I wanted to test out this cheap variable ND filter from Fotga (found here on eBay), and so I purchased a small 46mm filter to fit my Sigma E-Mount lens on the Sony NEX7. Major major fail today because I left that camera at home.
So today I just held it in front of the Canon 40mm F/2.8 pancake shooting with the T4i. I wanted to see if there was a purplish color cast (normally found on cheap variable ND filters), but was very relieved to see it didn't! My camera was set to manual at 1/50th shutter speed, F/2.8, and ISO 100. You'll see a section of the video where I pull the filter away. To keep this ideal shutter speed and still shoot at a wide aperture, you can see how overexposed the camera would be without an ND filter.
The next test everyone is wondering about would be loss of sharpness, especially when shooting at longer focal lengths. Unfortunately I only ordered a 46mm which doesn't fit any of my longer lenses. It looks pretty good so far, so i'll place another order for a 77mm filter size. I'm happy with this $13.00 dollar purchase, even though it took several weeks to arrive, it's a perfect little ND filter for my little Sigma 30mm F/2.8.