HD Pentax DA 1:4 15mm ED AL Limited
While Ive owned the DA12-24 f4 lens for some time and have often used it when travelling it tends to feel too large at times, so when the opportunity came up to purchase an excellent used copy of the HD Pentax DA 1:4 15mm ED AL Limited locally from another Pentax photographer, I jumped at the chance. The name may be long but the size and weight savings are 189grams vs 430grams and a length of 39.5mm vs 87.5mm. After taking it on several trips I feel I have a pretty good sense of its strengths and weaknesses. I will start of with what is its weakest point and that is chromatic aberrations. Despite having an Extra-low dispersion and a hybrid aspherical lens element it does suffer from chromatic aberrations away from the center of the image. 
Fortunately they can be greatly mitigated in software such as Adobe Lightroom which I use extensively.
The same can not be said however for the coma distortion in the extreme corners. If you can imaging a comet with its tail this is what is happening to the image in the corners which tends to look ‘smeary’ across the effected area.
When compared to the DA12-24 or the DA*16-50 at 15mm or 16mm respectively its a very similar result for the CA but slightly worse for the coma so there is a marginal trade off of image quality for size.
These distortion issues away from the center aside the lens renders a sharp image and a nice smooth background when focused on something close. This is enhanced by the use of rounded aperture blades that are the most useful and effective around f5.6. 
Like the other Limited series lenses from Pentax this is a wonderfully constructed lens made from high grade aluminum without a hint of play anywhere while still being silky smooth to manually focus.
As for its image circle it is definitively a lens for use with APSc cameras as you can see from this test shot on 35mm film.
In summary the HD Pentax DA 1:4 15mm ED AL Limited is a good lens optically and a great choice as a wide angle when small size is paramount.