And if you guys happen to have any practical experience of stacked reversing being better than single reversing.
For ordinary camera lenses, I don't recall ever seeing side-by-side test of a stacked lens pair versus the same front lens on extension. But the question piqued my curiosity so I shot one.
The front lens here is a ancient Mamiya-Sekor 55mm f/1.8. For the stacked configuration I've used as the rear lens an even more ancient Sears/Ricoh 135mm f/3.5. This gives a calculated magnification of 2.45, which I matched by bellows extension. Then using my Canon T1i (22.3 x 14.9 mm sensor, 4752 x 3168 pixels), I shot some focus stacks of a handy moth (
Noctua pronuba) and ran them through Zerene Stacker PMax. Here are the results.
First, image center, actual pixels . Stopping the Mamiya to f/5.6 seemed to be the sharpest aperture in both configurations. I'm hard pressed to see much difference between stacked lenses and bellows. (Presumably f/8 is too far into diffraction territory, at effective f/28.)
Now, image corner. It's a different story here. The stacked lenses are significantly sharper than either of the bellows versions, and stopping down the bellows to f/8 doesn't make much difference.
Re-reading your previous post, I notice mention of adding extension to get 1:1. Of course 1:1 is the worst possible situation for a lens designed with a longer conjugate. Lester Lefkowitz in "The Manual of Close-Up Photography" writes that
The particular reason of concern here is that image quality deteriorates when a lens is used much closer than its design limits. Trying to beat the system by inserting extension tubes between camera and lens, or by attaching supplementary optics to the front of a prime lens, will make that deterioration (called field curvature) evident, through fuzzy corners, slightly curved images of straight lines, and unsharp edges.
The higher the magnification, the more these aberrations manifest themselves, and the smaller the aperture required for good results. For magnifications of 0.5X (1:2), an aperture of f/8, and even smaller, is recommended. But if magnification is kept below 0.5X (1:2), and if small apertures are used, results can be surprisingly good for all but the most critical applications, such as photographing small flat objects or documents where edge-to-edge sharpness, uniformity, and freedom from distortion are needed.
This description is playing a bit loose with the term "field curvature", but the key point is just that the image gets soft in the corners. In the case shown here, even stopping down doesn't help because we're already at the point where diffraction is kicking in. This is only at 2.45X (0.41X from the other side). Pushing this lens to 1:1 with just extension seems like a bad idea, consistent with Lefkowitz's comments, and if I recall correctly I only tried that trick once or twice, decades ago.
Of course 1:1 is also problematic for stacked lenses because of vignetting. I have a couple of these 55 mm lenses, and when I reversed one in front of the other this evening, they vignetted pretty badly. That was stopping down with a regular aperture, not by adding a new aperture centrally located. It could be that adding a central aperture would take care of the problem, but that's quite a bit of trouble. My personal feeling is that the most practical way to do a good job at 1:1 is to use a lens designed for that application.
I was just wandering where I should look for adverse consequences of using single reversed lenses in the position they were obviously not designed for.
Corners, I think. If I only cared about center resolution, then at 2.45X as shown here, using this Mamiya on bellows seems just fine. In fact I used it that way for many years and was happy with the results. Of course that was way before computational focus stacking, so I was always fighting DOF and often had the thing stopped down to deep into diffraction territory. Times change...
--Rik
And the outcome with bellows seems to be consistent with image degradation at close focus in the forward direction - we'd probably see more of it if we could see the whole image circle. 