I am a keen exponent of star trail photography - something I have been doing for years with my D800E and am now doing successfully with the 645Z, which handles it very well.
However, since the transition to the 645Z, a strange phenomenon has been occurring that I am at a loss to explain, let alone solve. I am hoping that the brains trust here can help.
If you look at the following images, you will see a strange banding artefact. In this picture, it is most pronounced on the right hand side of the star trail area, showing up as a series of vertical stripes (it is also there to a lesser extent on the left hand side of the star trail area):
[IMG]StarTrailsFromFiles_IMG4165-4619LITVERSIONStep7CropSMALL by Ed Hurst, SpiffingPics (1/4million views -thanks!), on Flickr[/IMG]
In this image, the artefact shows up in the bottom left of the image as a series of radial bands:
[IMG]StarTrailsFromFiles_IMG3935-4000Step10SMALL by Ed Hurst, SpiffingPics (1/4million views -thanks!), on Flickr[/IMG]
In this image, it is once again visible as a series of radial bands to the left, but this time clearly extends beyond the star trail area and can be seen over the beach etc. on the left hand side; it is also there to some degree on the right just over the trees:
[IMG]StarTrailsFromFiles_IMG3363-3426Step11CropSpotSMALL by Ed Hurst, SpiffingPics (1/4million views -thanks!), on Flickr[/IMG]
So the issue occurs with these images made up of a large number of stacked files, most especially in the corners with (perhaps) a tendency to happen most in the lower left. The issue is not restricted to the star trail areas themselves (in the images where you cannot see the banding in non-star trail areas, that is because those areas may have been layered in from single [i.e. non-stacked] files).
Now, here is the most perplexing thing of all: this effect only appears once the stacked images are combined. When I flatten the image (or use 'Merge Visible), the banding artefact appears. Until that point, when viewing the image on screen, even zoomed in close, those areas are very clean. The stacked files combine beautifully with no sign whatever of the artefact. It is only the flattening (or merging) of the layers that produces this effect, which strikes me as very odd indeed. This seems to happen regardless of whether I include all the layers or only a smaller subset of them (I thought it might be PS getting confused by the huge number of layers, but seemingly not). If the effect was simply there on screen once the layers were stacked, then I would assume it's something to do with how the layers are interacting with one another, but how can the layers display perfectly when they are all stacked, but screw up like this once flattened or merged?
This is very odd indeed and extremely hard to rectify in post. I emphasise that I never had this problem with the D800E, though a friend of mine is having it with similar images shot with his 6D. At first I thought it might be something to do with moire caused by no antialiasing filter (combined with the high res sensor), but then how can the images look perfect before flattening/merging?
It seems to me that it has to be something to do with how PS flattens or merges files, but I am damned if I can think of what it is or how to solve it.
Any help or suggestions would make a silly man very happy!
Thanks in advance to you all,