Why is Photoshop's Photomerge stitch having problems correcting exposure on huge amount of photos?

Question

I have a problem. See photo 1 (with all stitched) and photo 2 with only top left. You see in the smaller it's OK. In the bigger it didn't correct any small difference in exposure (due to sun going behind and out of clouds). All images were shot M mode with same settings. Developed with same settings. This happens with pano's with MANY photos.

The bigger image was stitched from 80 photos. The smaller was stitched from 20 photos. But the thing is, I have also done stitches with 120 photos which has worked. Sometimes it doesn't, which is strange.

Any idea about this? As part by part works this must be some limitation? memory?

Using fewer source images is not a solution — I have my reasons to use this many.

HiIlaf · Answer

There might be a few problems that cause this:

Overlapping: if there is not at least 20-30% overlap between images (as I can see that in the bottom ones there aren't) then lens distortion is harder for the algorithm to fix, and might be even impossible.
Time: If it took a lot of time to take the set of images, then the lighting might have changed just enough (I can see it was a cloudy and brightly lit day so cloud movement could have made a dramatic change) to give the algorithm a hard time stitching.
Lighting conditions: Even though you were on M mode the entire time, sometimes, lighting differences cause be viewing angle (directly to the sun or to the opposite side) might be to much for the algorithm.

My suggested attempt at a fix is to do them in small batches (since you said they work fine that way) and then stitching the batches together (either with the automated tool or by hand).
Good luck!

Why is Photoshop's Photomerge stitch having problems correcting exposure on huge amount of photos?

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