I recently acquired the Pro Optic Adapter with Aperture Diaphragm manufactured by Kiron to mount my Canon EF 28mm to my E-P1. I bought it from Amazon. In the instructions, it says not to turn the aperture ring past “6”; there are no f-stops, just the numbers 1-6. Of course, because the f-stop is a function of focal length, it would not be possible to have them marked on the adapter unless you had a different adapter for every lens. Anyway, I noticed when looking at the adapter that the number 6 position was considerably bigger than what f16 should be. A comparison with my lens confirmed it; the number 6 position is about four stops bigger than f16 on my 28mm. However, if I continue to turn the aperture dial, I could approximate f16. I marked the position on the adapter with a Sharpie marker, and then did the same for f8. These two f/stops and wide open allow me plenty of DOF control in the streets. Using the Sunny f16 Rule, I verified that the camera was giving me the right shutter speed for what I thought the f-stop should be. Anyway, just wanted to pass this on to anyone who is using an adapter with a diaphragm in it.
Here's a shot i did with my E-P1 and the canon EF 28mm 2.8 of a kid playing guitar on the street-

I just ordered one to use a Sigma 20-40mm f2.8 that has an EOS mount. It tends to back focus on my 40D, but that won't be a problem on my E-P1. I'm looking forward to using the lens a lot more.

__________________
Love mercy, do justice, walk humbly - Micah 6:8
"A true photograph need not be explained, nor can it be contained in words." Ansel Adams