Canon EF 16-35mm f/2.8L III USM

For years the answer to "which fast ultrawide for a Canon body" was a coin flip between this lens and the Nikon 14-24mm f/2.8, and the Nikon usually won on corners. The Mark III is Canon's reply to that. Where the Mark II went soft and smeary toward the edges wide open, the III pulls the corners up to a level that finally competes, and Canon shooters who had been adapting the Nikon onto EOS bodies for clean edges no longer had a reason to. The across-frame bite is best at the wide end. Stop down to f/5.6 at 16mm and the whole field is crisp. At 35mm the corners still want f/8 before they fully tighten, so it is not uniformly sharp at every focal length the way the marketing implies.

It is an EF-mount full-frame zoom built late in the DSLR era, 2016, with the f/2.8 aperture and a bulbous front element that pushes the filter thread out to 82mm. That last detail matters more than it sounds. The earlier 16-35 II took 82mm too, but plenty of Canon shooters built their kit around 77mm filters, and this lens forces an upgrade of your whole ND and polarizer stack. The bulge also means no front-mounted grad systems without a wide holder, so landscape people who live on graduated filters plan around it.

Contrast is high and clean, flare is well controlled for an ultrawide with that much glass facing the sun, and the color is the warm, slightly forgiving signature that carries from this generation of L zooms. That is the reason to buy it. At 16mm there is the rectilinear stretch you expect, faces and bottles at the edges get pulled wide, so it is a tool for space and architecture rather than people up close. Distortion at 16mm is on the strong side, more than you would hope from a lens this price, and most people just lean on the software lens profile to straighten it. Vignetting is heavy at f/2.8, three to four stops into the extreme corners, and it eases as you stop down but does not vanish by f/4.

The people who pick it up are landscape and real estate shooters, the event and wedding crowd who want one fast wide that survives a dark reception, and astro shooters who push it open under the Milky Way. Coma is where the III earns its keep. The Mark II was notorious for smearing point sources near the edge into little wings wide open, and the III cleans most of that up, which is a big part of why it became a recommended ultrawide for star fields. There is still faint residual coma in the extreme corners at f/2.8 and toward 35mm, so the most demanding astro shooters still cross-shop the Sigma 14-24mm Art, but for terrestrial work you never see it.

On film bodies it gets interesting, because an EF ultrawide on an EOS-1V or an Elan gives you a genuinely fast, sharp 16mm on 35mm film, which is rare territory. If you are running a polarizer or a stack of NDs on that 82mm thread for long daylight exposures, meter for the filtered scene and let Zone Light Meter fold the filter factor into the reading so your wide-open shadows do not collapse. Today the III sits in the upper used tier, undercut by Canon's own RF 15-35mm f/2.8 on mirrorless and by Sigma's cheaper Art, but for anyone still on EF glass it is a wide whose corners hold up.