Equalize exposure levels of a set of images in a stitched panorama

Question

I've taken a set of pictures for a panorama. Unfortunately, I've forgotten to set the flash to manual exposure, hence I ended up with uneven exposure levels in the images.

When I stitch them together, the result looks like this:

Next, I've tried to use Lightroom 3 to adjust the exposure levels manually so that they match more closely, using the histogram as a guide. That did work somewhat, but still wasn't perfect: LR only lets me change the exposure in 1/3 EV steps, but I felt I needed finer control.

When I stitched the adjusted imgs with Hugin, it evened out the levels somewhat. See the result:

As you can see, this didn't work well, either: The image is brighter in the top center, growing darker towards the edges. Hugin's automatic exposure adjustment only seems to have applied the adjustments locally between neighboring imgs, dealing with the unevenly exposed tiles. However, it didn't succeed in evening out the brightness on a global level for all images.

How can I get this right?

I have CS4, LR 3, Aperture, Hugin.

Maybe I could edit the resulting gigapixel image with PS, adjusting the brightness regionally (i.e. brighten the borders more than the center). I believe I need to use a radial gradient mask, but I#ve not figured out yet how to get a gradient that's going only from light grey to white instead of black to white.

However, I'd prefer a solution that would help me align the exposure of the original imgs instead of trying to fix the stitched result.

An idea: Is there a way to see the histograms of all images in a set at the same time? LR seems to let me see the histogram only for one (selected) img at a time. Seeing all histograms in comparison at once would probably help me seeing which ones are out of line.

Asked by Thomas Tempelmann

Answer

Looking at the last result, I think that's as good as you'll get modifying each set, so I would do as you suggest and use a radial gradient.

To select grey to white, rather than black to white, set your foreground/background colors to grey and white before selecting the gradient tool.

Or you can use black/white, set blending mode to overlay and use layer opacity to fade the effect.

Another possibility is to create a solid black layer in overlay blend mode. Create a new mask, select the mask, then do Image > Apply Image. This will create a mask which matches the overall luminosity of the image. That means lighter areas will be lighter in your mask, and should allow the black overlay layer through, which will darken those light areas.

Answered by MikeW

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