Keith over at Northlight Images has completed his extensive review of the Irix Firefly 15mm f/2.4 lens. We seem to have an abundance of fast ultrawide lenses to choose from now. Canon, Sigma, Rokinon, Zeiss and now IRIX all offer competent lenses at different price points.
The Firefly version of this lens has a lower build quality than the Irix Blackstone 15mm f/2.4, which Keith touches on.
From Northlight
Optical performance is crisp and with good detail once you stop down a bit. My rough and ready testing suggests that on my 5Ds, f/6.3 or 7.1 give the best overall performance. That said if it’s a central subject the f/2.4 setting restricts DOF enough to soften the background for closer subjects. The amount of coma at the widest aperture may be an annoyance for astro photography, but it’s on a par with many other wide lenses I’ve looked at.
If you’re not used to it, then manual focus definitely will take some getting used to, but after a while you rarely need to give it too much thought. Modern DSLR viewfinder focus screens are pretty poor for manual focus, so even with a focus confirm indication, your distance estimating skills will improve too. Read the full review
Irix lenses are available at B&H Photo