Distortion, CA, Vignetting
Vignetting and Chromatic Aberration typically appear on wide angle lenses as it is not easy control these phenomena on the edges. And indeed, there is a fair amount of CA in the corners throughout the aperture values. Centrally, however, the CA is well-controlled. Not a problem for anybody who post-processes photos anyway and can get rid of the fringing with a few clicks in software like Lightroom and Photoshop.
Here is an illustration showing the lens distortion, as provided by Irix:
Vignetting is very strong at f/4, reaching up to -3.5 EV in the extreme corners, getting to acceptable values (-2 EV) once stopped down to f/5.6 and higher. I did not observe vignetting much when shooting landscapes, I could, however, observe it once photographing interiors. Bear in mind that at 11mm there is some natural light fall-off happening due to physical laws, regardless of the optical design of the lens.
Irix 11mm lens offers a relatively average level of barrel distortion (3.7 %). That is a better value than typically offered by wide-angle zooms (such as the Nikon 14-24mm f/2.8G with 4.6 % at 14mm, or 5% of the Sigma 12-24 f/4 Art lens at 12mm), and still much better than Rokinon / Samyang 14mm fixed prime (suffering from mustache distortion), the only lens offering a much better result is the Venus Optics Laowa 12mm lens. Anyway, I find the Irix barrel distortion quite OK. Adobe software does not offer any profiles for this lens, you have to download it on the homepage of the producer and install it manually to Lightroom or Photoshop.
Sharpness
Central sharpness starts pretty strong at f/4 and quickly gets to peak values between f/5.6 and f/8. It remains strong even at smaller apertures, but keep in mind that diffraction certainly does impact anything beyond f/11. The mid-frame sharpness is also quite good wide open, then quickly rising to very good values at f/8. The corners start a bit soft and smudgy at f/4 and f/5.6, getting to very good values from f/8 and smaller.
I performed a basic sharpness test in the following indoor environment:
And the crops are based on the highlighted areas to showcase center, mid-frame / partway and corner performance. Let’s take a look at the center performance first:
Next, here is the sharpness performance comparison in the mid-frame:
And lastly, the corner performance:
As you can see, unless you need sharp corners at large apertures, this is a pretty reasonable lens when it comes to overall sharpness.
Here is the MTF chart from the manufacturer:
Lens Construction
If you are wondering about the optical design of the lens, here it is:
Ghosting and Flare Performance
Utra-wide angle lenses, especially with a bulbous front element, tend to suffer from ghosting and flare in back-light situations. I was curious to see how the Irix 11mm f/4 would fare with direct and side sunlight, as it boasts about the power of its neutrino technology. In real use, I was positively surprised about the low level of flare. However, you should be aware that there are some angles where flare (colorful circular casts at the edges of the frame) gets a bit nasty.
Here is an example of what happens when you place the sun towards the middle of the frame and shoot straight at it:
And here is the worst case scenario I have encountered, when positioning the sun a bit off center, towards the top of the frame:
Changing the lens angle towards the sun by a very minor change in composition, however, helped to get rid of most of it. Due to the extreme angle, I did not find a situation where such changes were not possible. Overall, a fairly good performer.
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