Originally Posted by
DanK
It's hard to answer without seeing the image. Most often, this occurs because of parallax when the surface behind the edge is far behind it. The image changes size as you move the camera or change focus to create the stack, and therefore, the edge is blurred.
Sometimes, as Chauncy suggests, you can paint from one image in the stack to solve this problem. If the gap is wide enough, however, this may not work, since there won't be any image that has a clear edge for one in the right place to merge with another. In that case, you sometimes have to change stacking methods because some are more prone to this than others. I use Zerene, which has two algorithms. In many respects I prefer DMap, but PMax has less severe problems with halos, so I often create composites with both. I then paint onto a DMAP composite from individual images, from the PMax composite, or both.