A lens cap can be added to a camera lens to protect it from scratches or the outside elements when not in use. If you use a variable ND (Neutral Density) lens filter you may wish to add a lens cap to the filter, so that this is also protected.
Normally, when you add a cap to a lens, you look for a cap that’s the same size as the lens, to ensure a perfect fit. This isn’t the case, however, if you want to add a cap to a variable ND filter. It simply won’t fit.
The reason behind this relates to how a variable ND filter is made. Essentially, this filter comprises of two layers of polarising glass. The outside layer is slightly larger than the inside layer (which screws onto the lens), to prevent vignetting from occurring. Since the outside layer is bigger, you need to accommodate for this when purchasing a lens cap for a variable ND filter, by going up a size.
For example, if you have a 67mm variable ND filter, you should choose a 72mm lens cap.
It’s usually a good idea to remove a variable ND filter when not in use.