As the mirrorless camera battle reaches a turning point, three entrants are teaming up to help their cameras stand out. Leica, Panasonic, and Sigma announced today that they’ll all use the same lens mount format for full-frame and APS-C cameras, allowing lenses purchased for one company’s cameras to work on each other’s cameras.

That’s a huge benefit for two reasons: it makes their cameras more appealing to photographers who won’t find themselves locked into a single company’s lens ecosystem and it’ll help to expand the lens selections that are available for their cameras, rather than each company trying to build out their own exclusive lens lineup. Junichiro Kitagawa, Panasonic’s international marketing chief, called it a “win, win, and win situation.”

The agreement is reminiscent of what happened with Micro Four Thirds cameras, where Panasonic and Olympus created a lens mount standard that allowed any company to build a camera that supported the same format of lenses. It’s helped the standard to flourish, even as larger sensors grew in popularity.

The agreement will help them mount a challenge during the transition to full-frame mirrorless

Panasonic and Sigma will adopt Leica’s existing L-mount as part of the agreement. That’s likely because Leica is the only one of them to have launched full-frame mirrorless cameras so far, and lenses already exist for the format. Panasonic announced its first full-frame mirrorless cameras, the Lumix S1 and S1R, earlier today, and they include the new mount; Sigma has yet to announce a full-frame model but says that it plans to develop some.

While the announcement shows all three companies focused on full-frame cameras, the L-mount is meant to work with smaller APS-C sensors as well. Leica has differentiated the two lens types with the names SL (for full-frame) and TL (for APS-C).

The partnership, which the companies are calling the L-Mount Alliance, makes a lot of sense. For one, these companies don’t always directly compete with each other. Leica is at the very high end of the market and focused on photographers. Panasonic has made a lot of headway with video shooters in recent years. And Sigma... kind of does its own thing.

Then there’s the simple fact that, without this agreement, these companies were facing a difficult battle. Sony has already become such a name in full-frame mirrorless cameras that it essentially forced Canon and Nikon to come in and compete. And now that Canon and Nikon are joining the fray, it’s going to be much harder for other companies to get attention.

Leica, Panasonic, and Sigma likely would have struggled on their own. But together, they may be able to mount a more formidable challenge to Canon and Nikon, especially since both major camera makers are still establishing themselves in the mirrorless field.

Panasonic has already announced that it’s working on three L-mount lenses, and it says it’ll have 10 within a year of the Lumix S1’s launch, which currently planned for spring 2019. Sigma has said it will start to make lenses next year. And Leica, having debuted the mount in 2014, already has six lenses of its own, with five more planned over the next two years.