Correcting Photos with Color or Shading Issues

Lens Color Cast is a tool for dealing with the problem.

Here is a photo that looks like a catastrophe:

  • The white LPG tank has magenta and orange colors on it

  • The sky has some shading at the edges of the frame

  • The bottom of the image also has some shading problem (darkening as well)

In this case, the lens itself is a really good one, a Schneider 47mm Super Angulon XL. The problem is that it was designed for film, and I am using it on a digital camera. This lens has to be really close to the ‘film’ plane, but a digital sensor has a cover glass and filters that affect the incoming light. This can cause shadows (not all of the illumination makes it through to the sensor), and color casts (the low angle of incidence affects the color of the image in a bad way).

These are collectively called lens color casts, or lens casts. They can be remove with a method that my astronomy friends will recognize by another name: flat field correction.

The trick is simply to find a way to capture only the color/shading information, and to them remove it from the image.

The Schneider 47mm Super Angulon XL on my Actus camera.

Holding a white polyethylene (or other white plastic) sheet in front of the lens. The sheet is translucent, not transparent, so they only thing that gets through is the light that causes casts and shadows.

The plastic sheet (called an LCC card if you pay more than $10 for it) must touch the rim of the lens with no gaps, so that the only light getting in comes through the sheet. I was careful to keep my fingers out of the way; they cast shadows.

The image that I got holding up the white plastic sheet to the lens. This is a record of the color and lighting differences.

I use software called Capture One to process my images, and it has an LCC tool which uses the color/shade information to correct the image.

The result of correcting the image with the LCC tool.

This type of problem most often occurs with older wide-angle lens designs; modern lenses are designed to handle this situation (and in some case, a manufacturer may design their lens to bend the colors of light different to compensate for this effect!)

I was pretty sad when I first saw how bad the images were with the 47mm lens. Someone told me about the LCC tool, and the above was my attempt to try it out. As long as one is careful to capture the data with the white plastic touching the lens rim to keep out stray light, it works.

As noted, this is a problem that most often occurs when curious folks like me try to mix old lens technology with new digital technology. It turns out that the 47mm lens is amazingly sharp, and I can’t wait to try it out on an appropriate subject. With my $10 plastic sheet in hand, of course.