If you own a Pentax DSLR and are thinking about buying a portrait lens, here's your limited choice.
Most professional photographers recommend the 85mm lens for portrait photography. If you own a Pentax DSLR, your choices are limited to two 85's that are designed to work with Pentax's APS-sensor based DSLRs and covers the 35mm equivalent of a 127.5mm lens.
Samyang 85mm f/1.4 Aspherical Lens (Manual Focus)
(Adorama price: $279)
DxOMark Score: Not tested
Overview: With its low price and surprisingly high DxOMark overall score, the Samyang 85mm f/1.4 is a manual-focus only lens, but its aspherical multi-layer elements reduce flare and ghosting, producing sharp, contrasty images that in many ways (especially distortion) rival those produced by pricer name-brand optics. The lens has 8 diaphragm blades, is moderately light at 19 ounces, consists of 9 elements in 7 groups, focuses down to 3.3 feet, and takes a 72mm filter.
Web reviews: Mark Golstein at Photography Blog says focus is a bit "dreamy" at the widest aperture but sharpens up by f/4, and calls it "a very affordable alternative to the OEM offerings." Photozone says that while image quality is not super-sharp, it is "certainly good enough for portraits," describing the lens as a "very affordable introduction to shallow depth of field photography."
Sigma 85mm f/1.4 EX DG HSM for Pentax
(Adorama price: $969)
DxOMark Score: Not Tested
Overview: Indie lensmaker Sigma’s sharp large-aperture 85mm pro-level portrait lens features SLD glass and a special element coated for minimized chromatic aberration and reduced flare and ghosting, even when shooting backlit scenes. Its 9-blade diaphragm consists of rounded blades for natural-looking, rounded specular highlights and pleasing Bokeh, and is a go-to lens for wedding and portrait photographers.
Web reviews: SLR Gear's Andrew Alexander exclaims the Sigma 85mm f/1.4 EX DG HSM is "as sharp as any of its contemporaries, even wide open at f/1.4." As Camera Labs points out, the lens is the cheapest 85mm f/1.4 with AF, but "you have to be very careful to focus optimally."