I’ve been testing the Fujifilm 120/4 GF macro lens at 1:1, using the Fuji 45 mm extension tube. I found that the corners were soft. I’ve been using the lens for a long time, and I never noticed this before. But I’ve never used it for repro work, where such a flaw could well be a big problem. I was asked to post some images of three dimensional subjects where the soft corners showed up. I didn’t have any in my Lightroom catalog. In fact, I didn’t even have images at 1:2 where there was supposed to be sharp detail in the edges and corners.
I got to thinking about why, and realized that the math behind 3D work at 1:1 is cruel.
First, some basic principles. Depth of focus at a given f-stop is dependent on the stop and independent of focal length. Therefore, for macro work, depth of field at a given magnification and f-stop is independent of focal length. At 1:1, DOF equals depth of focus.
With excellent lenses, for critical sharpness, the circle of confusion (CoC) due to misfocus should be held about one and a half times the pixel pitch. For the GFX 100 and GFX 100S, that’s five or six micrometers (um). Let’s say we have a macro lens that we can get to 1:1, either on its own, or with tubes or bellows. Let’s further say that it’s a great — almost perfect — lens. To have acceptable diffraction for critical work, we need to use the lens at an effective aperture of f/8 or wider. If the lens doesn’t have internal focusing, that means that we’ll set the lens to f/4, since the bellows draw for 1:1 will make the effective f-stop two stops narrower than the f-stop on the lens. That means that the depth of field — and the depth of focus — will be 5*8, or 40 um.
For a worst-case CoC in a focusing stack, we’ll have to move the focal plane by 2*40 or 80 um between each shot. That means we need 12.5 shots for every millimeter (mm) of subject depth.
If the subject depth is an inch, we need 12.5*25.4, or a bit over 317 shots. If the subject depth is two inches, we need 635 shots.