Variable ND Filter Scale
This is a design of a scale for a variable neutral density filter.
Traditionally variable ND filters do not have a calibrated scale. They may have a series of equally spaced dots. However those are not numbered and therefore of limited use.
Here is a design of a calibrated scale for use on such filters. It would serve as a reference for repeatability of setting and as an aid to dependent exposure compensation settings.
The numbers show the amount of light reduction as number of stops.
Below are illustrations of this scale. The filter is oriented at 0, 45 and 90 degrees.
At 0° white solid mark
indicates minimum density.
At 45° f-stop reduction
is 2 stops
At 90° hollow circle mark
indicates maximum density.
The above filters are assumed to be 100% efficient. Their scales are calibrated according to an ideal filter. The minimum density is 1 stop below that of using no filter. That is the intensity of light is reduced by one half.
In practise and in the interest of affordability filters are perhaps closer to 80% efficient. This would alter the calibration. Here is an illustration of a 80% filter. Notice the scale is more stretched out. The minimum density available is a 1.3 stops reduction vs. no filter.
The particular scale used depends on the factory spec/quality of the filter.
Here is a tool to generate sample scales such as those shown.
The tool has been updated since these samples have been made. The scale now shows numbered ticks up to 8 instead of 10 for a less cluttered look.
Revised 12 Nov 2018