LED ring flash for Laowa 15mm macro lens

= Introduction =
Laowa (Venus) 15mm f4 1:1 is a one of a kind lens: it is the first ultra wide angle (UWA) lens with 1:1 macro capability. It allows you to bring a totally new perspective (both figuratively and literally) into the world of macro photography. In short, it allows you to see the world from the perspective of the macro world inhabitants (insects). To get a better idea about this capability, check the Laowa 15mm Flickr group:
https://www.flickr.com/groups/2784583@N25/pool/
Unfortunately, there is a catch: the working distance (distance between the front lens element and the macro subject) is very small for this lens (18mm for 1:2 macro, and the totally impractical 4.7mm for 1:1 macro), especially when compared to the large diameter of the lens' barrel (~80mm). 
Why is it a problem? Because of the huge shadow created by the lens, resulting in the macro subject being much darker than the background, plus with very uneven illumination (front facing parts are much darker than the rim). Be the judge (these are 3D simulations using the open source ray tracing software Blender; the magnification here is 1:1.6, working distance 13mm):
"Ideal" (no lens shadow) shot:
https://farm2.staticflickr.com/1453/24352196094_841cfebe06.jpg
Shot with the lens shadow:
https://farm2.staticflickr.com/1592/24982797345_e5f05ab4d6.jpg
Looks pretty bad. At higher magnifications it will look even worse.
The obvious solution seems to be to use a flash. The problem with this is that 
* Regular flashes are fairly large - it is not possible to put it close enough to the subject when your working distance is say 12mm and the lens barrel blocks the view for the flash. All you can achieve is to make the rim of your object very bright, but the lens facing part would remain mostly in shadow. In addition, with large flashes it is almost impossible to prevent some flash light going inside the lens, which creates flare and reduces dramatically the image contrast and saturation.
* Macro ring flashes seem to be more appropriate for the task, but all the ring flashes I've seen are not designed for UWA lenses; when used on Laowa 15mm UWA lens, they will either block part of the frame (vignetting) and/or produce a bright halo along the frame's perimeter. In addition, these flashes are designed to work at much larger working distances (usually 50mm or more); at <15mm they are totally useless.
The ideal light for this situation would be light send through the lens (the way it is done with some microscopes). 
As this is not an option in the current case (one cannot put a source of light inside the camera, between the lens and the sensor), the next to the ideal is the case when you bring your light source as close to the subject as possible: somewhere between the lens and the subject. 
The idea I had is that one way to achieve this is to design a custom LED ring flash, which would be
* Very thin (so can be used with UWA lenses)
* Will have all its LEDs facing down, towards the optical axis of the lens, and not forward, as is the case in normal macro LED flashes.
As there will be a lot of bright LEDs located very close to the lens, special precautions have to be made to prevent light bleeding into the lens (which would create flares and reduce contrast and saturation of images).
= Designing the flash =
If you don't care about the theory, and just want a practical guide on how to build this flash, just skip this section.
First I wanted to test my idea theoretically, before spending money and hours of my time to build a real flash. I wanted to get answers to the two following questions:
# Will my design work at all?
# If it will work, what design parameters are needed to achieve the stated goals? (And the goals are: good quality light, without too much rim brightening, and bright enough to be usable under direct sunlight conditions.) In particular, which common type of cheap LEDs (either clear - narrow light cone, or diffused/milky - wide light cone) will be the best for the task?
First I had some fun playing with calculus (solving 2D integrals analytically) - my background is in research (astrophysics / cosmology), so it was nice to dust off some of my old skills. For details check the first post in my thread here. Summary: the overall concept seems to be okay, and clear (narrow cone) LEDs seem to be a better choice than the milky ones, for two important reasons:
* The shadow created by the lens barrel is pretty sharp (the transition from "good light" to "deep shadow" is pretty abrupt). If one wants the flash' light to blend naturally with the natural light, it should also have a fairly sharp transition, but in the opposite sense (brightest where the shadow is the deepest). Clear LEDs can do this task much better than the diffused ones, as they concentrate most of their light where it matters (in the deep shadow), and don't spill too much light where it is not needed (further away from the lens).
* For the same distance from the LED and the same power rating, clear LEDs are significantly (by a factor of 30) brighter than diffused LEDs, because they concentrate most of its light in a narrow beam, whereas the milky LEDs scatter almost in all directions.
Also, it should be easier to control the amount of light spilled into the lens with clear LEDs - because the light cone is much more narrow.
As I wasn't 100% sure that my calculus derivations were correct (plus they used some simplifications - most importantly, they were 2D, meaning I could "measure" the light quality and quantity only along the optical axis of the system), next I decided to build a 3D virtual model of the lens, the flash plus the macro subject and the surrounding world, using a ray tracing software Blender. It took me some time, but eventually I wrote the Blender script where I had a realistic lens barrel + the ring of LEDs facing down (with the ability to either use narrow-cone LEDs or omnidirectional ones) + a realistic model of a macro subject (laser scanned 3D model of a real wasp) + very simplistic surrounding world model (basically, a world with the sky in every direction).
I already posted two Blender generated images of the wasp in the Introduction - for the ideal case (no lens shadow, no flash), and the case with the lens shadow (again, no flash). Compare those to this rendered image, where an LED ring flash of my design was used (using narrow-cone LEDs):
https://farm2.staticflickr.com/1718/24615181989_83912863bd.jpg
As it looked pretty decent, I decided to pull the trigger and start building the flash "in flesh".
= Flash specifications =
The flash head I built for Laowa 15mm lens has the following specs:
* Size (diameter x thickness): 96 x 8mm (10mm in the stereo socket area);
* Weight: 41g;
* Color temperature: 7550K (not great, but usable);
* Maximum illumination: equal to direct sunlight (sunny 16 rule: 1/200s at f/16 and ISO 200);
* Shortest exposure: the shortest your camera can produce (it is effectively a continuous light, so no synchronization issues);
* Maximum usable magnification (when used with a protective UV filter): 0.6:1 (at larger magnifications the front shadow becomes too strong);
* Number of LEDs: 62.
* Number of configurations: three (top half, bottom half, the whole ring).
* Brightness range (continuously adjustable): three full stops
* Power: ~3.6W
* Cost: ~10$ (just the flash head; ~50$ with the Aputure ring flash);
WARNING: I designed this flash for a Canon crop DSLR (1.6x crop factor: like all Digital rebels, Canon XXD series, and Canon 7D cameras). The design should work for Nikon crop DSLRs as well, but some changes might be needed. Specifically, the diameter of the hole in the black plastic ring (or more accurately, annulus) used to prevent the LEDs light reaching the lens will likely have to be made larger, in accordance with your camera's crop factor. As Nikon's crop factor is 1.5x (1.067x larger than the Canon's sensor), the hole's diameter should be larger by the same factor as well (49mm diameter). Same with any other camera brand - as long as it is a crop camera (crop factor ~1.5x or larger).
My design will likely not work with full frame cameras (like Canon 6D / 5D, Nikon D610, D750 etc.) without significant changes, or it might not work at all. Do some testing first, cutting holes with different diameters in black paper, placing it next to your Laowa lens, and checking if the hole is too small (if it is, you'll see vignetting through the camera's viewfinder). And then you have to find a step-up ring (from 77mm) large enough to host the LEDs and to eliminate LED's light bleeding into the lens. My very simple estimate (just scaling everything up using the crop factor as the scaler) suggests that for full frame camera you will need the annulus' hole with the diameter ~74mm (46mm in my design). Adding ~23mm of space radially (or 46mm in terms of the diameter) for LEDs and for the ring, you might need a 77-120mm step up ring, which can be very expensive, or might not even exist.
Another warning: for similar reasons, this flash will not work on a crop camera with the lens shifted all the way up or all the way down. (The Laowa 15mm is a shift lens.)
= Required parts, skills and tools =
I used the following parts (all cheap, ordered from China or from a dollar store):
* 41.30$: Aputure Amaran Halo AHL-HC100 macro ring LED flash. This is by far the most expensive part of the project. I only needed the controller part of the flash for this project. My reasoning for buying this versus building my own controller is that I probably wouldn't be able to build as nice a controller for much less money. Plus, I made my new flash ring an interchangeable one with the original Aputure macro flash ring (which is a great macro flash ring for macro photography with "normal" macro lenses - telephoto, not UWA like the Laowa). So in a sense this was free, as I wanted the Aputure flash anyway, for normal macro photography.
* 3.00$: 77-95mm aluminum step up ring.
* 1.50$: 100 pieces of clear (narrow cone, 25-30 degrees) white 3mm diameter LEDs. Unfortunately the original ebay seller no longer sells the LEDs. This seller seems to be selling a similar (or identical) product, but I can't verify that (but I'm pretty positive these are the same LEDs as mine): ebay listing. I ended up only using 62 LEDs out of 100, but it is always a good idea to have a few spares. Plus, you might end up packing the LEDs tighter than I did, which would require a few more LEDs.
* 1.00$: two pieces of 3.5mm stereo phono plug (I only needed one).
* 2.00$: five pieces of 3.5mm stereo phono socket (I only needed two; this is not the seller I ordered from, but the item looks identical to mine).
* 1.00$: two black plastic baskets (1 mm thick plastic), from a local dollar store. I only needed one.
* 1.25$: epoxy glue from a local dollar store
* 1.00$ (?): superglue from a local dollar store.
* 1.25$: acrylic black paint from a local dollar store.
Besides some pretty basic mechanical and soldering skills, no special skills are required for this project.
Required tools:
* Soldering gun.
* Decent scissors (to cut round holes in 1 mm thick plastic).
* Drill press (to drill the half-hole on the side of the aluminum step up ring). One can probably get away with using just a hand-held drill, but drill press makes it much easier. In either case, be very careful - wear protective gloves, eye googles etc.!
* Sanding paper, from coarse to very fine.
Optionally, a sharpening tool (like this one) can be very handy when shaving off excessive epoxy glue. Again, take all the precautions - protective gloves, eye goggles etc.!
Alternatively, one can simply use a metal file.
I estimate you'd need ~6-8 hours of your time to build the flash. (The total time will be longer - 3 days at least - as you have to wait between different stages - gluing, painting etc.).
= Building the flash =
== Making the ring (annulus) ==
The purpose of the ring is to prevent the light from LEDs from entering the lens. Its outer diameter should match the inner diameter of the 95mm side of the 77-95mm step up ring (so it should be ~93mm), while the inner hole diameter should be 46mm for Canon crop DSLRs (crop factor 1.6x), and likely ~49mm for cameras with the crop factor 1.5x (Nikon crop DSLRs etc.).
If your camera is not a Canon crop DSLR, I strongly recommend first making a test ring out of black paper, to check 
# if it produces any vignetting when used on Laowa 15mm lens + protective UV filter + the step up ring, at magnifications 0.6...0.4:1, and
# if you get any lens flare from a single LED placed on the top of the ring near its outer rim facing the centre of the ring's hole.
If you get problem #1, you'll have to increase the inner hole diameter of the ring, if it is #2, you'll have to reduce the inner hole diameter. Ultimately, the inner hole diameter should be a compromise between the two issues. On Canon crop DSLRs, choosing b=46 mm completely eliminates both issues.
I cut the ring from a black thin (~1 mm thick) plastic basket from a dollar store, using good scissors. Then I sanded it using the finest sanding paper I had, on both sides.
Finally, I glued the ring to the step up ring (on the 95mm thread side) using superglue:
https://farm2.staticflickr.com/1530/26457353931_b858a33ac6.jpg
== Attaching the socket ==
The power to the flash is delivered via 3.5mm stereo socket (you need stereo - 3-wire - version, to be able to control independently the two halves of the LED ring flash).
Attach the socket to the step up ring as follows:
* Find the convenient position for the socket on the step up ring (remember the socket will mark the splitting point between the two halves of the flash).
* Make a half-hole on the side of the step up ring for the socket. Use drill press for that. Be careful (protective gloves and eye goggles are a must).
* Make three tiny circular indentations in the plastic ring corresponding to the three plastic protrusions on the side of the stereo socket. This will allow you to flash the socket against the plastic ring without any gaps (very important for quality gluing).
* Apply superglue to the side of the socket, place it on the step up ring against the plastic ring, and right away secure the socket to the step up ring using the socket's screw (don't overdo it). After that press the socket tightly against the ring for 30 seconds, and then leave it for 24 hours.
== Soldering the LEDs ==
This is likely the most tedious part of the project (~2.5-3 hours of work). You'll have to bend the legs of ~62 LEDs at ~90 degrees not far from the LED's body, and cut the legs in the way that there will be just enough of them left to solder the LEDs in parallel. You'l have to solder two large semi-circular segments, ~31 LEDs in each, closely following the outer perimeter of the 95mm side of the step up ring. Remember that the LED legs should be close to the metal rim of the step up ring, but should never touch it. LEDs have to be facing towards the optical axis (the center of the step up ring). You don't have to be super precise here. You should assemble and solder the LEDs on a hard flat surface. My LEDs have one side flat (the other side round), so it made a total sense to assemble LEDs with the flat side against the plastic ring. The LEDs have to be parallel to the plastic ring (and step up ring), so all their light beams would intersect near the plastic ring's plane, at the center of the hole.
Solder together the two segments on one side only using one leg of the LEDs (not both). In that way one can independently operate the two segments via three wires - one common wire, plus two power wires.
Here is the schematic of the flash head:
https://c2.staticflickr.com/2/1686/26370935752_70ca891c2a_o_d.jpg
At this point you can test both segments by providing ~3V power to one segment, and then to the other one. They should be glowing bright. (If not sure about the correct polarity of the LEDs, test first with a spare LED.)
Glue both LED segments to the surface of the black plastic ring using superglue (do first one section, then the other one). Make sure LED legs don't touch the step up ring.
24 hours later, you can solder the three connectors of the stereo socket to the LED ring. Use the ground socket's connector for the common wire, then use "left" and "right" connectors for the two power wires. It should look like this:
https://farm2.staticflickr.com/1618/25676358194_6f57a9ae15.jpg
https://farm2.staticflickr.com/1569/26188753442_9a781bb358.jpg
== Modifying the Aputure Halo flash ==
You should cut the cable connecting the Aputure Halo flash ring to the controller, near the ring (leave just enough of the cable to be able to install there a stereo socket).
Solder the three wires from the controller to a stereo phono plug. In my Aputure flash, red wire is the common one ("+" polarity), and black and yellow wires are the power wires for the two flash segments (both "-" polarity). Consecutively, I soldered the red wire to the base contact of the 3.5mm socket, and black and yellow wires - to the collar and the tip connectors.
Do your own testing, using spare LEDs, before attempting to power the DIY LED flash with the Aputure Halo controller - your LED polarities, wire colors, etc. can be different from mine.
Once you are 100% satisfied that the polarities are correct, try to power the DIY flash with the Aputure controller. You should be able to independently operate both halves of the flash, to change LED brightness, and to use the flash mode of the controller.
It will look like this:
https://farm2.staticflickr.com/1535/25678464893_33cd52632e.jpg
https://farm2.staticflickr.com/1469/26188751822_d86dcd222a.jpg
https://farm2.staticflickr.com/1688/26281194495_a0439bbb54.jpg
https://farm2.staticflickr.com/1460/25676354204_8df6b79dd2.jpg
https://farm2.staticflickr.com/1713/26281192565_0c2d804bd8.jpg
At this point, the flash is already perfectly usable - you can start using it to shoot insects and other macro objects.
== Epoxy glue, sanding and painting steps ==
To make the flash more sturdy and protect it from the elements, you should fill the LED legs area with epoxy glue. First prepare the flash (make sure the epoxy glue won't get to the LED's curved tops):
https://farm2.staticflickr.com/1641/25754798243_6cf76f8c60.jpg
https://farm2.staticflickr.com/1491/26291317271_9d7f1b5a2f.jpg
Then mix the epoxy glue (I used the one from a dollar store), and fill up the gap with all the wiring. Be quick - you probably only have ~1 minute to do the mixing, and to pour the glue into the flash head. After that the glue will get too thick to do a good job.
After 24 hours you can sand the top and the side of the flash head, to remove the excesses of the epoxy glue. At the end sand the glue with a fine sand paper.
Finally, you can paint the epoxy surfaces (and also the plastic ring) with black acrylic paint (can be from a dollar store) - creates very nice black matte surface. It will look like this:
https://farm2.staticflickr.com/1644/26369220741_05e21927d7.jpg
https://farm2.staticflickr.com/1630/26343066122_1c4c497710.jpg
https://farm2.staticflickr.com/1565/26343065522_8a3921a4eb.jpg
https://farm2.staticflickr.com/1634/25830552824_749c3e00f3.jpg
https://farm2.staticflickr.com/1448/26409467236_1687eabd3c.jpg
Voila - your flash is ready!
= Modding the original Aputure flash head =
This is how I modded the original Aputure flash head to make it usable again, as an interchangeable flash head. All I had to do was to make a small metal plate with three holes (for the 3.5mm stereo phono socket and two M3 bolts), and make 5 small holes in the plastic case of the Aputure flash head - three holes for the three contacts from the stereo socket, and two holes for the M3 bolts. Make sure the wires are soldered the way it was before modding. It seems to be sturdy enough even without any glue.
https://farm2.staticflickr.com/1697/26510366102_c714dab809.jpg
https://farm2.staticflickr.com/1686/26510362842_7e52c20940.jpg
https://farm2.staticflickr.com/1461/26510356032_e30b7fbab4.jpg
https://farm2.staticflickr.com/1673/26000199323_ebee636491.jpg
https://farm2.staticflickr.com/1645/26510359462_35ac197b15.jpg
= Tests =
Just one test shot for now - indoors, an orchid's branch, with a mirror in the background:
https://farm2.staticflickr.com/1594/26357544985_ecd5379de8.jpg
I corrected the colors using an Adobe color profile which I specifically created for this flash (using ColorPassport target and software). It was made hand-held, using magnification 0.6:1. The shot parameters were f/16, ISO 200, and 1/400s (so the flash illumination was brighter than from direct sunlight - according to the "sunny 16 rule").
== Color correction ==
I used Color Passport to create an Adobe Dual Illuminant color profile for Laowa 15mm + my LED flash. The profile can be used to make a smooth transition from LED-only color profile (7550K, +26 for ACR) to daylight-only color profile (5450K, +56). In particular, one can use it at the half way point between the LED-only and daylight only, which will have 6500K, +41.
Here is one test of the dual illuminant color profile: the top image was corrected for daylight only, the next one is the half-way correction, and the bottom one is using the pure LED profile. Daylight-only has the LED lit parts too cold, the LED profile has the environment too warm, but the half-way case (in the middle) seems like a good compromise, with both the foreground and background having reasonably nice colors. Under other lighting conditions (e.g. daytime in the shadow, where color temperature can be much higher) this will be even less of an issue.
https://farm2.staticflickr.com/1674/26393218642_f1fdfc8816.jpg
https://farm2.staticflickr.com/1585/26459603966_930ce85d77.jpg
https://farm2.staticflickr.com/1675/26485512785_b815a953b2.jpg