  Cameras provide a vast amount of information at high rates and are part of
many specialized or general-purpose devices. This versatility makes them
suitable for many interaction scenarios, yet they are constrained by geometry
and require objects to keep a minimum distance for focusing. We present the
LensLeech, a soft silicone cylinder that can be placed directly on or above
lenses. The clear body itself acts as a lens to focus a marker pattern from its
surface into the camera it sits on. This allows us to detect rotation,
translation, and deformation-based gestures such as pressing or squeezing the
soft silicone. We discuss design requirements, describe fabrication processes,
and report on the limitations of such on-lens widgets. To demonstrate the
versatility of LensLeeches, we built prototypes to show application examples
for wearable cameras, smartphones, and interchangeable-lens cameras, extending
existing devices by providing both optical input and output for new
functionality.
