variable ND filters and "x" pattern

Qarik

been looking at purchasing a variable ND filter and have noticed across the board..singhray, tiffen, hoya, heliopan..there are some users who experience a large black x in the images typically near darkest setting. Anyone have an idea what causes this?:huh

ian408

I'd go with the two different coatings and their alignment. I'm not 100% sure how they work but I'm guessing it's something like that.

ziggy53

I believe that there are several factors which gang together to produce the effect. Partly it has to do with the fact that variable-ND filters are 2-stacked polarizing filters, and often they use a linear polarizer in front of a circular polarizer arrangement. (I believe this order preserves AF and proper metering.) Some variable ND filters are also made of 2-circular polarizing filters, stacked together.

Most of the complaints about the x-effect occur with wide-angle and super-wide-angle lenses, and even a single polarizer can yield uneven skies with those lenses. The effect is most pronounced in scenes with strong and direct sunlight with lots of clear sky (which is a highly polarized scene).

Indeed, you can make your own variable-ND filter just by stacking 2-polarizing filters. The trick is that while this is easy to do, you have to be exceedingly careful about the filter choice in order to preserve color neutrality. In fact even the best, and most costly, variable-ND filters have "some" shift in color as they get darker.

There is also a loss in sharpness with any polarizing filter, and stacking 2 polarizers together will almost certainly visibly degrade the image, especially around the periphery of the image.

While variable-ND filters offer a great amount of convenience, the problems associated with the "x" effect, color cast, and loss in sharpness may be more than many will accept.

To maintain the best overall image quality I suggest using single ND filters as possible, and stacking ND filters for additional ND effect (if you must). Reserve variable-ND filters for those situations where conditions require rapid ND changes and/or conditions which require multiple different exposures in rapid succession.

If you "must" use a variable-ND filter, you may wish to avoid the combination of a very-wide-angle focal length lens and the strongest ND setting, the combination of which will almost certainly produce the x-effect darkening pattern.

Mitchell

I have the new version of this filter from lightcraft.

http://lightcraftworkshop.com/fader-nd-mk-ii.html

Overall, I've been pleased with the results and the construction. The price is great compared with the other brands. I have not noted any color shift.

The dreaded "X" does appear at wider focal lengths when the filter is used above 5 stops. I believe this occurs with all filters above this type. Just a strange type of vignette that occurs at wide focal lengths due to the overlapping polarizers.

thegridrunner

Here is a good video on variable ND filters
http://vimeo.com/54971758