I have used a Raynox DCR-720 Wide Angle Conversion Lens. This provides a .72x widening of the pic. It's a screw-on type of lens for a 52mm mount.
Raynox provides good quality, so there is very little loss of detail. I would say it is insignificant. There will be some, however, simply because you are asking the lens to handle more view/data, and there are only so many pixels to go around!
Here is an example from my K-x using the 18-55 kit lens both without and with the Raynox. It's not the greatest of shots since it wasn't very bright in the room, but you can get a sense of what it can do.
Pics at 18mm w/out and w/ Raynox
You can see the vignetting, but you can also see how you could crop the photo and still end up with a rather wide angle view. (The 18mm effectually becomes a 13mm. Mulitply by 1.5 to get 35mm equivalents.) Some barrel distortion, but that's fairly easy to correct.
Here's another example at 28mm, and now the vignetting is not an issue. Of course now, w/ the Raynox on, the 28mm is giving a 20mm view, but that's not as wide as the 18mm the lens is capable of. I've also had good luck stitching photos together to get the wide angle effect.
So why would you use this lens?
If I'm trying to go light, it's easy to add this lens to the bag. (The Raynox is about the size of a 50mm lens.) So, let's say I'm just using my lovely F 35-70 lens on the camera. If I don't want to stitch photos, and I don't want to carry along another lens, I just use the Raynox, and now it's a 25-50mm lens.
Last edited by mgvh; 12-12-2012 at 09:53 AM.