Camera Accessories

Scheduled for release in October, at around the same time as the new Sony DSLR-A700 camera, the DT 18-250mm F3.5-6.3 zoom lens covers a 14x zoom range that is equivalent to 27-375mm on a 35mm camera. This well-built optic weighs only 440 grams and is reasonably compact. It can focus down to 45 cm to give a close-up magnification of 0.29 times.

Consisting of 16 lens elements in 13 groups with some ED (Extra-low Dispersion) glass elements, the DT 18-250mm lens has an internal focusing system that allows angle-critical filters, such as polarisers and graduates, to be used without re-adjustment as the focal length is changed. Zooming is very smooth and, on the new A700 camera, the lens supplied for our review focused quickly, smoothly and quietly. The seven diaphragm blades close to form a near circular aperture at wide aperture settings, producing attractive 'bokeh', particularly at longer focal length settings.

Build quality on the supplied lens was very good. Finely-ridged rubberised focusing and zoom rings provide a secure and comfortable grip. The zoom ring is much wider than the focusing ring (28 mm vs 9 mm) but both move smoothly and positively. No focus scale window is provided but distances (in metres and feet) are engraved on the lens barrel just behind the focusing ring. A lock behind the zoom ring prevents the lens from extending when it is carried facing downwards. A petal-shaped lens hood is supplied but it can be tricky to fit.

Maximum and minimum apertures are pretty typical for a zoom lens with such an extended range. At maximum zoom, the f/6.3 setting provides great scope for differential focusing, although the resulting depth of field may be too shallow for some subjects. No stabilisation is included as it is provided in Sony's Alpha (A-series) camera bodies.

Performance
Overall performance was very good for a zoom lens with such a long focal length range. Subjective assessment to test shots showed them to be generally sharp and detailed. With the aperture wide open, bokeh was generally pleasing, particularly at longer focal length settings. Stopping down produced a wide depth of field at most focal length settings.

Imatest showed the DT 18-250mm lens performed best at shorter focal lengths and with mid-range apertures. A graph showing the MTF50 results we obtained at different aperture and focal length settings is presented below.

Lateral chromatic aberration was generally low, although not negligible. Fortunately, we found little difference in Imatest results from different focal lengths and aperture settings; although wide-angle performance was marginally better than the figures we obtained at the longest focal length settings.

Traces of coloured fringing were detected on the periphery of shots taken with the widest focal length setting. However, they were only visible when shots were enlarged substantially and we don't feel they would impact on normal picture-taking. No fringing was found near the centre of shots.

Edge-to-edge sharpness was excellent in all test shots at all focal length settings and we found little difference between centre and edge and horizontal and vertical resolution in our Imatest analysis.  Some barrel distortion was apparent at the widest lens setting but this had turned into slight pincushioning by the 50mm setting. Distortion was negligible at longer focal lengths.

Conclusion
Affordably priced, the DT 18-250mm lens would be a good choice for photographers who want a DSLR body with just one extended-zoom lens. Its focal length range covers angles of view that will suit many different types of photographers. Its relatively light weight, compact size and robust construction make it particularly suitable for travellers. We found it an excellent partner for the new DSLR-A700 Sony camera.

IMATEST GRAPHS

SAMPLE IMAGES

18mm focal length.

250mm focal length from the same viewpoint as the shot above.

Selective focusing at 250mm f/6.3. Note the shallow depth of field and attractive bokeh.

Flare is minimal with strong backlighting.

Detail is well recorded.

Focal length range: 18-250mm
Maximum aperture: f/3.5-f/6.3

Minimum aperture: f/22-f/40
Lens construction: 16 elements in 13 groups

Diaphragm Blades: 7
Minimum focus: 45 cm
Filter size: 62mm
Dimensions (Diameter x L): 75 x 86 mm
Weight: 440 grams