Pentax's vintage constant-aperture telephoto — the SMC 70-210mm f/4 for K-mount systems.
The Pentax SMC 70-210mm f/4 is a vintage telephoto zoom for Pentax K-mount cameras with a constant f/4 aperture. The constant aperture design was a professional feature, providing consistent exposure throughout the zoom range. SMC multi-coating ensures good contrast. It covered the standard telephoto range popular for portraits, sports, and wildlife.
Optical performance is adequate for the era. Sharpness is respectable when stopped down with the constant f/4 providing reasonable viewfinder brightness. SMC coatings help maintain contrast. The build quality and constant aperture place this above budget variable-aperture telephoto zooms from the same period. Bokeh is acceptable at wider apertures.
Pentax K mount, manual focus. Filter thread is likely 52mm or 58mm. Weight is approximately 530 grams. Build quality is solid metal construction. Compatible with all Pentax K-mount bodies including modern DSLRs with stop-down metering. Benefits from Pentax's in-body Shake Reduction on digital bodies.
Available on the used market at low prices. Check for smooth zoom and focus operation. The constant f/4 aperture is the main advantage over variable-aperture competitors. A functional vintage telephoto for Pentax K-mount shooters. Modern Pentax telephoto zooms are dramatically better but this remains usable.