Micro fisheye 8 1/2: Image-Aperture swap

Micro fisheye home

Short summary: just look at the illustrations at the end, or at the Shortcut.

29/11/2020 NIGHT WARNING – THE CONTENT OF THIS POST MAY BE TOTALLY WRONG – I’LL REBUILD THE SYSTEM ASAP AND CHECK

30/11/2020 morning update: Tested, IT WORKS. Just, aperture placement and relay lens are very tricky.

You don’t trust the optical quality of arranged lenses, eyepieces, pinholes and relics from 1930? That can be comprehensible; but would be strange if you don’t trust the sharpness of today’s state-of-the-art f/1.4 prime objectives; their almighty perfection is praised all around the web.

Ingredients for this strong macro (but moderately) wide angle setup, with opulent working distance and sharp to the corners:

• A macro lens; it will work at about 1:1 or less magnification. Pick your longest and sharpest.
• A fast prime lens. An expensive 85mm f/1.4 would be ideal; faster will work even better. Mediocre 50mm f/2 and other primes manufactured in nominally communist countries are strictly forbidden. This is an exclusive club. Takumars may be allowed only if in mint conditions.
• The latest full frame digital camera. 2019 models won’t work.
• Bellows made in Switzerland.
• 3 Kilograms of spacer rings and adapters, original from some famous brand.

Before starting: take your prime lens; remove the caps. Look at it from the front, with one eye, at some distance. Do you see your wide-angle lens? We just need to mount it properly and focus closer. I searched a lot through the web, but no, it seems nobody described this setup before. Hard to believe; it is almost identical as using an inverted lens as macro.

Yes, it is that simple. In principle.

How to put the system together; you have two options:

Method 1: ask a consultant, that will find a certified technician to do the job and deliver the finished system for you.

Method 2 (Disclaimer: do at your own risk! I am not liable for your actions): put all the items into a large suitcase. Ask a servant to vigorously shake the suitcase 10^42 times. The correct lens setup will have formed spontaneously between the many possible combinations, thanks to the Monkey with Typewriter Theorem.

You can now enjoy this exclusive lens and post wide angle photos of your dog.

Instructions for the servant and the technician

Don’t shake the suitcase, or you’ll have to repay 10^42 broken lenses.

Mount the macro on the camera and focus it at close distance (1:1,5).

Take the fast prime and place it in front of the macro, inverted, with the front facing the camera. Close down its aperture, and find the distance at which the aperture blades are in focus. You may need to adjust the camera distance and magnification, so to tightly fill the sensor with the image of the prime’s aperture.

Fasten the prime with your favorite bellows; you may want to protect the exposed back lens with a filter or a dedicated window for inverted macro. Focus again at the prime’s aperture blades. Then, set the prime’s aperture at full open.

Look at the image: you should be seeing an (inverted) wide angle image approximately focused at infinity. We have swapped the prime’s image plane with its aperture. Congratulations, you have on your table a forward-aperture probe objective. The prime is working as a forward-pupil lens; the virtual aperture is placed where the lens normally focus (the sensor); about 40mm in front of the back lens for DSLR lenses (the “flange distance”).

You may then proceed to crank the focus of the macro lens, so to magnify as near as you want.

You will notice a problem with this setup: the macro’s aperture is at the wrong place; when at macro distances, stopping down it vignettes. But – why would you stop down an expensive fast prime? They should be used always wide open. Period. Otherwise you’re wasting your money. Problem solved.

If you insist on controlling the aperture, a cardboard with an hole, or better an adjustable iris, placed somewhere midway the two lenses, will improve the experience. Otherwise a third lens, or some diopters, right on the front of the prime (toward the camera), will provide the focus on the right place while restoring the macro’s own aperture place. The latter works near infinity in this configuration. It may also be beneficial to use as relay, instead of the macro lens, a large long-focus lens (An expensive 400mm f/2.8 would work best). So at the end the relay lens system is quite more complex than promised; but if done correctly, all should work nicely.

All truth said, I have tried a few relay combinations, and none worked perfectly yet, with either vignetting or field clipping. Those issues seem to get solved by stacking more glass (like, prime 50mm + large achromatic doublets (diopters) + 200mm f/3.5) but now collimation, transmission, aberrations and practicality start screaming. I invite the readers to try some more setups. The simple relay, inverted prime + macro, works, just with no aperture control and a narrowish field. Also holding the prime with one hand, and with the other hand take the picture with a phone, does something.

The angle of view is determined by the prime’s aperture: f/1.4 means a light acceptance cone :

Those 39 degrees are nothing wide to scream about, but this setup has its advantages; thanks to the forward-pupil and the abundant working distance, the center of perspective can be pulled as close as you want to the subject, with high magnification following suit. If you have a f/0.95 lens, go on and try it, it should give 53 degrees of FOV. The high quality and elaborate construction of the prime provides potential for great images; and weirdly, at high magnification it works at or near its optimum design focal distance, providing as-good-as-we-can-get sharpness on axis, and pretty good in the corners too. If you are not satisfied about the corners, don’t blame me: you should buy a more expensive prime.

This angle is the total light acceptance cone of the objective; both angle of view and aperture must fit in it; dig into the Theory Post if the concept is not clear. So we may either get the full wide view but work at small aperture (boo!); or alternatively work at large aperture but with a narrow view (boo!). What I do, employing the ancient technique called “capra e cavoli” (goat and cabbages) is to keep both aperture and angle wide; on the axis we get full aperture (where there is my subject, than needs details), while on the sides the aperture gets reduced but I don’t care (and lighten up vignetting in post production). If still in doubt, you may ring prof. Gross of Jena for his opinion about how to balance vignetting and sharpness.

Shortcut

We don’t actually need the relay lens; the prime is perfectly capable by itself of focusing directly on the sensor. This is a much simpler setup, but we lose the long working distance and the focusing capability of the relay lens.

So take your prime, or your favorite inverted-macro lens (with large aperture!), and mount it inverted on the camera, as usual, with a lot of spacers; I think is best with medium magnification, like 2:1, but that may depend on your lens temperament; you can fiddle afterwards with that. Now, set up the rig normally and focus at today’s bug.

We need just to add the aperture; it will be placed few mm in front of the bug. The ideal position for the aperture is at the plane where the lens, when mounted straight, has infinite focus (if the lens was a normal prime, optimized near infinity). In my case I used a lens for M42 SLR cameras, they have a flange distance of 45mm, so I placed the aperture there, 45mm in front of the flange of the inverted prime. How? If you have around some M42 ring spacers and and iris aperture with M42 thread, you’re lucky! If you even own an M42 helicoid focuser, fantastic, use it, you can fine-tune the aperture position. Otherwise, adapters, cardboard, glue etc. The aperture should be a small hole (~1mm) and well centered. The lens f number, object-side, is simply Hole diameter/bug distance (from the aperture). Center magnification should be the same as without this aperture. The original aperture of the lens must remain wide open. If the new aperture is at the right place, you shouldn’t see it in the image; instead if you see vignetting, change a bit the aperture position.

Done! Here is your (moderately) wide macro. What’s changed from a normal macro? Only the aperture position. And the angle of view. Chromatic aberrations may have changed color and direction, but they don’t get so bad. Make a smaller hole as aperture if the image is all foggy; as for normal inverted lens macros. I didn’t search on the web for this setup, guess that I’m not the first to re-discover it.

Early morning tabletop setup to re-test the field swap trick. Of course I’ve also mounted it properly on the camera, but here is much clearer what’s going on (apart from, where the heck is the physical aperture? In the eye of the observer?).

A manual ray tracing that I hope will make things more clear. It is visible from here that the relay lens has an hard job, as the fast prime objective outputs quite a large bundle of rays. The competition aperture-view is visible also, with vignetting. The physical aperture will need to sit where she decided, and may happen to be an unpractical place.

Still not happy?

Next: not satisfied with the f/0.95 prime? Crave for a faster lens, wider angle? We have to find an f/0.5, or better, an f/0.1.

Impossible? hmmm… I happen have a couple of those lenses.

Stay tuned for the next stage in lens tweaking and mediocre resolutions.

Micro fisheye home