Understanding ND filters – Optical Density Number
We’ll learn about Filter Factor Number in this discussion. There are many different strengths and varieties of ND filters. When buying these filters, you will be confronted by different abbreviations and terms. These can quickly get overwhelming because they tend to confuse buyers—especially someone with limited knowledge of ND filters and their types.
ND filters are optical tools, and they’re named and labeled using terms that are related to the world of optical technology. So, you can come across something like ND 0.3, which refers to the Optical Density Number of the filter. You can also come across numbers like ND 2 or ND 4 and so on, which suggest the Filter Factor Number. We’ll discuss Filter Factor Number some other time.
Photographers prefer an indication that tells them what the rough amount of light the filter stops is. So that when they’re composing and setting the exposure, they can consider the light-stopping strength of the filter and dial the exposure settings accordingly. From a look at the packaging, it’s rarely possible.
So, what is the alternative here? How do you negotiate around these confusing terms and acronyms? Familiarizing yourself with them is a straightforward solution. For example, an Optical Density number of 0.3 suggests that the filter stops one stop of light. You know that one stop is equivalent to halving or doubling the amount of light depending on whether you’re stopping down or up). So, mounting an ND 0.3 on your lens will halve the light; therefore, you can double the shutter speed to counter it. That opens creative opportunities.
Let’s take an example to understand this. Let’s say you’ve metered an exposure at 1/500 sec at f/8. With an ND 0.3 that stops half the light, you can use a shutter speed of 1/250 sec or an aperture of f/5.6 (never both simultaneously).
Let’s say you use an ND 3.0 that stops ten stops of light. You can now use a shutter speed of 2 seconds. That will give you enough time to blur the movement of water, clouds, and people. A ten-stop ND filter is a very dark glass. Once you mount that on your lens, you can no longer use your optical view finder or even the LCD screen at the back of the camera to compose.
You must compose the shot first, calculate the exposure, dial that in, screw the filter on your lens, and then readjust the exposure manually. Alternatively, you could calculate the revised exposure details, dial that into your camera and then mount the filter.