I have noticed that when using the Letus Extreme on the EX1 that I can see traces of the ground glass (even when it is vibrating) when I have the fixed lens set to a higher f-stop. For example, f8 leads to more artifacts than f4 and f 4 in not as clean as f1.8.
This is a well known limitation of most oscillating glass 35mm adapters. You don't want to close down more than f5.6. Shutter speeds should be kept low as well.
The Letus Ultimate can be stopped down as far as you want without seeing the ground glass because it spins vs. oscillates. However, even the Ultimate should not be used with shutter speeds higher than 1/500th.