What Does Transmittance Mean in Photography?

Transmittance is usually talked about in relation to filters that go onto a lens.  When used in this context,it is the amount of light that is allowed to pass through the filter, and is usually represented as a percentage.  For example, a filter with a 50% transmittance will let 50% of the light through it into the lens and camera.  Photographers are usually concerned about transmittance when they are using neutral density filters.  ND filters deliberately cut down the amount of light entering a lens so that a photographer can control the shutter speed, aperture or ISO beyond otherwise available limits.  For example, using a long exposure to blur moving water.

Below is a table of all the most common neutral density filters and their equivalent transmittance. If you’d like to delve further into the mathematics, please read my article, Understanding the Names and Numbers On Neutral Density Filters.

F-Stop Reduction Optical Density Filter Factor % transmittance
0 0 0 100
1 0.3 2 50
2 0.6 4 25
3 0.9 8 12.5
4 1.2 16 6.25
5 1.5 32 3.125
6 1.8 64 1.5625
7 2.1 128 0.78125
8 2.4 256 0.390625
9 2.7 512 0.1953125
10 3.0 1024 (sometimes called ND1000) 0.09765625
11 3.3 2048 0.048828125
12 3.6 4096 0.0244140625
13 3.9 8192 0.01220703125
13 1/3 4.0 10000 0.01
14 4.2 16384 0.006103515625
15 4.5 32768 0.003051757813
16 4.8 65536 0.001525878906
16 2/3 5.0 100000 0.001
17 5.1 131072 0.0007629394531
18 5.4 262144 0.0003814697266
19 5.7 524288 0.0001907348633
20 6 1048576 0.00009536743164