I recently purchased a video-capable DSLR (Digital SLR) camera and wanted some ND filters (neutral density filters). I purchased some Zomei ND filters from ebay. Zomei is Chinese, but they make lots of filters and the products are nicely presented. There was very little feedback about Zomei on the web and so I wrote this review.
I found that the Zomei ND2 and ND4 filters were satisfactory. For the ND8 filter, exposure was inaccurate and colour errors were relatively large.
I tested Zomei ND2, ND4 and ND8 58 mm filters. The filters are well made and the threads are good. The filters are not multicoated.
I immediately noticed that the Zomei filters felt different, because they are made from high-density resin. Some Zomei filters use glass, but these ND filters were plastic! The ebay seller had incorrectly stated they were import optical glass (and promptly removed the listings after my negative feedback).
I also tested Kenko SMART ND8 52 mm and ND8 77 mm filters to compare with the Zomei results. The Kenko filters have a slim frame and are not multicoated. They were made in the Philippines, I think using glass from Japan.
I photographed an X-Rite ColorChecker Passport in sunlight on a clear afternoon (uniform and stable lighting is required) with a Canon EOS 350D DSLR and EF 85 mm f/1.8 lens. I set the base exposure (1/500 seconds shutter speed) with a 18% grey card and photographed the ColorChecker four times with fixed aperture f/8 and fixed ISO100:
- No filter and base exposure before.
- ND filter and base exposure.
- ND filter and exposure compensation (slower shutter speed: +1 stop for ND2, +2 stops for ND4 and +3 stops for ND8).
- No filter and base exposure after.
The remainder of this section is very technical and for specific software. You might prefer to jump over to the results.
Linear device RGB is required. I processed each image in Raw Therapee version 4.1 as follows:
- Processing Profile = Neutral (disables most adjustments).
- Input Profile = No profile (device RGB).
- Output Gamma = linear_g1.0 (linear RGB).
- Custom white balance on the third-lightest grey patch of the ColorChecker (any neutral light-grey patch should work).
- For image 2, apply linear exposure White Point Correction (2× for ND2, 4× for ND4, 8× for ND8).
- Crop to ColorChecker target.
- Save as TIFF (16 bit).
Rather than applying exposure compensation to image 2, I could have simply used image 3. The shutter speeds should be accurate enough.
I used Argyll CMS to read the ColorChecker patches from each linear RGB TIFF image:
scanin -v -p -a -G1 -dipn input.tif ColorChecker.cht ColorChecker.cie
-p compensate for perspective distortion.
-a chart orientation normal (not upside down).
-dipn generate diagnostic image.
-G1 Gamma encoding of image (linear).
input.tif is the input image and input.ti3 is the name of the output text file.
ColorChecker.cht is a chart recognition file.
ColorChecker.cie contains the the chart reference data (which is copied to the .ti3 output file).
I copied the scanin output to a spreadsheet. I compared white patch average RGB values with- and without filters (they should all be the same if lighting and exposure were the same).
Next, I used Argyll CMS to compute colour differences relative to the ColorChecker reference data. For this, I had to match the exposure of images 1 and 3 to minimise lightness differences increasing the colour differences. I used image 3 because it has a better signal to noise ratio than the underexposed image 2.
I had previously made my own camera profile for the Canon EOS 350D and matched white patch average RGB values to that profiling image. Alternatively, one could simply profile image 1 as the reference. After applying linear exposure White Point Corrections in Raw Therapee, I ran scanin again and then profcheck to evaluate colour differences:
profcheck -v2 -k image.ti3 profile.icc
-v verbosiy level 2.
-k report CIE Delta-E 2000 colour differences.
input.ti3 is the output from scanin.
profile.icc is the camera profile.
Argyll CMS profcheck takes the patch RGB values from the .ti3 file, applies the camera profile and computes colour differences with the chart reference data in the .ti3 file. I repeat: to minimise lightness errors, the exposure of the image must match the exposure of camera profiling image.
I copied the profcheck output to a spreadsheet. I compared average DE2000 differences with- and without- filters.
After 3-stops exposure compensation, the Zomei ND8 filter was still underexposed −0.49 stop relative to the images without any filters. The ND4 filter was better (−0.17 stop) and the ND2 was good (+0.02 stop). The ND8 filter was effectively a ND11 filter (2^(3+0.49) = 2^3.49 = 11.2), i.e. the effect is stronger than 8×.
For comparison, exposure errors for two Kenko ND8 filters were much smaller at −0.14 and −0.12 stops.
The 3.3 average colour error for the Zomei ND8 filter exceeded a just noticeable difference (DE > 2) and the maximum was 6.4. Colour accuracy was satisfactory for the Zomei ND4 filter (average DE = 2.0) and good for the ND2 filter (average DE = 1.6).
For comparison, average colour errors for two Kenko ND8 filters were much smaller at 1.5 and 1.4 stops.
The following two charts highlight the colour differences when using the Zomei ND8 filter.
We can use a ColorChecker chart, Raw Therapee and Argyll CMS to evalute ND filters.
The Zomei ND filters were increasingly inaccurate for higher densities. Results for the Zomei ND8 filter were unsatisfactory. This doesn’t mean that filter is useless, however it is not the 8× (3 stop) and neutral density (DE < 2) filter that we want. Results for Kenko filters showed that accurate ND8 filters are achievable.
Disregarding the ND8, I purchased two Zomei ND filters for AUD22 (AUD11 each). The Kenko filter was AUD19 (for 58 mm diameter). I think it is worth paying more for better quality glass ND filters.