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Forum: Pentax SLR Lens Discussion
01-25-2014, 01:00 PM
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Thanks to ogl for your very useful test images, which immediately suggest that the optical problems evident in the review test shots are not representative of all copies of this lens. Both shots show good cross-frame sharpness considering they were taken at f4.5. I'm guessing that the small amount of edge softness which is present will improve on further stopping down.
Whether the problem in the review images was due to focus issues or a bad copy of the lens is hard to be sure, but this gives me heart that at least some copies of this lens are worth the money.
It is my opinion that for reliable evaluation of lens sharpness in test shots, it is essential to 'bracket' focus at every aperture tested. Focusing systems, whether via AF, the optical finder, or live view, are imperfect. Without taking several shots with slightly varied focus, it is impossible to determine whether optimal focus has been reached, and you are effectively testing the focusing system as much as you are the lens. Focus bracketing would also help to determine whether edge softness is down to curvature of field or actual lack of resolution. This is not intended as a criticism of this review because some form of focus bracketing may have been used, but I'm pretty sure that many 'sharpness test' images posted on the web have not had focus optimised in this way.
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Forum: Pentax SLR Lens Discussion
01-06-2014, 09:35 PM
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Good to get a detailed review of this lens at last - thanks to Adam.
Well, if the sharpness results are reliable for the lens model in general, then this is very disappointing. As some have noted here, there is some evidence to suggest that the performance at the longer focal lengths may not be representative of all copies of the lens (a number of user reviews on this site suggest that performance is actually strongest at the long end), so it is perhaps too early to condemn the lens on this aspect at this stage. However, even if this is the case, it suggests that quality control is pretty poor. Considering that the only real justification for a slowish lens with a high price and short zoom range is guaranteed image quality, this is a real let-down. I for one am not willing to take a pot-shot at this price level.
As primarily a landscape photographer, I feel that Pentax simply don't make any normal-range zoom lenses that meet my needs, and I am now considering moving to another brand. It should be technically possible to produce a lens which produces decent sharpness across the frame at f8 at least, through a 20-40mm zoom range, even in a compact design. Even taking into account the high resolution of the sensor, the tested lens fails dismally in this respect at the long end - in fact my 18-55 produces much better edges than this at 40mm. Some of the problem seems to be strong curvature of field rather than ability to resolve, but it's very difficult to mitigate for this in most scenarios. Unacceptable either way.
The K-3 is an excellent new camera release, but Pentax really need to support that hi-res sensor with some excellently resolving glass, coupled with good quality control, if they are to improve their credibility - particularly at a time where Sigma and Tamron seem to be reducing their support for the brand. I'm still hopeful that this is a 1-in-10 bad copy. If Adam ever has the time I'd like to see a couple more copies tested for sharpness.
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