Decoding Neutral Density Filter Designations
Last fall I posted a threepart article on essential filters. In discussing neutral density filters, I made passing reference to the fact that different manufacturers use different means of designating their strength in stops. There are basically three different systems for labeling neutral density filters. At the time, while I understood two of the three, I really didn't fully understand why the third one was the way it was. In an online email discussion list about Nikon cameras, I recently learned the missing piece of the puzzle.
Starting from the beginning though, the easiest to understand of the three methods of designating filter strength involves simply listing the number of stops the filter blocks. One stop, two stops, three stops, pretty straightforward.
A stop is a doubling or halving of any value, which brings us to the second system for labeling filter strength. A filter that is rated as blocking one stop passes one half of the light striking it. Similarly, a two stop filter passes one fourth of the light and a three stop filter lets through one eighth of the light. Some manufacturers choose to list the denominators of this series instead of the number of stops itself. The series therefore becomes just a list of the powers of two: 2, 4, 8, 16, 32, and so on. Manufacturers such as Hoya and B+W use this system.
The third system, and the one I recently learned the details of, relies on a standard from the German national standards organization Deutsches Institut für Normung (abbreviated DIN) that specifies the use of logarithms for certain kinds of numeric sequences. Finding yet another photographic use for high school math, let's go over what logarithms are all about. A logarithm is the power to which you have to raise a given number (known as the logarithm base) to get the desired number. In base 10 logarithms, 10 to the first power is 10, so the log base 10 of 10 is 1. One hundred is 10 squared so the log base 10 of 100 is two. Ten is the most common base used for logarithms and also the one used by the relevant DIN standard. So, what does all this have to do with filters? Well, if we take the logarithms of the series consisting of the powers of two used in the method previously described, we get 0.3, 0.6, 0.9, 1.2, 1.5 and so on. If at one point or another, like me, you've puzzled at how Tiffen and other manufacturers label their filters, you will immediately recognize this series as being what they are using. Mystery solved.
To summarize, here are all three methods listed side by side:

Stops 
Filter Factor 
DIN Rating 
1 
2 
0.3 
2 
4 
0.6 
3 
8 
0.9 
4 
16 
1.2 
5 
32 
1.5 
6 
64 
1.8 


Almost makes you want to get all your filters out and read them more closely, doesn't it?
