Just before I left the forum 6or so months ago, the variable ND filter was a relatively new and popular choice as an alternative to the rather pricey 10stop filters. I recently ordered the 7dayshop.com variable filter and thought I would share my thoughts to aid your decision whether or not to buy it.
It claims to be up to 8 stops of ND filtration. (normal exposure x28) This should be dark enough to get the blurry water and motion blur desirable in landscapes whilst also giving us greater flexibility using wider appertures even in harsh sunlight. At just £20 including delivery, I doubt you will find anything cheaper (trust me, I looked). This is for the 58mm thread size and though prices vary for the larger sizes, I expect they will continue to be cheaper than comparable filters elsewhere.
My initial reaction is good. The filter is reasonably heavy; reasuringly so. And there is a nice weight to the rotation of the filter elements. The filter came cased nicely though some movement of the filter is allowed so may cause some cosmetic damage to the frame over time. The filter glass itself is recessed a little so should be fine to be kept in there. The filter thread on the outer side of the filter is bigger than 58mm though so may be a little awkward if you intended to stack another filter on top.
at 18mm on kit lens
On placing the filter on. At the brightest visual adjustment of the filter, it cut roughly 2 stops metering from 1/13th to 0.3 of a second. Contrast & colour remained very good with no signs of vignetting.
At roughly half way, Image quality remained very good still without signs of vignetting.
At the darkest setting however, the filter seemed to have a greater effect in the middle of the frame than at the ends- producing a hugely underexposed centre of the image compared to the outside.
It is worth noting too that kit lens also had trouble metering at darker settings (50mm prime did not).
at 55mm on kit lens
Image quality remains good throughout.
Apparent problem of uneven exposure from edges seems to have gone.
on 50mm Prime
Image seemed fine.
These are just very basic tests in my room. I will endeavour to take the filter out later in the week and see if problems persist. On taking a quick shot out of my window, Skies appeared unnaturally blue- something which will need looking into.