Here's a little excerpt from my upcoming book, The Lighting Notebook, that you might find interesting. I hope it's helpful to you!
An ND filter is essential when you want to shoot in full sun, at f1.4, for example. Remember your Sunny 16 rule? On a sunny day, to achieve f16, use 1 over the ISO rating as your shutter speed. For example, if the minimum ISO of your camera is 200, you would use 1/200 second shutter speed to properly expose at f16.
Doing the math, and working backwards to f1.4, means I’d need a shutter speed of 1/51,200! This is faster than any current SLR camera shutter. A three f-stop neutral density filter would bring my shutter speed down to 1/6400 second – within the capabilities of most modern DSLRs. An eight f-stop ND filter would bring the shutter speed right back down to 1/200th, which is within my optimal flash sync range.
When I need more effective output from my flash systems, and I am working in a lighting situation that would normally require a faster than optimal shutter sync speed, I will also turn to a neutral density filter rather than using a speedlight and high-speed sync. The ND filter allows me to set my shutter speed to 1/250 second, or slower, and still keep my larger aperture. I’ve discovered, through my own testing, that I can get more useable output from my flash systems at full power coupled with an ND filter than I would using high-speed sync at faster shutter speeds. It also allows me to use my other flash systems that otherwise wouldn’t work with sync speeds over 1/250.
I keep a Vari-ND filter in my camera bag at all times. This is a neutral density filter that seamlessly adjusts from 2 to 8 f-stops of density with a simple twist of the filter. Buy one sized for your largest lens filter size (77mm for example) and then get some step-down rings for your smaller lenses.
The first image below was shot with a Lumiquest large LTP softbox on an SB900 speedlight on full power. I needed a shutter speed of 1/8000 second to properly expose my sky and clouds, and use an f2.0 aperture – so high-speed sync came in to play.
The second image uses an ND filter, adjusted to give me a matching exposure for the sky, resulting in 1/250 second shutter speed. The same flash setup, still on full power, gave me 2 f-stops more output! This is a clear indication of the efficiency advantage of the ND filter with a speedlight.
Here are a couple of ND Filter options that I've used and like: