Image quality is very good in most respects, but the loss of sharpness towards the edges and corners of images is a drawback.
Minimal barrel distortion
Wide angle of view
Sharpness drops towards the edges
Maximum aperture only f/4
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Mount: Pentax K | Construction: 8 elements in 6 groups, 7 diaphragm blades | Closest focus distance: 18cm | Filter thread: 49mm filter | Autofocus: driven from camera | Dimensions: 63 x 40mm, 189g | Tested on: Pentax K5 II
The current vogue for retro chic design in cameras and lenses hasn't been lost on Pentax. From its 'Limited' range of lenses, this 15mm optic almost drips with yesteryear charm. It's beautifully engineered with the barrel and screw-in front cap being made from hand-machined aluminium. Inside, there's an aspherical element and ED (Extra low Dispersion) elements, plus Pentax's new HD (High Definition) coating which is designed to reduce ghosting and flare while increasing light transmittance.
Taking the 1.5x crop factor of Pentax APS-C format SLRs into account, this lens has an especially wide angle of view, at 86 degrees. However, it's certainly not a 'fast' prime lens. Its widest available aperture of f/4 is three stops slower than the f/1.4 lenses, and two stops slower than an f/2 lens.
The autofocus system is nothing to get excited about either. It's driven from a motor in the camera body and, while it's quite rapid, it's distinctly noisy. Handling is impaired by the manual focus ring rotating during autofocus, but at least full-time manual override is available.
Considering that this lens such a wide viewing angle, it's a real achievement that it also delivers very little distortion. There's very little vignetting, even at the admittedly modest widest aperture of f/4. However, image quality is let down by poor sharpness towards the edges and corners of the frame at apertures narrower than f/11.
Sharpness: The sharpness chart provides an indication of lens performance across the focal range (for zoom lenses) and at different lens apertures.
Centre sharpness is good but you need to narrow the aperture all the way to f/11 for decent sharpness in image corners.
Fringing: Results for colour fringing are pretty average and quite similar to those from other wideangle prime lenses. Lower scores are better.
Considering it gives such a wide angle of view, the minimal barrel distortion is impressive. Negative values indicate barrel distortion, positive values indicate pincushion distortion, and figures closes to zero are best.
Verdict: Image quality is very good in most respects, but the loss of sharpness towards the edges and corners of images is a drawback.