Originally posted by jaywes That is an obvious shortcoming of pentax lens. But for someone that shoots architecture (like myself) and would not use the tilt feature, it would be interesting and worthwhile.
I cannot compare the two lenses directly as I have never handled the Samyang T/S. I used, and liked, the Pentax 28mm Shift a lot in the analog days. However, I find the focal length too long on APS-C for most architectural purposes. According to the test shots that I saw from the Samyang, I'm pretty much certain that the Pentax has a lot more lateral CA and probably also more distortion (which becomes half-moustache-style with extreme shift settings).
I'm sure you can get much better results with a fine wide-angle lens and PP perspective correction in Hugin or a similar tool. Unless you have used such software tools before, you may not be aware that, based on the given field of view of your shot, they can compute exactly what you do when shifting (much better than the rough approximation that Photoshop offers as perspective correction--at least up to CS6 which I still use). In the same workflow step you can get rid of lens distortion. However, during PP perspective correction you lose a little FOV, so you might start with a slightly wider lens. The Samyang 16mm would be a mighty fine lens for this purpose.
In order to get extreme shift (that is, perspective correction) possibilities with a software-based approach I use a panorama setup--not much more awkward that a tripod-based shift lens setup but with unlimited possibilities.
If you want to draw your own conclusions I can send you some test shots from the Pentax Shift (PM me in that case).