Review of the FADER ND2-ND400 filter (Green.L)
Filip A. Sala
http://if.pw.edu.pl/~sala
Introduction
FADER ND2-ND400 filter is a neutral density filter with variable density. You can change transmission of the filter from
ND2 (-1EV) up to ND400 (approx. -9EV) by rotating it. It is produced by Green.L. It is probably made out of two polarizers
and a quarter-plate in the front. Quarter-plate changes linearly polarized light into circular one so the filter does not act like
polarizing filter.
Testing
Filter was tested using Mir-24H lens (focal length f=35mm) on a camera with a crop 1.6x (so the field of view corresponds
to the focal length f=56mm). Filter thread: M58.
Resolution was tested for f/2 at the center and at the edge of the image. Up to ND256 (-8EV dropdown) no significant
deterioration of sharpness was observed. White balance was set to Auto.
Resolution was also tested for f/4. It is also visible that there is no significant vignetting for ND64.
For settings ND256 and higher, strong vignetting and a dark pattern appear. Such pattern is caused by the strain in glass
placed between crossed polarizers.
When it comes to color reproduction the filter gives a slight yellow cast on the image. It is visible on the comparison below.
Test was made with the white balance set to Tungsten. On the white area histogram it is visible that filter absorbs the blue
light which is a cause of the yellow cast. For settings above ND256 filter gives very strong blue/violet cast (see previous
page ) which makes it useless.
Summary
The filter gives quite good sharpness, no significant deterioration was observed. It is useable in the range ND2 –ND256.
Above this range strong vignetting and blue/violet color cast appear. Color reproduction is not bad but slight yellow cast is
clearly visible in a whole range. Significant drawback of such filters (build with two polarizers) is that the change in
transmission is not linear with the rotation angle. Between ND2 and ND4 there are 45 deg. but between ND4 and ND8
approx. 15deg. and so on. Between ND64 and ND128 there are only ~3 degrees so it is very difficult to set it precisely.
Such drawback is present in all fader filters made out of two polarizers and is caused by the nature of such setup. Light
intensity after passing through two polarizers is a function of a squared cosine of the rotation angle.
Summarizing it is a quite good filter for a very low price comparing to other fader filters.