It's a Photokina year, so it's not surprising that Panasonic would be announcing something besides its Lumix LX100 II advanced compact, but the company has been firmly Four Thirds since its first interchangeable-lens model came out over 10 years ago. So the rumor that the company plans to show a prototype of a full-frame camera on Sept. 25 at the show, for release next year, comes as a surprise.

There's also a countdown page for its press conference, which means something big is probably in the works.

The news was broken by 43rumors, which gives it an "FT5" probability rating -- its highest level. And it comes after similarly high-probability rumors in July (which I missed) of a high-end Olympus model with a new sensor format. Since Olympus is also a Four Thirds company, it's pretty compelling evidence for a new-size sensor.

To recap, full-frame sensors are signficantly larger than the Four Thirds sensors in Panasonic and Olympus cameras, with a 3:2 aspect ratio compared to the smaller's 4:3. Note that the Four Thirds sensor size is distinct from the Micro Four Thirds mount (despite how I labeled the graphic below). 

It's a notable development for a couple of reasons. The different sensor aspect ratios require different lens mounts for one. Micro Four Thirds, helmed by Olympus and Panasonic, is a licensable lens standard, compared to the proprietary mounts from Nikon, Canon and Sony. So it's possible that any full-frame mount might be open as well. 

If true, it also means that we may finally get much better low-light and higher-resolution images from Panasonic. Micro Four Thirds models are great in tons of ways -- system size is one of the biggest -- but low-light quality has never matched that of full-frame or even APS-C models, mostly because of the smaller sensor size, which has also limited the maximum resolution -- it currently tops out at 20 or so megapixels. 

However, it's not necessarily a full-frame sensor, which harkens back to the size of a frame of 35mm film. It could be a new, larger-size Four Thirds sensor that would encompass full-frame so you could use those lenses with an adapter using some kind of multiaspect mode.

Some people are speculating that since Leica is Panasonic's long-time lens partner that Panasonic might use the SL-Mount that Leica uses for its SL mirrorless, though I kind of hope not, since those lenses are really big. What would likely make sense is hybrid mount that could also take MFT lenses for a crop mode. 

Since its cameras are native mirrorless, they may not need an adapter the way you need to mount a DSLR lens on a mirrorless camera; the adapters adjust the distance between the sensor and the lens (the flange distance) to compensate for the absence of a mirror.

The full-frame mirrorless segment is currently owned by Sony's A series thanks to a several-year head start. But Nikon has just jumped in with its Z series and Canon's EOS R full-frame mirrorless announcement is rumored to be imminent. However, Panasonic has been innovating a lot in the mirrorless space and working it for a lot longer than either Nikon or Canon, so it would be very interesting to see what kind of developments it brings to the format.

Panasonic declined to comment.