Story and Photography by By Tom Horton
Human vision has a lot to take in and process, with the result that our brains are constantly on the lookout for low-risk shortcuts as they assemble our visual representations. Magicians and other entertainers routinely use this fact to trick us into seeing what should be there, rather than what really is there, and making a visual error.
Photographers are as prone to this problem as anyone. Several years ago I recognized a subtle way this was happening in my photos: Shadows in many were distinctly blue, whereas the shadows seen by my real vision were more like shades of gray. Biology reminded me what was going on. Reality had taught my brain that shadows are shades of gray, so when I looked at the camera’s representation of reality, my brain assumed shades of gray, when the camera was actually showing me blue. (Fig. 1)
- Fig.2 – Photoshop color balance functions showing enhanced yellows and reduced blues in shadows.
These blue shadows increased with altitude, and although some could be attributed to haze increasing skylight scattering, haze was not a factor in many others. Could this be an ultraviolet effect? Common wisdom is that all digital sensors have built-in ultraviolet filtering, but this is only partially true. (Rista, 2010) There are portions of the far ultraviolet spectrum, around 200 to 300 nm, that sensors typically don’t filter. And, the recent trend in sensor design has been to cut back filtration generally, in order to maximize dynamic range.
- Fig. 3 – Photoshop selective color functions, showing reduced yellow and black in the green tones, to compensate for “leakage” in the earlier color balance corrections.
Once this picture came together and I began seeing the true color of my shadows, I was blown away at how common blue shadows are. Fortunately, after some quick experimenting in Photoshop, I found a simple way to move shadow renditions back towards grayscale. Photoshop’s color balance menu (Image > Adjustments > Color Balance) allow selection of shadows, midtones or highlights for correction. (Fig. 2) Check the Shadow button and move the yellow/blue slider a tad toward yellow. Trial-and-error will show you the right amount, and it is easy to go too far. You’ll easily see the blue come out of the shadows and the neutral come back in.
Unfortunately, this Photoshop filter is rather imprecise and you’ll see some correction leak over into the midtones, particularly saturating the greens. There are a couple of ways to compensate for this. One is to use Photoshop’s saturation control to select the greens and pull the saturation back a few degrees. The method I like to use is Photoshop’s Selective Color menu (Image > Adjustments > Selective Color)to do something similar — to pull some yellow out of the greens and perhaps lighten them up, getting them back to normal. (Fig. 3)
Following this path on the shot above yields the result below. Shadows are much more natural and the photo has gained depth and realism. You probably won’t be able to kill all the blue in shadows without affecting other yellows and greens in the photo, but you’ll see a significant improvement.
- Fig. 4 – Corrections have rendered shadows less blue, more neutral and realistic, without significant effect on other parts of the image.
In summary, incomplete ultraviolet filtering in most digital sensors put blue casts on many outdoor shadows. Our brains’ visual habits allow us to assume these shadows are gray, when they are not. Using Photoshop or other editing software to subtract blue from these shadows by adding yellow brings them more toward neutral, and reality, making the prior blue cast even more obvious by comparison. When this correction affects other colors in the image, separate corrections can be applied specifically to those tones to bring them back to normal.
Rista, Jon; 9 August 2010, http://photo.stackexchange.com/questions/2262/are-digital-sensors-sensitive-to-uv
Tom Horton is the proprietor of Further To Fly Photography in Park City, Utah. He specializes in very-limited-edition photo art of nature and ethnographic genres. His portfolio coves the western US, Asia, Africa and the Middle East.