Today we are going to be testing our ND filters to determine their exact properties. If you recently purchased some ND filters, this should really help! Even if you've had your filters for years, this will come in handy. I recommend shooting in RAW and using Adobe Camera RAW for the following tests. If you are new to Long Exposure Photography, read my tutorial for more information.
First, you need to identify the color cast of your ND filters. Depending on which filter you buy, you may have a significant color cast in your images or none at all. When I use my Lee Filters, I see a heavy blue cast in the photos. Click here to see how bad the color cast really is. Once you know the exact values of the color cast, you can easily remove it every time! Otherwise, it could take hours to tweak the colors in Post-Processing!
Ideally, you can find a white wall in your house. You want it to be a neutral grey for the easiest processing. Take a photo of the wall, set the White Balance so the wall is grey / white. Once you have a good Before image, add your ND filter and take the After photo. It is critical that the White Balance is the same for the Before and After photos.
Take both of these images into Adobe Camera RAW. Use the White Balance sliders to remove the color cast from your filter photo. For example, my Big Stopper needed to be 4600K warmer and +20 Purple Tint to remove the color cast. Once you have removed the color cast, write down how much you needed to adjust the White Balance and Tint.
For this process to work correctly in the field, make sure the camera is always set to a Manual White Balance (Daylight, Cloudy, etc..) when doing long exposures. You need to have a Before image, where the White Balance looks correct. I always take a normal photo, without filters, before taking the Long Exposure image. This will help in Post-Processing, to verify the color cast is removed.
16 Stops vs No Filter
For example, I take a photo at Daylight White Balance without a filter. The colors look good. I add my filter, leaving my camera on Daylight White Balance, and take the long exposure. When I bring the images into Adobe Camera RAW I can now add 4600K and +20 Purple Tint to remove the color cast from the long exposure. Then I compare the Before and After filter images to make sure the colors match up.
I've been shooting with ND filters for 3 years now. Since I was self-taught, I never really understood what a Stop was until last year. If you are confused about Stops, read my Stop Guide Tutorial. Hopefully this will clear things up. You need to have a firm understanding of Stops before testing the density of your ND filter(s).
Most ND filters actually have a different density than what's listed on the box. For example, a 6 Stop ND might actually be 6 and 1/3 stops. Since the shutter speed needs calculated manually when doing Long Exposures, you need to know the exact density of your filter. If you don't, your images will likely be underexposed. It could literally mean the difference between an 8 minute shutter speed and a 16 minute shutter speed!
You should do this test while you are doing the color cast test. For this test though, it's critical to have constant lighting. The light levels cannot change between exposures. Take a Before photo, without filters. Pay attention to what the Shutter Speed is. Add your filter. Now determine what the new Shutter Speed should be using the Lee Exposure App. For example, if your shutter speed without any filters is 1/100 sec, and you add a 10 Stop ND, the new shutter speed should be 10 seconds.
Bring both images into Adobe Camera RAW. Look at the Histogram of each photo. If the filter image's histogram is further to the left, it is underexposed. Likely by 1/3 or 2/3 Stops. Increase the Exposure slider by 0.33. Do the histograms line up now? If not, try +0.5 and +0.66. One of these values should align both histograms. The histograms below show my 6 Stop ND was underexposed. I had to increase the Exposure slider by 0.33 to compensate. I could have also taken a 5 second photo for the same result.
No Filter Histogram 6 Stop ND Histogram, underexposed
There's an alternative way to do this test. Again, let's assume the 10 Stop ND shutter speed is 10 seconds. Take a second photo at 13 seconds and one more photo at 15 seconds. (1/3 stop longer and 2/3 stops longer) Bring all of these images into Adobe Camera RAW, including the image without a filter. See which image has a histogram aligned with the No Filter image.
Write these numbers down so you can refer back to them in the field and when post-processing. I have a note on my phone that states:
For more information on this process, watch the video below. I cover both processes in more detail and show exactly how to change do these tests in Adobe Camera RAW.