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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:

enter image description here

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:

enter image description here

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.

share|improve this question
    
Flash for panorama stitching? Never heard of that one. I better update my tutorial to mention that flash has to be OFF for shooting a panorama. Even if illumination power was even, it would be almost impossible to stitch in the general case since shadows, highlights and the texture of things in your scene will look completely different and will cause the matching algorithm to fail. Back to your problem, reshoot if you can and do it from a tripod without flash. –  Itai Feb 13 '12 at 3:53
    
@itai I disagree. A fixed positioned external flash that's not moving with the camera, or even two of them, shouldn't be a problem. Besides, I've had a rather long distance from the object, and the stitching software had no problem whatsoever. –  Thomas Tempelmann Feb 13 '12 at 11:03
    
Ah you mean the camera is not on camera? That may not be as bad as I thought. If you are quite a distance though, most flashes contribute little to nothing though. –  Itai Feb 14 '12 at 0:57
    
@Itai In my case I actually had the flash on camera because I didn't have the full eqipment with me and had to do with what I got . The artwork was being disassembled soon after, so I had no choice. And it was a rather strong flash. I had also tried without the flash first but exposure times ran into seconds, and the contrast was rather low. In the end, with the flash I fared better. –  Thomas Tempelmann Feb 14 '12 at 10:21
    
@Itai But my particular explanations are besides my point to you: Your claim never to use a flash is quite inconsiderate, as you rule out that people may have special cases where the flash is the better option, like in my case. If you want to be a good teacher, I suggest you don't deal in such absolute and final statements. –  Thomas Tempelmann Feb 14 '12 at 10:23

1 Answer 1

up vote 3 down vote accepted

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.

share|improve this answer
    
Thanks, Mike. Your various explanations on how to apply the gradiant were helpful as I'm still learning these tools. On the question of aligning the exposure of the tiles - I'm now planning to write my own tool for this, and maybe publish it later, as I believe this could be useful to others as well. –  Thomas Tempelmann Feb 14 '12 at 10:25
1  
I've tried some of my suggestions on the above images. I found that adding an exposure adjustment layer at about -0.70 then a radial gradient in the mask worked well. For the gradient, I set to white/black and clicked at the top middle of the image, and dragged almost, but not quite, to the bottom left corner and that evened out the image pretty nicely. –  MikeW Feb 14 '12 at 10:53

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