All rumors suggest that Canon and/or Nikon is going to get into the high-end mirrorless interchangeable lens camera market, in some way.

I don't have any insider knowledge of this, or I wouldn't be able to write this, but it looks pretty likely at this point. It also seems pretty likely to me that both brands will do everything to maintain compatibility with their existing lens mounts. Avoiding the need to design a whole new lens range, maximizing the value of the investment in the existing lineup and side-stepping the kind of anger that Canon generated when it abandoned its previous 'FD' mount (all the way back in 1987) are all major motivators.

Let's imagine what would happen if they decided to make a system that sat alongside their existing DSLRs, rather than heralding their obsolescence

Planning for a future, F-mount compatible mirrorless camera might explain why all of Nikon's recent lenses have finally abandoned mechanical aperture control from the lens mount, since it means they could be fully controlled by an adapter with electronic mounts.

So what's the alternative? Let's imagine they didn't tie themselves to their existing lens mounts. In fact, let's imagine what would happen if they decided to make a system that sat alongside their existing DSLRs, rather than heralding their eventual obsolescence.

What if they decided to make a system designed to be companion camera? A system that offers something your DSLR doesn't do, rather than trying to mimic what it already does?

A small camera, designed for enthusiasts and pros to be used alongside a DSLR or in circumstances where you don't want to lug a DSLR around. A street shooter's camera, a carry-everywhere photographer's camera. Let's think about the potential benefits.

Canon's EOS M range has drifted towards the kind of camera I'm talking about, but doesn't have the lens range to match.

It's not necessarily true that mirrorless promised to be smaller but, not least because it started with Four Thirds and APS-C sensors, that was one of the differentiators when MILCs first appeared. Yet the expectation that a mirrorless camera must replace a DSLR has resulted in an unfortunate convergence.

Demands (including from us) for more battery life, along with the need to handle and act as a like-for-like DSLR replacement has led to mirrorless cameras getting progressively larger. This has helped create a generation of cameras that are nearly as big as their DSLR rivals. And, with the exception of better video implementation and the mixed blessings of electronic viewfinders, little else to set them apart.

So what do you propose?

Essentially, I'm asking for a full frame, interchangeable lens Fujifilm X100. Ok, that might not sound much like an X100, but the common thread is of something relatively small, that by design, doesn't try to do everything. A camera that will sit happily alongside your existing camera (mirrored or not).

You can cover a lot of styles of photography with a couple of short-ish prime lenses. After all, it works for Leica

As with everything photographic, it quickly comes down to a question of lenses. This is the key element to it not being a DSLR rival: don't try to build a full lineup of lenses. I'd propose a camera with a limited number of lenses, starting with a 24mm, a 35mm a 50mm and a 90. And nothing longer than that.

This is because the size benefits that come from removing the mirror from between the sensor and the mount can only be realized with short focal lengths. Stick mainly to the shorter focal lengths and you can keep the camera and lenses smaller.

The lineup should be designed with the expectation that most people will only buy the one or two lenses that suit them. You can cover a lot of styles of photography with a couple of short-ish prime lenses. After all, it works for Leica.

Taking the long view

For me, telephoto lenses ruin mirrorless cameras. There, I've said it.

Telephoto lenses for mirrorless are just as long as their DSLR counterparts, so there's no size benefit to throwing away your mirror. Worse still, these long, heavy lenses demand that mirrorless cameras develop the bulky, bulbous grips that SLRs have evolved since the 1990s.

Creating a limited, dedicated set of lenses relieves a lot of pressure. It means you don't need to build an extensive, open-ended lens range from scratch. No tele zooms, no mid-price 24-70s. Hell, no zooms at all if you don't want to. This is something every mirrorless maker has struggled to do, both in terms of the time it takes to flesh-out a new lineup but also because mistakes get made in any learning process. Every mirrorless system has at least one lens that either isn't as optically good as you'd expect or that focuses much more slowly than you'd want.

For me, telephoto lenses ruin mirrorless cameras. There, I've said it

However, building a lineup of any size is better than building a camera with a full-depth DSLR lens mount in the name of backwards compatibility, since this condemns its users to carrying an empty mirror box around with them for eternity. And that's a punishment with a level of pointlessness right out of Greek mythology.

Nikon's 300mm F4 'Phase Fresnel' or Canon's 'Diffractive Optics' designs provide a route to providing compact telephotos.

Just produce a handful of great, dedicated primes that take full advantage of the new system without any compromises that come from maintaining compatibility with DSLRs. That way you don't have to split your R&D resources trying to keep two full lineups up-to-date.

This also has the advantage that you can sell your camera to photographers with commitments to other systems, because you're not forcing them to choose. But it still gives your existing, faithful users the benefits of full compatibility with your flash systems and other accessories, along with familiarity with your menus.

A small, self-contained system solely aimed at a subset of photographers, rather than trying to be all things to all people. A camera that complements, rather than competing with the existing lineup.

As I say, it'll never happen. But it'd be nice, wouldn't it?

As well as the desire to mesh with the existing lens lineups, the other reason we won't see the camera I describe is because Fujifilm has already effectively invented it