Best full-frame DSLR
Pentax's 55mm f/1.4 lens is outside the price bracket of this group, costing a hefty £600/US$800 (around AU$880). That leaves the 50mm f/1.8 but, on a direct comparison of maximum aperture, it's three times more expensive than the Canon 50mm f/1.8. So what do you get for your money?
Build quality is quite tough overall but the mounting plate is plastic rather than metal. The lens is fairly compact and lightweight, as you'd expect from a 50mm optic with a maximum aperture of f/1.8 rather than f/1.4.
There's no lens hood, but the front element is well recessed at the infinity focus setting. However, it ends up quite close to the front of the lens at the shortest focus setting, as the inner barrel extends as you focus down.
The lens doesn't have an internal autofocus motor, instead being screw-driven from a motor in the camera body. It's not particularly fast and is very noisy.
One plus point of the 'quick-shift' focus system is that you get full-time manual focus override, but it's stiff to operate unless you switch to manual focus mode on the camera body. At least the aperture is quite well rounded, based on a seven-blade diaphragm.
Sharpness is very good at f/1.8 and is excellent in the range between f/2.8 and f/11. Colour fringing is fairly low and barrel distortion is slightly less apparent than with the Canon and Nikon 50mm lenses on test.
The upside of a reduced maximum aperture is that the Pentax offers extremely good sharpness at wide and medium apertures.
Colour fringing is impressively low at large apertures, although it rises slightly as you progress through the aperture range.
Like the Canon and Nikon 50mm lenses, there's a little barrel distortion, but the amount is a bit less and barely noticeable.
Image test verdict
It's rather expensive for a 50mm f/1.8 lens with only a basic feature set, but all aspects of image quality are very pleasing.