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I have owned this lens for over two years now and, as my favourite type of photography is wildlife/birds, it's my most used lens. It's enabled me to take some very good photographs during this time due to its versatility and high quality optics.
The build quality is, for a lens in its price range, perfectly acceptable. The lens, although extremely lightweight (for its size), appears well put together. After two years hard use, there's no sign of any wear at all in my copy.
In use, this lens is easy to hold, and to adjust for focal length. The body has a knurled grip which is easy to turn, even when wearing gloves. It can be set to auto or manual focus by means of a switch near the connection to the camera body. Due to its light weight, it can be used handheld, without a tripod.
Autofocus, although accurate most of the time, is a little slow and noisy. In poor contrast lighting, the focus can 'hunt'. Occasionally, focus can be lost just as the shutter is pressed, leading to out of focus shots being obtained. Generally, however, the autofocus works well.
Image quality is good to very good. One word of warning, however, this lens is soft wide open. I've found that, for my use, anything under f/8 will show poor sharpness. The lens is at it's sharpest at f/11. Stopped down, this lens is very sharp.
Contrast is good, as is the lack of lens flare (the slightly recessed lens, even when not using the supplied hood probably helps here).
One disadvantage of this lens it the lack of image stabilisation. Long focal length lenses can benefit from image stabilisation, particularly comparatively slow (f/5.6) lenses such as this one. It can be a struggle to obtain a suitably short exposure, to get a sharp picture, without resorting to high ISO settings.
I have used this lens at times with a teleconverter (the Kenko 1.4x teleplus pro 300). The lens will attempt, and sometimes achieve, autofocus with the teleconverter fitted, but only in very good light. The images obtained with the teleconverter show much reduced sharpness. My opinion is that this lens does not work well with a teleconverter.
Overall, this is a very versatile, high quality lens that is extremely good value for the price.