Making the grad(e)

If you read the popular photographic magazines as we do, when you see images of landscapes there is often mention made of using a neutral density graduated filter to control the density of the sky.

What is a Neutral density graduated filter?

As the description says the filter is neutral in colour, reducing all colours equally, graduated in density from top to about halfway down the filter.

That got me thinking, maybe it is time to take some neutral density graduated filters out into the field for a test.

What was I aiming to achieve?

I wanted to see how accurate these filters were in terms of density, graduation, colour and whether they affected sharpness in any way.

The filters used in the test consisted of 4 Z size soft graduated filters – 3 optical resin filters – Zomei ND8, Zomei ND4 and 84.5 ND8, plus 1 glass filter – Zomei ND4. (Incidentally 84.5 state that all their filters are hand made and individually checked.)

Prices for the filters range from £15.99 for the resin Zomei filters, £39.99 for the glass Zomei filter and between £26.89 and £36.30 for the resin 84.5 filters. (84.5 filters are priced in euros and are currently on special offer.)

Test filter line up

All the filters were fitted to a Cosina 19-35mm lens via a Zomei Z filter holder to a Nikon D800, exposure was set manually, everything was mounted on a tripod and shutter was actuated by wireless remote. The scene was metered for the central area of the image the camera was then set to manual using these settings for all the test images. Actual settings were – daylight balance, 100 ISO, shutter speed was 1/250, focal length 19mm and aperture was set to f8.
It is worth noting that the wider the aperture on the lens the more subtle the graduation effect is.
To test the graduation of each filter, they were placed in the filter holder firstly equal to to bottom of holder, then the filter was moved down the holder so the graduation covered more of the image.

In practical use a graduated filter would not be place so far down in the holder, however in this instance it was to test how the filter affected the white clouds in terms of obvious colour shift.

The Results

The initial NEF files were converted to Jpeg format in Adobe camera raw, no adjustments were made to the images. Thee subsequent images were compared to the control image on a colour profiled monitor.

There were a few surprises in store!
All the resin filters caused a red cast to some degree or other.
To be more specific, Zomei ND8 made the sky very warm whilst warming the clouds to a slightly lesser degree, graduation was quite sharp. 84.5 filter kept the sky more neutral but made the clouds noticeably red, again a fairly abrupt graduation.  Zomei ND4 warmed up sky and clouds a little but not as excessively as the denser previous filters and exhibited a reasonably smooth graduation. Only the Zomei ND4 glass filter kept the image neutral and had a very smooth graduation.
In terms of density of each of the filters, if we use the 84.5 filter as correct density since this filter is claimed to be individually tested, the 2 Zomei resin filters are within specification, whilst the Zomei glass filter appears to be a stop lighter than claimed.
None of the filters seem to have affected the sharpness of image, although, the camera having been set in auto focus mode was noted to shift focus when the resin filters were repositioned in the holder whilst with the glass filter focus remained static.

Control image

Filters in mid position

Filters in full down position

Conclusion

If used with care and a degree of post processing any of the resin filters are borderline acceptable, the Zomei filters offer reasonable value for money if only occasionally used. Sadly the same cannot be said for the 84.5 filter, being almost as expensive as the Zomei glass filter but without the neutrality. This makes the Zomei glass filter the winner in terms of value for money and being way ahead in terms of neutrality, the only reservation with the Zomei glass filter is the inaccuracy in filter density.

I cannot honestly say that I will be frequently using the resin filters, I shall probably just invest in a further denser glass graduated filter.

C

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