Extreme Macro Focus Stacking Tutorial Part II - Calculations
This is the second part of tutorial on extreme macro focus stacking -- some basic calculation on various optical setups. Some important notes: When using infinite objectives, it is highly desirable to use a lens of which its focal length is just enough to avoid vignetting. Though this might reduce magnification and out of its design specification, the effective aperture will much larger (lower f number), thus help to avoid diffraction.
This is particularly important for APS-C sensors as diffraction will occur around f/11. Keep in mind, when using photographic lenses as tube lens, keep its aperture wide open and set focus to infinity. When extending or stacking lenses, the aperture of front lens should be set to as wide as possible so that effective aperture is large (lower f number), but some lenses start to have chromatic aberration at wide aperture, so there is balance between diffraction and chromatic aberration.
Whether reversed, extended, or stacked, the shorter the focal length of front lens (or the only lens), the easier to get higher magnification. When stacked, set both lenses to focus to infinity and keep the back lens wide open.