In the fall of 2018, we took a trip along the Blue Ridge Parkway in the hopes of capturing fall colors.  Typically, waterfalls on the Parkway are less than impressive in the fall, but this year, the waterfalls were quite elegant, while the fall colors were disappointing.  The impressive waterfalls begged for me to use my neutral density filters to give the wonderful creamy appearance.

Singh-Ray Variable Neutral Density Filter

I had a Singh-Ray 77mm (Thin Mount) Vari-ND Variable Neutral Density filter with me.  Fortunately, I had done some previous evaluation of the performance of this filter on my Sony a7RII with a 24-105 f4 lens, but I had failed to remember that my 16-35 f4 lens was only 72mm.  With a little bit of work, I was able to get some nice waterfall images.

Upper Lindeville Falls: 0.5 sec, f13, ISO 50 (Singh-Ray filter)

Rake Mill Pond: 1.6 sec, f11, ISO 50 (Singh-Ray filter

While these were satisfactory, in the top picture I had to push the f-stop up to f13 to slow the shutter speed to 0.5 seconds (sort of at the very low end for shooting waterfalls for the creamy look).  

In previous evaluations of this variable density filter, I knew a couple of very important things.  First, a bit of an explanation about the Singh-Ray filter.  The density of the filter (according to published specifications) is from 2.4 to 8 stops.  This variable density is accomplished by rotating the front right (below) from min to max.  I am assuming that the first line on the left represents about a 25% adjustment, the midline about a 50% adjustment and the right line about a 75% adjustment.

1. At a focal length of 24mm, there was some significant blue cast vignetting apparent on the Sony 24-105mm f4 lens at 24mm which improves at 28-30mm.  

1/160, f7.1 ISO 200 at 24mm

2. At a focal length of 16mm (72-77 step up plus Singh Ray Vari-ND), the vignetting issue gets much worse (see below) on a Sony 16-35mm f4

3. At a density setting of 75%, a purple hue effects the image (see below and compare to the one above)

4. At near maxium density setting, a horrible X shaped color cast forms (see below)

Singh-Ray Conclusions

1. The full range of 2.4 to 8 stops is not useable as color casting becomes a significant problem above the 75% mark.  I am guessing this is about 6 stops

2. Vignetting on the Sony 24-105 is evident at 24mm and disappears at 28-30mm.  On the Sony 16-25mm f4 (with 72-77 step up ring), vignetting is a severe problem at 16 mm and improves(nearly gone) at 20mm

Breakthrough X4 Neutral Density filters

Because of the Singh-Ray limitations, I decided to evaluate Breakthrough X4 neutral density filters.  I tested the 6 stop and the 10 stop filters on both the Sony 16-35 f4 (with a 72-77 step up ring) and the Sony 24-105 f4.  

Vignetting: Neither the 6 stop for 10 stop filters caused any vignetting on the 24-105mm f4 lens.  On the Sony 16-35mm f4, the 72-77 step ring did not cause vignetting (see below)

Adding the 6 stop filter caused a very small amount of vignetting that was easily corrected in software (see below)

Sony 16-35mm f4 with 72-77 step up ring and Breakthrough 6 stop X4 ND @ 16mm

Stacking the 72-77 step up, 6 stop and 10 stop neutral density filters did cause considerable vignetting at 16mm (see below), but the vignetting disappeared at 18mm (second picture below)

Sony 16-35mm f4 @ 16mm (72-77 step up ring,6 stop and 10 stop X4 ND stacked

Sony 16-35mm f4 @ 18mm (72-77 step up ring,6 stop and 10 stop X4 ND stacked)

No Color Cast with Breakthrough X4 ND filters

I did a very simple test of color cast with the Breakthrough filters.  I took three images with no filter, with the 6 stop filter and then the 10 stop filter using the Sony 24-105mm f4 lens.  The images are below and they are without any correction.  It appears that there is no to very minimal color cast with either of these two filters.

No filter

6 stop Breakthrough X4 ND Filter

10 stop Breakthrough X4 ND Filter

From each of the images above, I pixeled peeped to evaluation sharpness.  At a 2:1 magnification, I saw no evidence of loss of sharpness (see below)

Breakthrough X4 Neutral Density filters conclusions

1. On either the Sony 16-35mm f4 or 24-105mm f4 there is excellent vignetting control

2. There appears to be no to minimal effect on sharpness of the image.

3. There is no obvious color cast with either the 6 or 10 stop filters.