How to match colors on photographs stitched together?

Question

I wish to create a high resolution picture of two paintings, each in three parts for printing in a book, each across two pages that each measure around 12.5"x10". One painting is a triptych of three panels and another that is very wide (about 8 feet) and short (about 1 foot) street scene. I'm taking the photos with a Nikon D90 in RAW format at 2848x4288 pixels each using a Sigma 50mm f1.4 set to f5.6. I've taken the pictures outdoors in cloudy skies, moving the tripod sideways to be centered on each section being taken. When I go to stitch the photos together as three in a row, left to right, the colors and levels never quite match between the photos. I also get issues with more reflection on the left than the right, causing the abutted right of one to look more different than the abutted left of the next photo.

How can I use Photoshop CS6 to adjust one image to match the various light levels of another image so that they look the same when placed adjacent to each other? Perhaps I should instead leave the tripod unmoved and swivel the camera around tripod axis to shoot left and right of center and use perspective crop to correct for the distortion?

Answer 1

Normally you would include a an object known to be white in each frame. There are special things for this, most commonly White-Balance Cards Then, you use the software to pick the WB from that point. Repeat for each image and the colors will now be the same.

For levels, I suspect you mean that you got a different exposure. To fix that, you must shoot all the set with the same EV, which is easiest to do in M mode by selecting an aperture, shutter-speed and ISO and not changing this between frames. In any other mode, the camera will choose an exposure based on the content, so different painting are likely to give different results.

The same thing for WB can be done for exposure using a gray card. That is an object which is known to have a certain reflectance, usually 30% grey. You include that in each shot and then you can use Photoshop to set levels by sampling from a point on the grey card to be the mid-point.

NOTE: Unless you did the above, swiveling the camera will not solve your problem. The camera will see something different in each frame and therefore will pick different exposure and WB settings. This happens even is shooting RAW where the WB is not baked but usually stored as default.