I just bought one of those adjustable Neutral Density filters (ND2 – ND400) that I noticed having surfaced on the market for very reasonable prices (I paid £13 for mine in 77mm size). Of course, one needs to be careful what one puts in front of an expensive lens as a bad filter will make a great lens look mediocre.
For the lenses of my 35mm cameras and DSLR I have sets of different ND filters: 0,3, 0.6, 0.9 for my prime lenses and it is always difficult to pick the right density factor straight away and sometimes I feel like I really need to add two or more of them together to get the reduction in light I want. The Adjustable ND filter covers a whole range and more and getting just one filter does make a lot of sense for people like me who carry lots of filters around in different colours and sizes.
I took my Pentax 67II and loaded it with some Fuji Acros 100 as I know that I wouldn’t need to correct for reciprocity failure with exposures of up to 30 seconds. My first trip took me to the Grand Union Canal around Rickmansworth.
The weir in the photo above was shot with an exposure time of 15 seconds. The water flowing over the weir turned nicely white and the rest of the water developed that nice, undisturbed quality. To take this shot, I turned the filter until the camera’s TTL meter showed the desired exposure time in the viewfinder.
This second photo shows the lock at the same location, again the exposure time was set via the TTL meter and by turning the filter until an exposure time of 15 seconds was displayed in the viewfinder. As a result of the long exposure, the whirlpool in the front of the photo became a lot more noticeable.
The results were very promising, even in full sunlight the adjustable ND filter allowed me to chose exposures of 15 seconds. The next day it was one of those days you have occasionally in autumn: very clear but loads of wind. I figured that I could go to the Ruislip Lido and capture a couple of clouds in motion with an Orange Filter and the adjustable ND filter.
In the bright sunlight that would often suddenly change when a cloud covered the sun, the handling of extreme long exposures became tricky. But even if the lighting stayed constant, with the two filters, it looked like the TTL results where not reliable any more. To be investigated!
To summarize: The adjustable/variable Neutral Density filter I used did everything it promised on the box and I haven’t seen any drawbacks yet. It allowed me extreme long exposures that the camera could handle via the TTL meter. Only time will tell if the filter is durable but the construction does like solid and certainly not more flimsy than any polarizer filter that I own. The only (minor) remark that I have about this particular filter is that the filter size of the front of the filter is not the same as the filter size of the lens which means that once the filter is on the lens, you cannot use the lens cap for protection of the filter.