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Lens for Pentax K systems
I own a Pentax K20D and a K-5 body, along with a range of interchangeable lens, most of them on the cheaper end.
I'm interested in Gigapixel photography and also plan to write (and possibly sell) software to assist the involved tasks.
To get good results for gigapixel images, I figured I needed a sharp telephoto lens more than anything else. While practically all lens are shart at least in the center, for my purpose I need a lens that is sharp up to the corners, as much as possible. I quickly figured that I need to get fixed focus lens because they're generally sharper in this regard compared to zoom lens.
On this page I like to share my findings about the sharpness of all the lens I own so far, or had briefly for testing.
My "reviews" here will not be as detailed as those you might find at the excellent photozone web site, though. However, my simple tests with a printed Siemens Star chart will already reveal a lot about the weaknesses of these lens.
Telephoto test samples
All pictures show the Siemens Star (»Siemensstern V1.1« copyright Dieter Bethke | fotofreaks.de, used with permission) which is printed borderless on a DIN A4 sized sheet. I've adjusted the distance between sheet and camera so that the sheet always fills up the entire picture area. The images were then scaled down to a quarter of the original size for easier online viewing here.
Pictures were taken at f/8 or f/11, to get optimal results. Any optional filters have been removed from the lens.
Admittedly, they pictures are not pretty, not evenly exposed and don't have the same exact size. But they show what I indended to check on: How sharp they are.
The fixed focus M lens are very sharp from center to corner. Practically no distortion. Only weakness: Cyan/Green CA towards edges. The DA 55-300mm is fairly sharp in the corners as well, but shows quite a heavy pincushion distortion. There's plenty of CA as well. The Sigma, the cheapest zoom I own, is not so bad either in regards to sharpness. The 18-135 WR is quite sharp in the center but rather unsharp towards the edges.
Update 23 Apr 2012: Turns out that my 18-135 WR is defective: The unsharpness it shows is not at all corners but actually only towards the right side, suggesting a tilted tubus or an out-of-center lens. I had sent this in to Pentax service who notified me now that this is indeed out of the normal and that I'll get a replacement unit.