Neutral density filters help you “Take it Slow”. These filters allow you take longer exposures and thus slow down the world to add movement by letting less light to the sensor. This means the longer the shutter remains open the more movement is exposed to the image. We used to use low ISO film and a small aperture to achieve this. Old wet plate cameras had an ISO of ONE so using the Sunny F16 rule you would expose the image at One second at F16.
“Sunny F16 rule states that on a clear sunny day your exposure is One over the ISO at F 16. ie: ISO 200 is 1/200 sec at F16”
Anything that moves can be a subject for ND filters; clouds, water, etc. Just remember that if you have too long of an exposure and there is lots of movement in the scene then the movement may not be registered. Let’s say that you are at the Parthenon in Greece. There are thousands of people all over the site. You set up and shoot a 4 minute exposure and the only people that show up are the two lovers sitting on the bench; the rest of the people blend into the scene. The rest of the people are moving fast enough that they will not register on the sensor.
The first thing I would like to review is Stops. A stop is the movement of the exposure either up or down by Half or Two Times. Moving a stop can either be done by altering the F stop or the shutter speed. Both need to be moved to retain the correct exposure. (If one is moved up, the other has to move down)
ND filters will stop a specific amount of light reaching the sensor. There are two ways that they mark these filters; .3 is one stop, .6 is two etc. Or they will state how many stops the filter will move the exposure. (1 to 20 stops) This is important when you are determining the exposure for a scene. If you have a 4 stop filter then you would have the following calculation:
Original exposure: 1/125 sec at F16 (A bright sunny day with ISO of 125)
Addition of a four stop ND filter = New exposure of 1/8 second at F16:
1st stop = 1/60 sec.
2nd stop = 1/30 sec
3rd stop = 1/15 sec
4th stop = 1/8 sec
This works well, but when you get into 10 and 11 stop filters, things get interesting. Meters will not register the light coming through the filter, so you have to calculate the new exposure using the exposures you get from the meter before you install the ND filter. As most cameras will only shoot down to 30 seconds you will also need to have an electronic cable release and a way to time the exposure. As they say these days “There's an App for that”. Search ‘ND filter app’ and choose one that is compatible with your phone. You simply enter your original exposure and the number of ND stops that you are using and it will give you the new exposure.
The thing to remember is that every time you add one stop of ND you double your exposure. So a 4:30 exposure with only one more stop added will become a 9 minute exposure, both of which you will have to manage with a stop watch or an App.
Some ND filters will produce a colour cast on the image. This is usually with the less expensive ND filters. A way to get around this is to do some tests with the ND filter and see if you can find the white balance that cures this problem. Once you know the white balance then every time you use the filter, dial in the custom white balance to correct your image. A lot easier than doing it with software when you get home.
Hint: Buy filters that are larger than your largest lens filter size then use step down rings to bring the filter to the correct size. Saves you money on buying more than one filter for your kit of lenses.
Variable ND filters are available – one filter giving you from 1 to 10+ stops, but I have found that they can be tricky to work with especially with wide angle lenses.
Now let's walk you through the procedure of getting an exposure:
A) Set up the camera on a tripod for the images and composition that you have chosen.
B) Stop down the lens. I usually go down to the smallest aperture I can.
C) Take note of the exposure; Let’s say you get 1/30 second at F22. Determine the ND filter you want to use and how many stops it is.
D) Punch the exposure into the app. along with the number of ND stops. Or get the calculator out. Mine is in my head. :-)) Working with time is a lot easier than it seems!
OK I have an 11 stop ND filter so here are the steps:
5.) 1 second
6.) 2 seconds
7.) 4 seconds
8.) 8 seconds
9.) 16 seconds
10.) 32 seconds
11.) 1 minute 4 seconds
E) Turn off the Auto Focus! This will stop the camera from searching for focus when you install the ND filter as it will not be able to “See” when it is installed. High number stop filters are too dark to be able to see or meter through. Install your electronic cable release. Place the camera shutter on Bulb.
F) Install the ND filter
Open the shutter when you start the timer and close it when it is done. Now see how it looks and make exposure adjustments if they are needed.
Check out Robert Chisholm's Newfoundland images he had an 11 stop filter with him!! You’ll easily spot the photos where he put it to good use.