You can ensure the Lens starts in the rear camera by configuring your project info as such:

1. Click Project Info at the top left.
2. Select that your Lens works on Only Rear Camera

Note that the Lens Works On drop down allows you to set which camera needs to be open in Snapchat for the user to see the Lens in your carousel. Here you can set if you want your Lens to appear on the Front Camera (for Face Lenses), Rear Camera (for World Lenses) or Both Cameras (for Face and World experiences).

Lens Works On won't actually disable any components, but rather indicates which carousel your Lens will appear in. We recommend you use Both Cameras so users can find your Lens in both carousels. That being said, if you select Both Cameras it will default to opening in the front camera. If you select Only Rear Camera, it will open in the rear camera when you unlock the Lens as originally desired.