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I'm just starting to experiment with focus stacking using a rail to adjust the camera to subject distance but in my very first test setup I encountered something that made me go D'oh and now I'm not sure if it is an issue or not.

With a fixed magnification, moving the camera closer to the subject makes the subject appear larger! (who would have thought that?!?!). Here is an example where I have moved the camera about 40 mm from the nose of the car to the tail of the car.

enter image description here

enter image description here

These images are straight from the camera and scaled identically, and the image where the camera was closer to the subject is obviously larger.

My big question is will this change in subject size confuse typical focus stacking algorithms? And/Or should I change my set up (move further away) so that the change in subject size is minimized? Or would adjusting the zoom to keep the subject the same size be better? (I'm not sure how coupled the zoom and the focus are - but doing so would change the geometry of the setup)

I am aware of this and this question, but they don't appear to address my question.

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  • \$\begingroup\$ Might be worth adding a tag wiki for focus-breathing. \$\endgroup\$
    – MrUpsidown
    Sep 5, 2022 at 9:17
  • \$\begingroup\$ @MrUpsidown It's not what I was thinking of when I wrote the question, but in hindsight it does make sense if I change my capture methodology. \$\endgroup\$
    – Peter M
    Sep 5, 2022 at 15:12

2 Answers 2

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Taking the images for focus stacking by shifting the camera position, or by shifting the focus distance, both cause a change in the size of the subject.

Because the maximum magnification only occurs at the minimum focus distance, there really is no way of avoiding this issue and it is expected... it should not confuse the focus stacking algorithm. But if you only used the first and last images of the sequence it might (you might need to scale the images separately/differently before stacking).

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  • \$\begingroup\$ As this is an expected outcome, is there a preferred workflow to minimize the issue? (and I'm only showing the first and last images because they show the extents of the issue) \$\endgroup\$
    – Peter M
    Sep 5, 2022 at 14:54
  • \$\begingroup\$ No method/workflow is inherently better... the main thing is to avoid changing the angle, which is harder to do when shifting the camera position unless the subject is elevated. \$\endgroup\$ Sep 5, 2022 at 16:18
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You can try using align_image_stack.exe (comes with the Hugin distribution) with the -m option to correctly stack images with slightly different magnification. However, the focus stack will have parallax issues since you are changing the camera view by moving the camera position. Better to keep the distance constant and change focus. Use align_image_stack.exe to correct for the slight magnification change with the change in focus settings (magnification of the lens changes with focus setting which is more apparent at close focus distances). BTW, Hugin also comes with Enfuse.exe which can focus stack image sets.

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  • \$\begingroup\$ That's an almost 150% change in magnification, hardly what I would call "slightly different magnification". And I use a mac, so an exe doesn't quite cut it (and yes, I know there is a macOS version) \$\endgroup\$
    – Peter M
    Sep 5, 2022 at 15:07

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