The flexibility of the G12 is greatly enhanced by using a lens adapter. With the right lens adapter, the G12 can be fitted with filters, close-up lenses, wide angle converter lenses and telephoto converter lenses. Although Canon makes its own adapter, the LA-DC58K, I decided to use the Lensmate adapter, Canon G12/G11/G10 Adapter, which comes in three sections: A, B, C (see above photo, left-to-right). Lensmate developed their system in response to vignetting complaints about the Canon system and answers some questions in a FAQ page. I decided on the Lensmate system based on my favorable experience with Lensmate adapters on my G9.
In the Lensmate system, Part A is the base and is always used as the connection to the camera body. Part B terminates in a 58mm filter thread and is the same outside diameter as Part A. Part C flares to a diameter larger than Part A and terminates in a 72mm filter thread adapter. The basic idea is that Part B is used to mount telephoto adapters and close-up or macro lenses whereas Part C is used to avoid vignetting at wide angle focal lengths. Part A can actually be used alone to mount filters or wide angle converters; however, care must be taken to avoid zooming the lens into the filter or conversion lens. I bought the full set of A, B, C adapters.
With the A and B parts, filter size is the same as for the other G series cameras so I already have the polarizing and graduated neutral density filters. The 72mm filter for part C is quite large but I have 77mm filters used with DSLR lenses. I could get adapter rings but am considering a Cokin system with adjustable graduated neutral density filters. It seems to me that polarizing filters do not create even effects for wide angle lenses.