@Bill_Fach , that is a darn good question and I found an explanation in an article from Photography Life: What is Focus Breathing and How You Can Mitigate It . When we focus through an object during image generation for stacking, the magnification and frame area change from near to far focusing and this phenomena is called focus breathing. This is caused by moving the internal lenses and it alters the optical behavior of the lens optics. All macro lenses have a fair degree of focus peaking while focusing at close objects. Moving the entire rig on a focusing rail should give better results because the lens optics are set at a single focus point. Some lenses like shorter focal length lenses may have negligible focus breathing. Lenses designed for shooting movies are designed to reduce the “zooming-like” effect of focus peaking. My guess is that the stacking software we use also have algorithms that minimize the perspective and magnification changes from focus breathing. I think that using a rail should produce better stacking results versus using the focus ring. Somewhere in my bag of tricks, I have a focusing rail and should give that a try on my next flower shoot…Jim