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I'm taking pictures of a D&D miniature. I'm using an iPhone with an app called VSCO that allows me to change the focus point. I've taped a 10x jeweler's loupe over the camera. My goal is to focus stack the images. However, the images are not aligned correctly, because changing the point of focus slightly zooms out the image.
Does anyone know why this zoom occurs? The edges of the jeweler's loupe are slightly visible around the edges of the image, maybe that's causing the problem?
Focusing by moving a simple lens or an entire compound lens as a single unit will change the field of view. When the effect is seen in a camera lens, it's known as focus breathing. Some camera lenses, use "internal focusing", moving only certain groups within the compound lens, to avoid focus breathing. Since you're using a loupe and don't have control of the individual elements, the apparent change in focal length is likely unavoidable.
I have not attempted focus stacking. However, I would consider using a mirrorless system with any internal-focus lens and extension tubes with pass-through electrical contacts.
You can also check your focus stacking software for settings to align images despite being taken at slightly different focal lengths. For instance, align_image_stack has options to optimize for field of view, radial distortion, center shift, and XYZ position. Optimizing for field of view (-m) is the main option that appears to be applicable for focus stacking. Optimizing for Z position (-z) also seems as if it would be helpful. I would just try different options with reduced resolution images until I found something that seems to work.
