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Pentax f/4 SDM 17-70mm Zoom: Pentax’s New Lens Has a Wide Aperture with Fast, Quiet Autofocus

Pentax continue on their quirky way with the release of their latest general-purpose zoom. It sits between the upgraded 18-55mm kit lens, and the top level DA* range. The new lens covers a focal-length range from 26mm wide angle to 107mm telephoto in the 35mm format.

SDM Only

This is the most significant development as Pentax make a late commitment to SDM. The full title is Supersonic Drive Motor. This is Pentax’s name for the piezoelectric motor used in the autofocus system. Their small size allows them to be included in the lens, and their speed and accuracy improves autofocus operation. Pentax initially relied only on a camera body mounted motor with a mechanical coupling to the lens focusing system.

Long after most manufacturers followed Canon’s lead in introducing the system, Pentax’s latest top of the range DA* lenses now have SDM. The Pentax DA 17-70 breaks new ground by incorporating SDM in an ordinary DA general purpose lens, and there is no provision for autofocus drive from the camera motor. This lens is SDM only.

A Pentax representative said this indicated a new policy for Pentax of making SDM only lenses, or those using only the camera mounted motor. Although their recent DA* lenses have both systems, the DA 17-70 marks the start on a one system only policy for any new lenses. According to Pentax one of the reasons for this policy is to accommodate their extremely compact Limited range of lenses. These exceptional quality prime lenses are very compact and do not have room in the lens body for the SDM system.

The Pentax DA 17-70mm retains the quick shift clutch in the focusing mechanism so photographers can make a final quick manual focus adjustment simply by turning the focusing ring on the lens barrel.

Constant f4 Aperture

One of the compromises lens designers make with zoom lenses to cut costs is the effective aperture reduces as the focal length increases. The Pentax DA 17-70mm lens maintains the same aperture over its entire zoom range. The larger maximum aperture at full focal length gives it an advantage over other general-purpose zooms for better performing autofocus and image quality in low light situations. Typically, other zooms have a smaller f5.6 maximum aperture.

In Between

It is still not up to the size of the DA* zooms with their f2.8 apertures, and this is partly why the DA 17-70 is an in between lens. It also lacks the weather sealing of the DA* lens range so a bit more care is required in adverse conditions, although Pentax say their new SP (Super Protect) coating protects the exposed lens elements and repels dust, water and grease. The SDM does set it apart from the ordinary Pentax lens range.

Comparing the DA 17-70 to offerings from Canon and Nikon, the Pentax lens still falls in between. It has a larger maximum aperture and shake reduction capabilities give it an advantage over competing consumer grade lenses. It does lag behind the oppositions pro grade models with their f2.8 apertures, but generally at about half the price. The Pentax offers value for money with above average performance potential.

Camera Body Compatibility

The Pentax representative said the company will continue to develop bodies that support the body based focus system in addition to the SDM system.

Pentax highlighted the advantages of the SDM focusing system, such as silent and accurate AF, making his an ideal lens to be used with their latest DSLR bodies. Specifically the K20D, K200D, K10D and K100D Super with the latest firmware installed will use the SDM system autofocus drive system. However, the DA 17-70 is still usable with older DSLR bodies in manual focus mode.


  • Lens mount: PENTAX KAF3 mount
  • Lens construction: 17 elements in 12 groups
  • Angle of view: 79° – 23° (with PENTAX D-SLR camera body)
  • Number of diaphragm blades: 7
  • Minimum aperture: F/22
  • Minimum focusing distance: 0.28m (0.92 ft.)
  • Filter size: 67mm
  • Maximum magnification: 0.31X
  • Dimensions (diameter x length): 75mm x 93.5mm (3.0 inch x 3.7 inch)
  • Weight: 485g (17.1 oz.) without hood; 515g (18.2 oz.) with hood

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