Halo reduction

[Adam Long]

Charles, Rik, many thanks for your help and encouragement. I've now got a much clearer idea of what direction I need to be going in, and what I can and can't achieve - just what I needed at this stage. Hopefully I'll get back to you soon with some better images.

[Lou Jost]

Rik, I imagined that if I took a stopped-down photo focused at some point X on the subject, and made that the first photo in the stack, then open the lens up and run a stack (perhaps starting at point X on the subject, or perhaps getting point X into sharp focus midway through the stack), for most of the stack the lens will see the subject from a different perspective than the perspective it saw when it made the stopped-down picture.

So when the wide-open frames are fitted onto the stopped-down frame, they won't match exactly, unless the magnification is high enough that each frame in the stack has negligible depth of field.

Am I thinking incorrectly about this?

[rjlittlefield]

Lou, thanks for the additional information.

One of the Zerene Stacker FAQs says this:

Does it matter if I shoot the stack in order?

For DMap, it's definitely best if the frames are processed in order, either front-to-back or back-to-front. If they're out of order, then the result may still look OK at first glance but it's almost certainly degraded. Often there will be obvious artifacts like nasty halos.

PMax itself does not care about order, but going from one end straight to the other is still the safe way to get good alignment. Sometimes when source frames are processed out of order, enough error accumulates in the alignment process that two frames, focused at nearly the same depth but far apart in the stack order, end up aligned slightly differently. When that happens, you get artifacts like “echoes”.

When people talk about shooting extra frames stopped down, I think they mean to shoot extra frames at the rear (and possibly at the front), and then to process those in spatial order, either front to back or back to front.

Putting the stopped-down frames at other positions does seem to be asking for trouble. I'm not sure that the trouble can be specifically attributed to changes in perspective, since most of the time every frame has a slightly different perspective anyway. But out-of-order definitely gives more opportunity for things to go wrong.

I'm also missing the point about what benefits you think might accrue from shooting stopped-down in the middle, even with some setup in which changes in perspective are not an issue. Can you say more about that aspect?

--Rik

[Lou Jost]

Rik, I don't know that there would be any benefit from having the stopped down image in the middle, except that this would minimize perspective differences between the stopped-down frame and the individual wide-open frames of the stack.

Maybe the best way to do this would be to take the stopped-down frame at a middle distance, then stack the nearer wide-open frames from far to near, and then use that result as the new first frame to stack the farther wide-open frames from near to far.

In other words, start each half-stack from the center point.