35mm 1.8 & extension tubes
David Lal wrote:
...Therefore the reproduction ratio is 68/35=1.94x lifesize.
If there is any working distance left. When adding extension tubes, the working distance decreases.
You should stop the aperture to F/8 or more to get useful images, both to get some DoF and useful image quality. But not to far if you mind diffraction softening.