Sony has been pushing hard in the mirrorless camera realm for about seven years now, and they have undoubtedly made major progress both in terms of technology and market share. Still, Canon and Nikon hold a major advantage over Sony that if leveraged correctly can help them significantly in the long run.
I have been primarily a Canon shooter my entire life. I shot on Canon film cameras growing up and ended up in their digital ecosystem purely by chance when I found a good Black Friday deal and kept growing from there. The geek in me certainly was tempted by Sony when it exploded onto the scene with quickly advancing technology. So, in 2017, I found a used a7R II body. There was only one problem: all my glass was made by Canon.
However, one of the big promises of mirrorless when it first hit the market was the ability to adapt lenses. Due to shorter flange distances due to the lack of a mirror box, the promise was that pretty much any DSLR lens could be adapted to a Sony mirrorless camera and that that would rewire how a lot of photographers thought about and worked with their gear. I certainly jumped on that hype by buying a Metabones adapter. At first, it seemed great: my notoriously finicky 85mm f/1.2L lens focused perfectly on my big toe when I tried out the adapter for the first time sitting on my bed. However, the limitations quickly came into focus: the autofocus speed of adapters was frequently very slow and sometimes glitched out completely depending on the attached lens. It became clear that it could not be relied on for crucial work and certainly not for anything that required continuous autofocus — fast or slow. Adapters were great for video shooters or anyone wanting to experiment with vintage glass, but the promise of a system with universal compatibility wasn't all we had hoped.
What is important to remember is that a decade ago, the photography landscape looked very different. DSLRs ruled the land, and Canon and Nikon ruled DSLRs. In fact, in 2010, Canon had a 44.5 percent of the market share of interchangeable lens cameras, with Nikon at 29.8 percent, and Sony at 11.9 percent. Furthermore, Canon and Nikon had (and continue to have) deep lens libraries, with many professionals deeply invested in those systems.
This created an appreciable amount of intertia for a lot of photographers in terms of gear. Switching brands is tedious and often costly, and even if another brand offers clear advantages, a photographer will often stick with their original brand due to the depth of their investment in that system. Nonetheless, Sony managed to win a lot of users over by aggressively pushing mirrorless technology forward (as did Fuji), a testament to just how exciting their cameras have been and a symptom of Canon and Nikon's hesitancy to dive into mirrorless seriously.
Canon and Nikon's Advantage
However, things are different now. Both Canon and Nikon have acknowledged the mirrorless future and entered the market space seriously. And while they have released lenses for their respective mirrorless mounts (in particular, Canon has aggressively developed their RF glass), their advantage lies in those deep lens libraries and Newton's First Law. Photographers are generally reluctant to switch brands, and the more deeply they are invested in a specific company's equipment, the deeper that inertia goes.
Eventually, the industry is going to be entirely transitioned into the mirrorless realm, and eventually, the majority of photographers are going to have to follow it. If first-party adapters (e.g. a Canon adapter for adapting a Canon DSLR lens to a Canon mirrorless camera) did not maintain the full performance of the lens (like my Metabones experience), this would level the playing field quite a bit, as a photographer would have the opportunity to reevaluate their brand investment when the time to switch to mirrorless came. However, working with a first-party adapter is a vastly different experience.
Reviews of Canon's EF-to-RF adapters and Nikon's FTZ adapter have almost universally praised autofocus performance, indicating no loss in autofocus speed and just as importantly, equivalent (or often even increased) accuracy and precision due to the fact that mirrorless cameras do not have separate autofocus sensors and thus do not need autofocus microadjustments. This is huge, as it means photographers already invested in Canon or Nikon DSLR lenses can buy a mirrorless body and complete the transition to mirrorless at their own pace or even just stick with their DSLR lenses indefinitely.
Of course, Canon and Nikon want people to purchase their new mirrorless lenses, and Canon, in particular, is doing a fantastic job of creating show-stopping lenses that offer performance and features a step up from their already great latest generation EF lenses and thus giving reason and temptation for photographers to upgrade, but before a photographer decides to purchase new lenses, the company has to convince them to buy their specific lenses. Having first-party adapters that make the mirrorless transition so smooth help to lock users into a brand all the more and leverage that aforementioned inertia.
Canon is doing an especially great job of leveraging this key benefit. While the Nikon Z 6 and Z 7 and Canon R5 and Canon R6 all have in-body image stabilization, Canon is getting innovative with their adapters. There is the standard adapter for $99, which allows you to adapt EF lenses and maintain all the standard functionality. There is the Control Ring Mount Adapter EF-EOS R for $199, which adds the Control Ring found on all RF lenses that allows one to control parameters like ISO using a dial on the lens.
Then, there are the Drop-In Filter Mount Adapter EF-EOS R with Circular Polarizer Filter ($299) and Drop-In Filter Mount Adapter EF-EOS R with Variable ND Filter ($399). These are especially interesting for a few reasons. First, they can be fantastic options for landscape photographers, as they save money and bulk by obviating the need to carry filters of different sizes and make it especially easy to work with lenses with bulbous front elements like many ultra-wide options. This gives photographers an easy and affordable way to maintain compatibility of their lens libraries or to explore more interesting options that enable new ways of shooting.
10 years ago, Canon and Nikon ruled the DSLR world, and there are literally hundreds of millions of their lenses in the libraries of photographers around the world. Combined with the fact that their first-party adapters provide a seamless experience and significantly ease the financial pressure of transitioning to mirrorless, Canon and Nikon have the chance to significantly leverage their previous DSLR market shares as we move into the mirrorless future.