on 18 January 2014
Have shot off several hundred images and first impressions are that focussing is very quick, convenience is great (not having to carry a bag with several lenses), no worries about dirt ingress when changing lenses. Printing at A3, I find some loss of edge definition at ends of zoom range. Overall very satisfied, this lens will get a lot of use because of the convenience.
on 16 February 2013
hyper-zooms like this one suffer from a number of design constraints, usually making them under-perform optically, weigh a lot and be big physically.
Sigma have used low dispersion glass and aspherics to largely overcome these constraints.
Modern design technologies continually improve lens design. This lens is optically better (less chromatic aberration) than an older Sigma 70-300 I own with much simpler design.
The aperture is limited but this is to be expected from such a compact hyper-zoom.
Note that the Pentax version does not come with optical stabilisation built into the lens since this is built in to the camera body. However this lens does have a hypersonic motor driving the internal focus mechanism resulting in quicker and quieter focus acquisition.
This lens is now my standard lens for my Pentax M30.