is with the lens at infinity focus (excuse my shadow in the middle of the frame). The second shot is with the lens at the closest focus distance. The vignetting is there but it's soft and workable in post processing. I believe (but may be wrong) this is mostly the natural fall-off (hope that's the right term) of the lens wide open.