Bests & Worsts Reviews from Amazon

As a pro photographer I carefully researched variable ND filters beforehand. The Tiffen and Singh-Ray seemed to come out in the top of the heap, and so I ordered both and tried them out. My primary goal was to be able to shoot headshots in bright light outdoors at wide apertures blurring backgrounds and using fill flash. I also wanted to be able to use the filter for landscapes, too (so for the Singh-Ray I got the thin mount to avoid vignetting). I was going to be testing for COLOR CAST, SHARPNESS, and VIGNETTING. I tested both lens under different lighting conditions on different days, with and without filters on, and on different Canon lens (I shoot with a Canon 5DmkIII). First thing I realized was the reviews were right, you cannot use your lens hood with either filter. I believe you can buy a lens cap that will cover one or the other; otherwise you have to be really careful. The greater problem outdoors though is without the lens hood you can get horrible flare. BOTH lens have a COLOR CAST; it's described as "warm" but it's a warm green cast which might be fine for landscapes for some people, but even with custom WB using a gray card and color checker, and shooting in RAW, I was not happy with the cast. It wasn't fully correctable on skin tones with either filter; especially nasty at greater density. At the lightest density the cast was more correctable but still not as accurate as without the filter. BOTH lens have a VIGNETTE at anything below 35mm, especially the more density you use. The Tiffen was worse on some lens, achieving the infamous X very clearly and quickly, whereas the Singh-Ray had a more muted X only at the higher density. The vignetting wasn't limited to corners, both crept in at the sides in landscape mode or top/bottom in portrait mode in anything greater than light density. That was a deal breaker. As you can imagine, when you darken the background you darken everything, and you have to light your subject. First problem was not being able to use my lens hood with either filter! This created horrible lens flare that ruined shots, so I repositioned the subject, but the near-sunset shots on an overcast day were just ghastly with lens flare and a green cast that was impossible to correct. When I got the background dark enough the subject was so dark my 580exII in an EZbox at max power was not strong enough to light the subject unless the box was right beside him. Lighter density was great, it brought down the intensity of the light perfectly and allowed for fill flash just fine. But, the color cast was a deal breaker...so much so sharpness wasn't even a consideration at that point, so I can't comment on sharpness! I returned both filters (thank you amazon and Adorama). I know others have had joy and success with both filters, but for my needs these just did not work. Even the landscapes had a cast I did not like, and I do professional retouching, too, so I know all the tricks. And why have a vari-ND filter if you can't use it below 35-40mm on your 17-40mm lens at 17mm? I think if someone just wants to use one for a very light density and doesn't care about skin tones, then they'd be ok with it. You will find portrait and landscape photographers who love or hate the vari-ND filters. Each to his/her own.