Redstone update adds lock screen camera button to some Windows 10 Mobile devices6
There is talk that the new feature will appear in next week's Insider build. The idea is to allow those sporting a phone without a dedicated camera button, the opportunity to have a shortcut that allows them to get to the camera quickly.
At this point, the lock screen camera button is not customizable, but we could see new features added prior to the release of the final version of Redstone. In addition, only Windows Phone models with on-screen keys have the camera button available. This also is subject to change before the Redstone update is final.
source: Winbeta.org via WindowsCentral