A little more than a year ago I wrote a rather popular blog post on how to correct the colour cast caused by the Lee Big Stopper. If you’re not a regular reader but clicked through to this post, odds are that you are familiar with the blue cast caused by the filter and are looking for a way to correct it. If not, I suggest you start by going through my earlier post.
In my earlier method, I had taken two correctly exposed photos of the same scene, with and without the Lee Big Stopper. From these I created a custom curves layer with three additional control points, each correcting for readings from white, gray and black cards in the scene. This worked well enough, certainly much better than the alternatives advocated elsewhere (such as auto white balance, or a high-temperature manual white balance). However, it could not be applied in the first stage of the workflow (i.e. during RAW conversion). I tried creating the same curves layer in DxO Optics Pro by using the same control points in the tone curve tool. Unfortunately, using the same control points does not guarantee the same overall curve, as anything between the control points is at the discretion of the software.
I also thought a more precise curve can be obtained by having more control points, ideally across the whole range of tonalities. The logical way to obtain this is to shoot a colour calibration target, a flat sheet with a number of patches of different colours. With so many control points it’s also easier to automate the process, so this project was born.
As a first step, I took photos of my scanner colour calibration target, with and without the Lee Big Stopper. Ideally I wanted the exposures to be in the same range as what I would normally use in practice, which usually start at 1/30s without the filter, and 30s with the filter. I used natural light, but since the photos were taken indoors, I had to increase my camera ISO setting to 800 to avoid excessively long exposures.
I processed these with DxO Optics Pro, using my usual settings but without any exposure compensation or ‘smart lighting’. As you can see, with the Lee Big Stopper, the output has a distinctly cooler white balance.
Next, I wrote some software to determine an optimal curves transform. The topic of image restoration was always a personal favourite, so it was nice to apply my skills to a problem of direct interest. I will spare you the technical details involved in aligning the images and selecting corresponding pixels from the two images, to use as control points. Once I had these (millions of) control points, I fit a polynomial curve that best approximates these control points. You can see the result below.
Actually, to make sure I had enough coverage throughout the tonal range, I used two pairs of photos – the ones shown earlier, at ideal exposure, and another pair underexposed by one stop. Finally, I built 256-point tables for each channel’s curves. I saved these in two formats, one for DxO Optics Pro and one for Photoshop and similar tools, which you can download further down.
Applying the correction curves in DxO Optics Pro to the photo taken with the Lee Big Stopper brings it very close to the photo taken without the filter. You can compare the two below.
There are slight differences, particularly visible in the light blue range. These cannot really be fixed without a 3D look-up table. I may look into this in the future if I find the difference to be worth pursuing.
Comparison with 3-point method
For completeness, you can see the curves for the new LUT below (dashed lines). On the same axes I also plot the curves from the older method with three control points (solid lines). I assume that Photoshop creates the curves with a polynomial fit of the required order (4 in this case). The older control points themselves are also shown as circles.
You can download the newly created LUT in one of two formats, below.
- IRIDAS / Adobe CUBE format, with one-dimensional tables [cube, 11k]. This file can be added as a 3D LUT layer in Photoshop CS6 or later, and other software (including video compositors, editors, and colour graders).
- DxO Optics Pro preset [preset, 24k]. This file can be applied to a photo in DxO, changing only the ‘tone curve’ tool settings. You may need to activate the tone curve manually.
The correction curves assume:
- White balance is manually set to daylight (either in the camera or in the raw conversion).
- Colour space is Adobe RGB (both in the raw conversion step and as subsequent working space).
If you use these, let me know how it works for you. I am curious to see how much variation exists between different Big Stopper filters. The same method could also be applied for other colour casts, or to characterize specific looks (say from different cameras, etc.). Get in touch if you’re interested.