Back in the large-format film era, there were many images that showed pronounced motion blur from moving water. In many cases, it looked really nice. When films were slow and lenses stopped down to f/90, it was pretty easy to get moving water looking creamy, especially since midday was usually a time for siestas and loading film holders, not photography.
These days, we see a lot of that look to water. In addition, many photographers are making images with a lot of cloud motion. Those images aren’t all made at night, although Michael Kenna has done some marvelous night motion-blurred images. With lens diaphragms wimping out at f/16 or f/22, and camera ISOs generally running from 100 up, we often need assistance in making long exposures under more light than a full moon.
Enter the neutral density (ND) filter. I’d use them more if I could find ones that work well at infrared wavelengths, but I have played with them to good effect.
Today, I’m presenting an aid to planning for shoots using ND filters, with a little table I’ve concocted based on the hoary old “Sunny 16” rule: set your camera’s aperture to 16 the shutter speed to one over the ISO setting, and you’ll have a good exposure for a typical sunny day. I’m not suggesting that you use this table in actually setting the exposure. but it could be useful if you know what lighting conditions you are likely to encounter, and roughly the range of shutter speeds that you want to use. You can then decide what ND filters to take.
Across the top are ND filters you might consider, with stops of attenuation as the sort criterion. The first column is your f-stop, and the rest of the table is the exposure time in seconds using the Sunny 16 rule. The column with 0 at the top is the shutter speed for no filter.
I don’t know how many of you remember the exposure instructions that Kodak used to include with every roll of film that gave the SUnny 16 exposure — although they didn’t call it that — and then the exposure for a bunch of lighting conditions a stop apart. I’ll see if I can get close from memory:
- Snow or beach scene: stop down one stop.
- Hazy sun: open up one stop.
- Cloudy bright: open up two stops.
- Light overcast: open up three stops
- Heavy overcast or open shade: open up four stops.
I hope this helps a few people figure out which ND filters to buy and/or take to a shoot.