Summary: Pentax or Samsung versions. I have a Pentax version on a Samsung GX20. Mine is very sharp wide open. It equals or betters the Canon 17-40 f4 but at less than half the price. Images are undistorted with good colour. CA is a rare event and the bokeh is buttery. It has full time manual focus too. One of the best value lenses around - for any mount. As with any lens, you might need to try a few copies to get a good one.