Tamron has announced it is to offer its 17-50mm f/2.8 mid-range zoom for Pentax mount.

The lens was previously available only for Canon, Nikon and Sony systems.

The Tamron SP AF 17-50MM f/2.8 XR DI II LD Aspherical IF is designed for exclusive use with DSLRs with an APS-C sized sensor.

Tamron emphasises that this lens is not designed for use with 35mm film cameras or DSLRs with image sensors larger than 24mm x 16mm.

It delivers 17mm wide angle coverage and covers virtually the same angles as focal lengths of 26-78mm when converted to a 35mm format.

Since the lens is designed exclusively for APS-C sized digital cameras, the image circle diameter has been reduced.

"Moreover, careful adjustments to the design have achieved a lightweight, compact body with a filter diameter of 67mm, overall length of 81.7mm, and weight of 434g", adds the manufacturer.

To counter aberrations, the lens uses XR (Extra Refractive Index) glass as well as two hybrid aspherical elements and an LD (Low Dispersion) glass element, which compensates for on-axis and lateral chromatic aberrations

The new lens achieves a minimum focus distance of 0.27m over the entire zoom range, which is claimed to be one of the best in its class among fast standard zoom lenses with an f/2.8 maximum aperture.

The maximum magnification ratio of 1:4.5 at the 50mm setting is also said to be the top level in this class of lens.

In terms of structure, the lens has a new triple cam structure where cams zoom and focus within a three-layer lens barrel.

Additionally, says Tamron, the engineering of the lens has been completely overhauled to downsize its parts and to increase their precision and durability.

Key features at a glance for the Tamron SP AF 17-50mm:
- Focal length: 17-50mm
- Maximum aperture: f/2.8
- Minimum aperture: f/32
- Maximum magnification ratio: 1:4.5
- Lens construction: 16 elements in 13 groups
- Minimum focus distance: 0.27m
- Dimensions: 74x81.7mm
- Weight: 434g