I recently borrowed a LightCraft Fader ND filter. I wanted to see how a variable ND filter would perform, specifically on my 10mm-22mm Canon lens.
The following test was performed at 10 mm using a continuous florescent light source. Min and Max refer to the markings on the filter, which did correspond closely to the lightest and darkest settings. At 10mm the filter has significant pattern issues on the dark end.