If you do any work with Neutral Density Filters, here’s a helpful reference chart of exposure times:
For example, let’s say you’re looking for a target exposure time of around 1 minute and have a 10-Stop ND filter with you.
Set your camera and lens up on the tripod for the composition you’re seeking, then dial in your aperture to f/8 (for argument’s sake we’re starting at f/8) in aperture priority mode, lowest ISO, and check to see what your camera recommends for shutter speed (remember, in aperture priority mode, you choose the f-stop and the camera will choose the appropriate shutter speed for the “correct” exposure).
Arbitrarily, say the camera sets the shutter speed at 1/30s. Looking at the chart above, if you dial back the shutter speed to 1/15 seconds without a filter on, adding the 10-Stop filter will give you the target shutter speed of 1 minute. You can close down the aperture one stop to f/11, which would increase shutter speed to 1/15s.
Now put the camera in manual exposure mode, and manually focus the shot (auto focus gets weirded out and may not focus properly if you use an ND filter on the front of the lens, so manually focusing before adding the filter is the best way to achieve proper focus). Next, if using a Nikon, you set shutter speed to “Bulb” (on a Nikon, selecting “Bulb” means you manually control shutter speed depending on how long you press the shutter release…I don’t know what other camera systems name it).
Finally, you mount the ND filter on the front of the lens, press and hold the remote shutter release for one minute, and you’re done!
Here’s a real-life example of what an ND filter can do for a photograph: I set up two cameras right next to each other: A Sigma Quattro DP0, and a Nikon D810 with the Nikon 14-24mm lens at 14mm.
This is a shot taken with the Sigma DP0 Quattro, no filter attached. This camera has a fixed lens at a full-frame focal length equivalent of 21mm:
I was in aperture priority mode, and the shutter speed was 1/6s at f/22, ISO 100. (Can you see the car in the clouds?)
The shot below (which is the same one I shared a few weeks ago), used a 5 stop ND filter on the Nikon D810 at 14mm (you can see how much wider the composition is than the 21mm shot above):
Both images were taken at exactly the same time. As you can tell, the clouds were moving pretty quickly that evening…and moving in the same direction (west to east) as the lawn was cut. Had the clouds been moving in a different direction, it would have made the photograph look geometrically awkward since the clouds wouldn’t have lined up with the mowed grass.
In my experience, Neutral Density Filters have proven to be the single most valuable type of filter I’ve used. Not only can ND filters allow you to shoot at extremely long exposures which would not be possible without these filters, but an ND filter can also allow the photographer to control depth of field, regardless of the amount of ambient light available. For example, on a typically bright sunny afternoon, using a fast prime lens wide open (at, say, f/1.4) would be difficult, if not impossible, due to an exceedingly fast shutter speed beyond the capability of the camera’s shutter (usually 1/4000s or 1/8000s). But, with an ND filter on, you can slow down that exposure, without any loss in image quality…and grab that portrait with a crisply thin DOF.
You can also use an ND filter to remove any moving objects from a scene. For example, if you’re shooting a waterfall, and there is no possible way you can set up the shot without people walking into your composition…if you use an ND filter to extend the exposure time, any moving objects will literally disappear and become invisible (provided the objects aren’t a source of light, like flashlights or a mirror reflecting the sun). It’s MUCH more efficient to use an ND filter to clean up the scene rather than trying to clone out everything in Photoshop later.