When asked which ND (Neutral Density) Filters to get, I always tell people to get a 3, 6, and 10 F-Stop. A 6 F-Stop ND Filter means that this filter will reduce the amount of light entering your camera by 6 F-Stops. However, not all ND Filters label the intensity of their filters in such a simplified way, and more so, different brands have different labeling methods. In my eBook, The Ultimate Guide to Fine Art Long Exposure Photography, I explain all the ins and outs of ND Filters. But, for your convenience, I will provide you with a short and easy-to-understand guide that will provide you with all the information you need when purchasing your next ND Filters, so please, keep on reading below.

Overall, there are three different methods for indicating the intensity of an ND Filter:

1. Density, is measured by this formula, as in Tiffen and B&W filters, for example: 

Density = (0.3) x (F-Stop Reduction)

 Therefore, when shopping for say a Tiffen or B&W filter that reduces the amount of light by 6 F-Stops, you will look for a filter with a Density of: Density= 0.3 x 6 = 1.8, and
3 F-Stops will be 0.3x3=0.9
10 F-Stops will be 0.3x10=3

3. Filter Factor, is a very popular method amongst filter brands and refers to the multiplicative amount of light a filter blocks. The number of F-Stops required to correct the exposure with a given filter may also be calculated using the formula:

 Where ‘x’ is the number of F-Stop increases required.  i.e. a filter that reduces the amount of light by 3 F-Stops will have a Filter Factor of:

I will take a B+W filter for example: All of these three measurements are printed on the filter as such (example for a 10 F-Stop Reduction Filter) and also shown below.

To read more about ND Filters, please refer to my blog on Which Filters to Buy? Part I