The concept of creating a mirrorless system is very similar in the case of all the camera manufacturers: get rid of the mirror and create a new lens mount closer to the sensor. What if there was another option to do it, arguably a better one?
When we started moving from DSLRs to mirrorless camera systems, we entered uncharted waters, not exactly knowing what to expect. The first instinct was to make the cameras smaller, more portable — tiny body with a pancake lens, a hiker's dream setup we still can get in many of the systems (e.g. the Fujifilm X-E4 or Sony a7C). But then came the realization: while we can make the bodies smaller, it will happen at the cost of ergonomics. And it's even more difficult with the lenses, as laws of physics cannot be bent to the will of even the best marketing division of a camera manufacturer. And so, our mirrorless journey, instead of smaller setups, led us to 2 lbs+ behemoths like the Panasonic Lumix S1 or Canon RF 85mm f/1.2 L — great and capable tools, but no different in size from their DSLR-era counterparts. Or even worse, they were bigger and heavier.
If the size of a mirrorless camera isn't its advantage, then what is? That's a fairly easy one: the autofocus system, with subject tracking, eye detection, animal detection, and so on. Modern electronic viewfinders are a technological marvel as well. The thing is: we didn't need new lens mounts for any of those. We even had Sony SLT cameras for a while, a crossbreed of a DSLR lens mount and an electronic viewfinder. As sensors got the on-chip PDAF systems, this could have been developed in a totally different direction.
The Two-Sensor Camera
Imagine we take a DSLR or SLT camera and replace the focusing screen (in a DSLR) or AF module (in an SLT) with a second sensor, leaving the mirror to split the light between them. Here's a diagram to illustrate it.Such a device would need a modern sensor (two of them) with on-chip autofocus and an electronic viewfinder. IBIS would also be a welcome feature. It would technically be a mirrorless camera with a mirror.
The first advantage of such an approach is pretty clear: we can keep the old lens mounts and the lenses we've gathered over the years. But that's the least important one. Imagine what can be done with two sensors and a mirror between them that splits the light evenly or at a certain ratio. Here are some concepts; let's start with those where light is split evenly (1:1):
- Two Bayer mask RGB sensors shifted by 1/2 pixel. This is a fairly simple option focused on increasing the resolution. Think of it as pixel shift technology without the limitations of multiple exposures, which means it would also work in situations like sport or wildlife photography.
- A Bayer mask RGB sensor and a monochrome one. Combined, they could provide for a better resolution and lower luminance noise. Such a camera could also be turned into a monochrome one with the flick of a button (that would lift the mirror up). No more dedicated monochrome models would be needed.
- A Bayer mask RGB sensor and an infrared one. A similar solution to the one described above, but allowing for simultaneous capture of color and IR information may be useful in some cases.
If we go for a mirror that splits light unevenly (e.g. 16:1, which translates to a 4 EV difference), we could, on the other hand, achieve what computational photography is doing in smartphones: expand the dynamic range. And doing it with two sensors and simultaneous capture would introduce the benefits of HDR to genres of photography that weren't compatible with that technique before, like sports or action. Videographers could also harness this.
The sensors wouldn't need to be the same: one could be a high-resolution module wired to capture the details in highlights and midtones, and the second one could have bigger pixels and a base ISO chosen with the purpose of capturing shadow details in mind. Imagine having a Sony a7R IV and an a7S III in one body.
There's one last possible variant is a non-translucent mirror, that either reflects 100% of light to sensor one or moves up, revealing sensor two. Those sensors could differ in size, such as a 24-megapixel full-frame sensor and a 24-megapixel Micro Four Thirds sensor. Such a combination could be a nice option for bird photographers: a larger sensor with bigger pixels for low-light situations, and a smaller, more packed one, to get the extra reach when light is sufficient. No need for a second, crop-sensor body in your camera bag.
While such a dual-sensor camera could widely expand the creative possibilities for both photographers and filmmakers, there are still some issues to be considered. Let's discuss them:
- Synchronizing the sensors. If such a camera offered IBIS (which would be very welcome), the stabilization modules would have to work in perfect sync. The same goes for on-chip autofocus units, assuming both sensors had them.
- Manufacturing. If we take several sensors with different resolutions and/or capturing range (RGB, monochrome, IR, etc.) and multiply that by several mirror variants (1:1 light split, uneven light split in several variants, no split), the resulting number of camera models would be overwhelming, both for the manufacturer and the customer. Getting it to work would probably require an approach used in the car industry, where the customer orders a certain engine, trim package, and color, and picks up the car several months later. Not everyone would be happy with such a sales model replacing a store shelf with cameras on it.
- Old lens mount. While keeping the EF, F or A mount would be great for the customer, it wouldn't be for the manufacturers. With new mounts, they can sell us the same (or better) lenses again, but with a higher price tag.
Are we ever going to see such a camera? With the industry shifting so rapidly towards new lens mounts, it's not very likely. But it's nice to have dreams. Maybe Pentax will come to the rescue?