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the person who did the In Depth Review comparison of the 35mm 2.4, the 40mm Limited and the 40mm XS stated:
" Pentax DA 35mm F2.4 vs 40mm XS vs 40mm Limited . . .
Our test copy of the DA 40mm XS produced excellent images in all conditions, ranging from close-ups to distant landscapes. It wasn't quite as sharp at the edges and corners as our DA 35mm, but it was more consistent across the frame, and seemed to be the best all-around performer. Chromatic aberrations were comparable to the DA 35mm, though it displayed nothing near the control of the DA 40mm Limited—differences in coating, perhaps? Distortion was a non-issue. Bokeh is marginally improved over the DA 40mm Limited thanks to the nine rounded aperture blades. Autofocus was fast and accurate. The build quality was surprisingly tight and rugged for an all-polycarbonate unit, and its tiny size definitely made the camera easier to carry on long, hot summer days. I could see this lens living on my camera for large chunks of time, and I get why it was packaged as the kit lens with the K-01: it's a very good compromise choice.
As with the DA 35mm, frills are kept to a minimum. While it retains a metal mount (somewhat surprising, since it's definitely in the budget lens camp), it doesn't include a lens hood, and its insanely compact dimensions preclude any distance scale. As noted above, the construction, while rugged, is predominately plastic. Its looks are a subject for debate; personally, we liked them, but your taste may vary. Anyway, as small as it is, the looks are easy to overlook if you disagree. . . . "
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Pentax DA 35mm F2.4 vs 40mm XS vs 40mm Limited Review - Conclusion: DA 40mm f/2.8 XS | PentaxForums.com Reviews