Now that the Vari-ND is leading more and more photographers to explore the realm of slow motion imaging, many of them want to go even slower. The new Mor-Slo filter is a quick, easy way to add even more density to the Vari-ND Filter without losing the ability to "dial" whatever degree of density you want.
Simply by placing the Mor-Slo 5-stop ND Filter in front of your Vari-ND, or behind your Vari-ND thin mount, you can achieve densities at any point between 7 to 13 f-stops.
The Singh-Ray Mor-Slo is available in 77mm or 82mm, either a standard filter ring with mounting threads on both front and back or a thin-mount ring with no front threads.
Here's an example of how the Vari-ND and the Mor-Slo Filters can be used to produce extremely long exposures ...
let's begin with a typical outdoor scene at mid-day. The shutter speed would be slowed from 1/500 at f8 (with no Vari-ND on the lens) to 1/125 at f8 (with the Vari-ND on your lens and set at its minimum density) to as slow as 1/2 second at f8 (with the Vari-ND set to its maximum 8-stop density).
At this point, if you add the Mor-Slo Filter in front of the Vari-ND, you can slow your exposure to as much as 30 seconds at f8 -- or 4 minutes at f22.