Pentax DA* 60-250mm F4 Review

Sharpness

Resolution (the capacity to render details, also called sharpness) is of paramount importance for any lens. The following pictures allow comparison of resolution for both lenses at various apertures. The target is a Imatest.com test pattern. The target was photographed with a Pentax K20D set on a tripod, with 3 seconds mirror lock-up. Our test setup was not accurate enough to evaluate centering defects and corner differences should be blamed on the target alignment instead of lens manufacturing. The test represents a graphical evaluation of the modulation transfer function (MTF) of the lenses. The target does not provide a scale so no quantitative measurements have been made.

The target was placed at roughly 100x the focal length (i.e. at 25m for a 250mm focal length). The target was first placed in the center of the frame, then at the top left corner. Images were cropped to show only the target itself. The image below illustrates the position of the target at 250mm and F8.

At 60mm, results are excellent even at F4. Performance peaks between F5.6 and F11. F16 is comparable to F4, F22 is barely usable and F32 should be avoided whenever possible. Corner performance is slightly worse than center wide open and gets slightly worse when zooming in.  This is noticeable even when stopped down.

Sharpness at 140mm was very good even wide open, especially in the center. F5.6 shows some improvements in the corner. Results become excellent from corner to corner at F8 and remain so up to F16. At F22 sharpness is still good in the center, less so in the corners, and F32 is no more than barely usable. In general, results at 140mm are better than at 60mm but not overly different.

250mm shows impressive results, even wide open. Most lenses on that range (usually 70-200mm) are significantly softer at their tele end. This is not so with the 60-250, at least in the center: it is very good wide open, becoming even sharper as the aperture closes, again up to F16. F22 is barely usable and F32 is very soft. The corners are a different matter however. They are soft wide open, becoming very good at F5.6 and beyond, but never excellent. F16 is usable but F22 and F32 are extremely soft. Considering that long focal lengths are often used to isolate a particular subject (instead of capturing a whole landscape) the softer corners should not be too bothersome in practice.

Verdict

The most impressive aspect of this lens is that it is surprisingly sharp wide-open.  This means that in everyday use, you shouldn't ever worry about shooting at F4. The lens feels equally reliable at all apertures larger than F16.  The results at both ends of the zoom range (where the lens is most likely to be used) are stellar, especially in the center. Sharpness wide open at 140mm is excellent even in the corners; the lens really shines at this focal length. Corners at 250mm are the weak point of the lens, but center sharpness is excellent.