Originally Posted by Jeffrey A. Steinberg
Each 0.30 of neutral density blocks one stop of light. To get from f/16 to f/4 you would need a -4 stop neutral density value of 1.20 units. For f/22 to f/4, a total of 1.50 units would be needed for -5 stops. The filters can be purchased in different increments to reach the exact values needed. They can also be stacked so if you had a -3 stop filter (0.90) you could add a -1 stop filter (0.30) to come up with a total of - 4 stops (1.20). Depending on the type of filter you purchase (glass vs gelatin, etc.,) you may find the filter density designated in either stops (e.g., -2 stop filter) or in neutral density units (e.g., 0.60 nd) or both.
Each stop (0.30) of neutral density will require an exposure adjustment of 2x which is the "filter factor". You can either change the shutter speed by that factor or open the lens by the stop value. For example say you have metered an exposure without the filter to be 1/8 second @ f/5.6. You can either give 2/8 (=1/4) second @ f/5.6 or use 1/8 second @ f/4 with the filter in place. If the factor was 8x for a -3 stop filter using the same example, the new exposure would be 8/8 (=1) second @ f/5.6 or 1/8 second @ f/2 (or equivalent) with the filter in place.