Review 1 of 4
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$0.00 Summary: This lens has been called the "undisputed king of superzooms" (http://www.pentaxforums.com/forums/p...tili-zoom.html).
For specific applications (corner to corner sharpness, 1:1 macro, available light shooting, etc.) you'll obviously find specialist lenses that will outperform the 18-250mm. However, they will also be much more expensive and you'll have to be prepared to change lenses all the time. This is not only cumbersome and may make you not take a shot sometimes, but also increases the risk of getting dust/pollen on your sensor when changing lenses outdoors. In terms of versatility I think the 18-250 is hard to beat and it provides remarkable optical performance, not only with respect to its zoom range, but also in absolute terms. If your print sizes are 11x14 and lower, I believe you'll be very happy with this lens all the time. Used within certain parameters, it will provide outstanding performance for even bigger print sizes. Strengths: I like it a lot for its versatility. Having 18mm at the low end of the focal range is great. The 10mm difference compared to a zoom that starts with 28mm is dramatic. In comparison, 50mm more or less at the end of the focal range do not matter that much. In many tests the 18-250mm has been found to be a great improvement over the Tamron 18-200. (Note that the Tamron and Pentax versions of the 18-250mm lens are identical modulo cosmetics.)
With body shake reduction activated, I found that I could take tack sharp shots even at 250mm without a tripod, with proper holding techniques.
I love its close focus ability of 45cm throughout the entire zoom range. Strictly speaking its not a true macro (1:1 magnification factor) as its magnification factor is "just" 1:3.5. This is due to the fact that changing the focus also changes the effective focal length. Bob Atkins reckons that the effective focal length of the 18-250mm is ~125mm at its close focus distance of 45cm. (See his Tamron 18-250mm F/3.5-6.3 AF Di-II LD Aspherical (IF) Macro Review (http://www.bobatkins.com/photography...on_18_250.html) for his findings on this matter.) Still you can make some great macro shots with it with an incredibly low barrel distortion of just 0.01%. I've heard one person moan about focus influencing focal length -- a common side effect with internal focus lenses -- but frankly I couldn't care less. I get maximum zoom for objects that are far away, plus great macro capability. A 300mm telephoto with a close focusing distance of just 150cm gives less magnification.
An advantage of the internal focusing system (IF) is that you can use a polarising filter, which won't turn when you are changing the focus.
Like many modern designs from Tamron/Sigma, the 18-250mm optimises the centre sharpness in favour of better average across the field performance. That means that you'll get excellent centre sharpness but corners won't be near up to that level, in particular not with higher apertures. I believe this is a good decision as you'll get the chance to get great IQ for where it matters. Often things that happen in the corners are not important and will not be in the plane of focus anyhow. If you want to take landscape pictures that are sharp from corner to corner, near to far, then you'll need much more expensive lenses or, best, a view camera. For a print size up to 8x10, the 18-250mm will deliver A/A+ performance in almost all focal length and aperture settings and even with 11x14 prints you'll only get down to a B with some settings. (Source: http://www.popphoto.com/cameralenses...-ii-macro.html)
My copy doesn't suffer from zoom creep. The barrel will extend on its own when you point it downwards and shake it hard but in normal shooting conditions, it will stay at the same focal length. Weaknesses: At this price point and the very high zoom range, there are obviously some downsides:
You'll observe vignetting across the whole zoom range unless you stop down the lens sufficiently.
The lens isn't fast to begin with (f/3.5-6.3) and the need to stop it down in order to avoid vignetting / increase corner performance doesn't make it faster. However, with body shake reduction and very good low light performance from my K100D I've always been a very happy user so far. Even low light indoor shots worked well for me. I have never tried a prime yet so I don't have the comparison but on an absolute level, I'm more than happy.
The barrel distortion at 18mm is quite high but in normal scenes you'll only really notice it by comparison with a corrected version. This, and some CA (chromatic aberration) can be easily corrected with post processing (e.g. Photo Laboratory or Raw Therapee).
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