giving 11.2mm f/5 full frame fisheye
Use of this extender require taping of extender pins as shown at bottom to avoid ERR01. Without the extender the 8mm lens gives black corners on the image and suffers a bright ring of flare when used in the sun. The extender simply removes the offending parts of the image still giving a much wider view than the Zenitar 16mm. Samples can be found on my Peleng Gallery:
http://www.pbase.com/dougsmit/peleng
Taping the extender is not necessary if you use a Focus Confirmation M42/EOS adapter which fools the camera into thinking there is a real 55mm f/1.4 lens attached.