Irix 15mm F2.4 Firefly & Blackstone Review

Distortions

Distortion refers to the capability of a lens to represent straight lines as...straight lines. Wide angle lenses often create barrel distortion, where lines look like they've been put on a sphere. Telephoto lenses can create the reverse effect, named pincushion distortion. The following images illustrate the various types of distortion:

Barrel distortion, left; no distortion, center; pincushion distortion, right

It is very difficult to create a zoom without some distortion. It is also very hard to design a wide lens, even a prime, which is distortion-free. On the other hand, normal and short tele lenses can more easily avoid the problem.

The way to evaluate distortion is pretty straightforward: take a picture of straight regular lines, and look if they curve. Calculating the ratio of curvature yields an evaluation of distortion.

Cameras such as the K-3 and K-1 include built-in distortion correction, which can be automatically applied to files captured files. These corrections are limited to the company's lenses, however. Irix lenses will not benefit from any correction.

Distortion Test results

The following images show how the distortion pattern looks when photographed with the  Irix 15mm, for APS-C and full frame sensors.

APS-C

Full frame

In a rare move, the manufacturer provides distortion figures on its own website. Our findings are in accordance with these values, with 2% of distortion. While this number is large, it is not surprising for an ultra wide lens.

How does this translate in actual shooting scenarios? This first image illustrates what occurs when the lens is tilted a few degrees. In that case, distortion is dramatic.

This second image shows the same building and demonstrates that, when the composition is properly adjusted, the effect can be kept under control.

Verdict

At 2%, distortion will be visible in many scenes, especially if the lens is tilted.