Sigma's early constant-aperture standard zoom — the 28-70mm f/2.8 EX for Pentax K mount.
The Sigma 28-70mm f/2.8 EX is an early constant-aperture standard zoom for Pentax K mount. The EX designation indicates Sigma's professional tier. At f/2.8 across the standard zoom range, it provided Pentax shooters with a fast standard zoom at a time when Pentax's own options were limited. The 28-70mm range covers the essential standard zoom focal lengths.
Optical performance is decent for the era. Centre sharpness is good from f/2.8. The constant f/2.8 aperture is the main advantage — providing consistent DOF and exposure throughout the range. Corners are softer wide open. The EX build quality is solid. Autofocus speed depends on the Pentax body's screw-drive motor in this mount version.
Pentax K mount. Filter thread is 67mm. Weight is approximately 530 grams — moderate. Build quality is solid with the EX designation. No image stabilisation — benefits from Pentax's in-body Shake Reduction on digital bodies. The Pentax K mount ensures compatibility with all Pentax DSLRs. Screw-drive AF on Pentax bodies.
Available on the used market at low prices. Check AF accuracy and smooth zoom operation. One of the few affordable constant f/2.8 standard zoom options ever available for Pentax K mount. The in-body SR on Pentax DSLRs compensates for the lack of in-lens stabilisation. Adequate for Pentax shooters on a budget wanting fast standard zoom coverage.