Here we'll compare some popular teleconverters on the G12/G11/G10. The image on the left shows maximum optical zoom, and the one on the right shows how far you can zoom back toward wide before vignetting starts to appear.
All samples were taken with the Lensmate G12/G11/G10 lens adapter, sections A + C combined. Two of the converters (the Canon TC-DC58B 1.5X S3/5 and the Sony VCL-DH1758) require a 58mm spacer ring to be added to this setup to provide adequate clearance for the zoom lens. More information with the image samples below.
To calculate 35mm equivalent of actual focal length in exif, multiply x 4.59. Example: The maximum optical focal length of the G12/G11/G10 is 30.5 (actual). 30.5 x 4.59 = 140mm 35mm equivalent.
To calculate the teleconverter factor, multiply x the rating of the converter. Example: Maximum telephoto of the G12/G11/G10 is 30.5mm (140mm equivalent) x 1.5x (teleconverter rating) = 210mm.
Focal lengths given have the teleconverter factored in. The samples on the right show how far you can zoom back toward wide before vignetting starts to appear.
Camera Settings: Manual 1/320 f4.5, iso 80, Sunny WB, IS turned off.
Original Large/Fine jpegs cropped in CS3 and saved as jpeg 10 to minimize file size with minimal image quality loss.
(The difference in exposure between the first and second samples is attributed to a G12/G11/G10 Manual mode quirk - as the focal length decreased, the aperture got larger as the camera would in auto mode in low light. We'll reshoot these when weather permits.)