Pysical dimension of optic, in mm.
Listed as length x width x thickness
R in 25mmR stands for Ringed.
Most of our round filters are housed in anodized aluminum rings.
25mm denotes the outside diameter of the ring.
The clear aperture will be smaller than this.
NR in 25mmNR stands for Non-Ringed.
Some, but not all filters are available without rings.
Here, 25mm denotes the outside diameter of the filter, no ring included.