Edges of the stacked image is distorted

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ayyappanm
Posts: 7
Joined: Tue Dec 29, 2020 10:05 am

Edges of the stacked image is distorted

Post by ayyappanm »

Hi,

Upon stacking images, I always see the edges of the images are distorted. I like to know if this is expected or I'm missing something while taking & stacking images.

I have attached the portion of a stacked image for reference.

Infinity Plan 10x / 4x microscope objective, attached to canon 80D using zoom lens (180 mm f8).
Cognisys Stackshot with step size 8um.

Thanks in advance.
Attachments
upload.jpg

Scarodactyl
Posts: 1617
Joined: Sat Apr 14, 2018 10:26 am

Re: Edges of the stacked image is distorted

Post by Scarodactyl »

That's coma and/or astigmatism (both I think?). Details blur in a line towards the center (coma) and in a line perpenicular to that (astigmatism) depending which way they're out of focus, and stacking algorithms can't help interpret it as these starbursts. It's inherent to the optics, not the program's fault.

rjlittlefield
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Re: Edges of the stacked image is distorted

Post by rjlittlefield »

Adding to Scarodactyl's explanation...

These cross-shaped patterns are typical of what happens when (1) the lens has astigmatism in the corners, and (2) you stack using one of the "pyramid" methods, like PMax in Zerene Stacker or Method C in Helicon Focus. They happen because the astigmatism causes each subject feature to appear sharp but stretched radially in one frame, and sharp but stretched tangentially in another frame. Then the stacking method combines those two images to produce the characteristic cross pattern.

The crosses can be avoided by stacking with a depth map method, such as DMap in Zerene Stacker or Method B in Helicon Focus. However, the corners of the image will still look distorted or blurred to some extent because the lens aberration prevents it from being focused both radially and tangentially at the same time.

The only way to get dramatically cleaner corners is to use a lens that is free of aberration clear out to the corners.

--Rik

ayyappanm
Posts: 7
Joined: Tue Dec 29, 2020 10:05 am

Re: Edges of the stacked image is distorted

Post by ayyappanm »

Thanks for your response rjlittlefield & Scarodactyl

physicsmajor
Posts: 108
Joined: Sun May 10, 2020 12:56 pm

Re: Edges of the stacked image is distorted

Post by physicsmajor »

As an interesting aside/continuation - depending on your subject, in particular if it is in part translucent, you may have to deal with these stacking artifacts even if your optical system is perfect. They'll appear from details behind or included in the translucent material; that translucent material then acts as part of the optical system.

Many of my mineral subjects have some degree of translucency and some of the subjects have substantially different indices of refraction, resulting in these artifacts despite excellent lenses. Fluorite under high magnification is a prime offender (there's a reason expensive lenses have fluorite elements), as is Selenite. Very irritating to clean up in post.

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