Removing the Colour Cast on the Lee Big Stopper

[Update 8-Jul-2016: a more recent post gives a more accurate method to correct the colour cast on the Lee Big Stopper.]

[Edited 22-May-2016: included a table with the RGB control points used; this allows the use of this data in other applications.]

Today I’m finally writing about the method I’ve adopted recently to remove the colour cast on the Lee Big Stopper. It’s a bit of a technical post, so it’s likely to only be of real interest to a few people. Still, for those, here’s hoping it will be really useful.

Let’s start at the beginning. The Big Stopper is a high-density (about 3.0 optical density, so reduces the light coming through by 10 stops) neutral filter, intended to allow long exposures in daylight. It’s very popular, and when I bought it was constantly on back order through Lee, whose supply rate couldn’t keep up with demand. I just checked and it seems to be in stock right now through my usual supplier, so perhaps the supply problem has been resolved. Anyway, this is a fantastic filter for its intended use, and I couldn’t recommend it strongly enough. But it does come with an unfortunate caveat, that it’s not really neutral. It gives a bluish cast to everything, but this is something that can easily be corrected in post. I know people seem to expect a colour cast to be a general problem with ND filters, particularly if you stack them, but to be blunt I’ve never had that issue with any of my filters except this (and I knew about this going in). I bought myself both sets of ND grads from Lee, soft and hard, and they’re in regular use and I never had any colour issues with them. Pricey when compared to the lenses, but I have no regrets and would buy them again if I had to.

Anyway, back to the Big Stopper. This is known to cause a bluish cast, kind of like when you shoot daylight on a tungsten-corrected film. I’ve read about a few ways people use to correct this, and tried some alternatives myself. Allow me to list them for completeness, with some comments:

1. Perhaps the simplest method that I’ve seen recommended is to set a white balance point at a very high temperature (9000-10000K). This corrects some of the imbalance but to me never seemed quite right.
2. Take a neutral gray reading with the filter in daylight, and use that as a custom white balance. This is the method I’ve used in the past, and it worked reasonably. Not quite perfect, but usable. The way I did this was by taking a photo of a gray card with the filter on, using my (daylight-calibrated) strobes to light up the card. (As an aside, this was the only time I had to set my strobe at their highest intensity.) I took the white balance reading as a custom reading on Canon Digital Photo Professional, and would then apply this on any shots I took (in real daylight obviously) with the Big Stopper. Works ok, but I had to stop using this when I stopped using DPP.
3. Use a curves layer in Photoshop set up in a way to invert the colour cast, restoring a true daylight calibration. This is the method I’m using now and it works great. It’s a simple method once the calibration is done, and I don’t expect I’ll need to change it any time soon. The idea behind this is largely similar to option 2, but we’re just doing things manually ourselves, rather than using the white balance corrector. I’ll explain how this works below.

Broadly, what we want to achieve is to amplify the colours that need amplifying (red), and attenuate the colours that need attenuating (blue). We want the adjustment to be such that for the input we get with the Big Stopper, the curves layer gives us as output what we’d have without the Big Stopper. To get the calibration values, I set up a simple scene with my gray (and white and black) cards, and some fruit & veg, all light by the soft daylight coming from the balcony. I took two shots, first without the filter, setting exposure manually. Next, I dialled down the shutter speed by 10 stops and took the same scene with the filter. In both cases, I had the camera white balance set at daylight. This is how my camera is usually set up, and this is the same process I use whenever I take long exposures outside. You can see both images below.

Daylight white balance, no filters. (Click on the photo for added detail.)

Daylight white balance, Lee Big Stopper. (Click on the photo for added detail.)

As you can see, the Big Stopper gives a noticeable colour cast. You’ll also notice that the gray card is not quite neutral in the first image, even though this was taken with indirect daylight and with a daylight setting on the camera. It’s probably the light bouncing around in our kitchen, but for this experiment it won’t matter.

I processed both RAW files with DxO Optics Pro, making no adjustments to colour. I loaded both files as separate layers in Photoshop, and placed three eyedroppers, one on each of the black, white, and gray cards. This allowed me to read the RGB values for each of these cards, and depending on which layers I had active, I could get the RGB values with and without the Big Stopper. I wrote these down; this is all the data I needed to set up my correction curves layer. At this point I did the following:

1. Created a new curves layer in Photoshop, with default setting (i.e. a straight line, no adjustment)
2. Editing each of the Red, Green, and Blue curves separately:
   1. I added three points, one for each of the black, white, and gray cards.
   2. For each point, I manually set the input value to the reading I had with the Big Stopper, and the output value to the reading I had without the Big Stopper. For the red channel, for example, what I had was the following:
      In this example, the red channel reading I had for the gray card was 145 with the Big Stopper and 172 without.
3. Once I was done, I had entered nine readings, three for each of the RGB channels. Result was the curve below:
   
4. I saved the curves layer so that I could easily use it in other images.

Now any time I want to correct the colour cast on a photo taken with the Big Stopper, all I need to do is to process the RAW as usual, keeping the daylight colour balance, then after importing in Photoshop I just need to add a curves layer with this saved preset. To give you an idea how well it works, here’s the same photo I took with the Lee Big Stopper, with this curves adjustment layer applied (and nothing else).

Daylight white balance, Lee Big Stopper, corrected. (Click on the photo for added detail.)

If you want to try this for yourself, it’s easily done. All you need is some white, black, and gray cards. They don’t have to be perfectly neutral either, as all you’re doing is using them to set three points for the adjustment curve. It’s good to have one middling gray, one whitish and one black (around zones I-II, V, and VIII-IX if you’re a zone system fan).

Otherwise, if you really don’t want to mess around, feel free to try mine [curves preset]. I have no idea how consistent Lee’s process is, and therefore how similar the colour cast is across batches. But it wouldn’t surprise me if it’s very consistent. The table of control points for the red, green and blue channels is also given below to allow the creation of similar correction layers in other software.

Channel	Dark	Middle	Light
Red	22→25	145→172	205→223
Green	25→21	160→167	215→220
Blue	27→15	181→158	229→217

[Note: since I wrote this, I developed a more accurate method to correct the colour cast on the Lee Big Stopper.]