We have seen that with a forward-pupil objective, we can go as near as we want to the subject; the magnification increases as the inverse of the pupil distance, so when we focus at the pupil, zero distance, infinite magnification. With such infinite magnification, do we see now the Quark knots, The Pico-God, or Planck laughing? No, we see just a shapeless blob of dim light. (Some may argue that this blob is The actual infinitesimal building block; the Ultimate Truth is just a meaningless single bit: uninspiring! Larger, more complicated half-truths are surely more interesting and colorful. But for now, this shapeless light blob is just a bad photo).
Let’s be brave and go beyond that border, focus even closer, and an image will appear again! A weird image indeed; not inverted anymore, and embracing our subject. We have made an Pericentric (or Hypercentric) objective.
The resulting photos are totally unnatural and probably useless for artistic macro photography; those objectives find some use in industrial inspection since the object can be imaged from all sides with a single shot. See also the usual article by Littlefield.
The downsides of this method are that we are quite near (40-10mm) to the subject with the lens: extreme risk (certainty) of touching and scratching the lens. Illumination gets tough; and we need to work at small aperture (or focus stack) in order to show well the bizarre pericentric field depth. Focus stacking must be done with the relay lens, the front lens shouldn’t move or we change the perspective, as for the macro wide angle. Anyway it’s fun and easy, just tweak the spacings and magnification of the relay system set up for the wide macro.
Actually the setup is even simpler than the full relay; the forward-pupil wide objective can be installed right in front of a normal macro lens (no spacing needed), the macro works at moderate extension, the frame gets filled with no issues, the aperture of the macro is (maybe, quite) at the right place. The larger, wider and faster the front objective is, the better this Pericentric trick comes out. Works well with an inverted fast prime, BUT the delicate rear lens of the prime will get its share of scratches. With a telescope eyepiece, the front lens is protected by a rubber ring and also already scratched.
This hypercentric trick can be done also with the inverted prime alone, don’t need the macro in the middle; just er have to put an aperture between lens and sensor. The angle will be less steep though. Another implementation is with a large Fresnel lens, looks intimidating but in reality it is simple, and c an be used with large subject.
Half cooked dreams by Giorgio Pattarini 