I am surprised to see such a poor account of this lens, given how well it has done in other reviews, e.g. Photozone.de:
"The Pentax SMC DA 40mm f/2.8 exhibits a quite harmonious optical performance combined with excellent build quality. It is capable to produce impressively sharp images across the frame straight from the max. aperture. Distortions are marginal and CAs are a non-issue. Vignetting is visible at f/2.8 but not a show-stopper. The bokeh (out-of-focus blur) is slightly sub-average and bokeh fringing can be an issue at very large apertures. The lens does also suffer from a focus shift when stopping down - this can have an impact in close focus scenarios".
I wonder if DXO's boffins are really as good at reviewing lenses as *real photographers* who recognise the practical applications of lenses.