Geek Articles

Assembling the C-4 Optics 4.9mm f3.5 Hyperfisheye Prototype

Published February 19, 2019

You guys have watched us gut a lot of lenses and cameras over the years. So I thought it would be fun for you to see us put one together from scratch. Compared to many of the lenses we’ve taken apart, this is all mechanical lens is rather simple: no focus motors, image stabilizers, etc. But even a simple lens is a very complex structure. This post will probably give you a good idea of how much mechanical design is required to make even a very basic lens.

The lens is also unique; it’s a prototype C-4 Optics 4.9mm f/3.5 circular fisheye. It’s a massive lens giving a 270-degree field of view, meant for immersive video and specialty shots. To give you an idea of what 270 degrees means, the lens sees behind itself. An ultra-wide 15mm fisheye lens gives a 180-degree field of view while an 11mm rectilinear lens is less than 120 degrees.

HyperFisheye Lens

Image Courtesy C-4 Precision Optics, 2019

 

The closest thing that’s existed to this is the 1970s classic Nikkor 6mm f/2.8 fisheye, which gave a 220-degree field of view, weighed 5 kg, and can be rarely found for $100,000 and up these days. The C-4 optics lens weighs every bit as much as the Nikkor, but should be far sharper, have less distortion and vignetting, and cost somewhat less than those do today. (‘Somewhat’ being defined as ‘less than half’.)

So let’s put stuff together!

Potting Elements

Several of the elements need to be potted (glued) into their metal holders before we begin assembly, then let dry for 24 hours before we really get started. This requires a steady hand, which is why I write the posts and take the pictures while Aaron does all the work.

We start with the smallest element in the lens, a tiny one that sits right by the aperture when assembled.

Fisheye Ultrawide assembly

C-4 Optics, 2019

Fisheye Ultrawide assembly

C-4 Optics, 2019

 

Once the lens is placed in position, a syringe full of potting cement . . . .

 

is used to lay a bead around the edge of the glass, cementing it in place.

Fisheye Ultrawide Assembly

 

The much larger second element also gets potted. This is a two-syringe-worth-of-cement job. (Yes, everyone, we know it’s dusty. Cleaning comes during assembly.)

UltraWide Angle Lens

C-4 Optics, 2019

 

The third and fourth (already in place) elements both get potted into the main support plate.

Fisheye Lens Assemble

C-4 Optics, 2019

 

Fisheye Assembly

C-4 Optics, 2019

 

This all dries overnight and then we get on to the actual assembly. Aaron laid out the parts for the assembly before we quit for the day.

Fisheye Assembly

C-4 Optics, 2019

 

Base and Cap Assembly

You probably just think of these as accessories, but on a lens of this size, they are critical pieces that took up as much design effort as the lens itself. Actually, most of the parts you saw in the image above are for them.

Each extending leg/handle is six pieces.

Fisheye Ultra Wide Assembly

C-4 Optics, 2019

 

They go together like this: The inner leg threads get lubed, the lock ring inserted, and then the inner leg inserted into the barrel.

 

After handle’s locking ring is tightened down, the rubber foot gets put on and the leg screwed onto its mount.

 

The bottom platform just needs its three adjustable feet covered, their rubber bumper added, and then screwed in.

 

Finally, the lens cap (it really should be called the lens helmet) needs to be assembled. This isn’t just a cover; it has to be capable of supporting the entire lens since that’s the only way to place it face-down to attach cameras, follow-focus, and other assorted gear to it. The cap itself weighs almost a kilogram and has to have air-release vents; otherwise, it can become immovable if it suctions down around the lens. (Yes, there is a story about how we figured out the vents were necessary.)

C-4 Optics, 2019

 

The latches have a ‘ball-bearing screw’ inserted on each side as a latch-lock, then the latch itself goes on over an inserted rod which is then locked in place with a set screw.

 

After that, we screw on the rubber stopper, add another rod containing the tension springs, and then mount the latch assembly to the hood. The plastic of the hood is plenty thick enough that it has nice 5mm molded latches to attach the latch rods with.

 

 

Fisheye Lens

C-4 Optics, 2019

 

Now to the Lens

Focusing Barrel

We start with the focusing group, a doublet that comes to us cemented with black lacquered edges.

 

This gets placed into the larger focusing barrel and an internal retaining ring used to fix it in place.

 

It doesn’t show well in the above pictures, but the outer barrel is threaded to receive the focusing link; the metal arms that will mount it to the mechanical focusing apparatus. Once this is started by hand, a spanner is used to thread the barrel to a measured position. Then the focus lock ring is placed to keep everything there.

 

Main Barrel

The main barrel, which contains the aperture and several optical elements, is the most complex part of the lens. The first piece in is a tiny doublet that goes right behind the aperture.

 

And then gets locked in place with a retaining ring.

 

Behind that goes a cemented triplet that is slightly larger.

 

A flat element gets mounted in its holder and then placed at the very rear of the lens. You can see by the knurled finish that this can be removed and replaced with various filters if needed.

 

Turning the barrel over, the aperture assembly gets placed in the other end, right in front of the doublet we inserted first. It has to be rotated to the proper orientation, then it’s locked down with a hex screw.

 

We finish up the barrel with an old friend. The small element we potted yesterday mounts to the front of the barrel. It’s hard to see in the pictures, but its mount is threaded and screws into the barrel.

C-4 Optics, 2019

 

Main Barrel and Gear Assembly

 

We start by lubing the focus gear and inserting the focusing helicoid, which is then adjusted to a measured depth.

 

The aperture fork is mounted to the aperture ring. The fork comes in two pieces that overlap. The reason for the two pieces is that these are adjusted to be the aperture stops on either side.

 

A hard stop is screwed in place in the focusing barrel and two more into the main barrel. The ones in the main barrel are eccentric; adjusting them sets the focus stops accurately.

 

Now the focus gear is slipped over the main barrel and the stops all aligned.

 

Then the focusing group that we assembled earlier is placed into the focusing helicoid and screwed in place.

 

Following that, the aperture / rear optical assembly gets put in place.

 

The aperture pin then gets screwed in place with a really nice, expensive tool that does nothing other than screw the aperture pin in place. That’s it.

C-4 Optics, 2019

 

Then a nylon spacer goes on (the green shim), which keeps the focus smooth and with just a bit of resistance. After that, the index barrel is placed and screwed down, and then the aperture gear can be slid on.

 

We’re getting close to being done with the rear part of the lens. The mounting ring goes on, then an infinity spacing shim (we just guess the thickness at this point, it gets reshimmed later), and finally the bayonet mount.

 

There’s only one more step to completing the assembly of the rear part of the lens; calibrating the aperture. We use an occulter, basically a semitransparent disk with a blackout that will be the size of the aperture at f/16.

The lens is placed on a light table, the occulter placed over the rear element, and the iris adjusted until the f/16 setting just occludes the occulter’s central black disk (it’s not quite there in the image).

 

Putting It All Together

All of the complicated parts of the lens are in the rear we’ve just assembled, but that’s not really the part you notice when you look at the lens. It’s time to put those together now.

First, we get the base plate (AKA cheese plate) that we had potted the third and fourth elements to. The top side of the plate is the base of the large front elements, the back side functions as a cheese plate for inserting rods and other support items. The second element, which we also potted yesterday, then threads on.

 

Finally, we unwrap the first element. These cost about $5,000 each so we tend to be a little paranoid about them.

Ultra Wide Fisheye Lens

 

Once it’s set in place, it will be held by what we think is the world’s largest retaining ring.

 

How do you tighten such a big ring? You temporarily put some big-ass screws in the locking slots to serve as handles. Once the ring is mounted, the front assembly gets put into its cap, which is the only reasonable way to hold it while we work on the back.

 

Now we can align and mount the rear assembly, the tripod legs, and finally the bottom platform.

 

With the legs in place, we turn it back over and put in a leverage screw for one last torque of the retaining ring. Then the ‘handle’ screw is removed and set screws placed in those three holes to keep the ring in place.

 

And assembly is done.

 

We’ll have to take the bottom platform off and fine tune the shims under the bayonet mount for infinity focus before we start taking pictures, but that’s about it.

For those of you who are curious, a smaller Sony camera (A7xxx) fits nicely protected within the legs. For larger cameras the bottom platform can be removed, the legs extended or removed, and various apparatus (rods, follow focus, aperture control, etc. ) mounted directly to the cheese plate.

This lens is one of two developed for C-4 Optics, so it likely won’t ever show up on our rental list. But it is a passion project of mine, so I wanted to show off some of the things I’m working on on the side. If you are looking for an ultra-wide, I’ve gone ahead and listed the widest lenses we have available for rental below.

Lens Angle of View
Canon 8-15mm f/4L Fisheye 180 degrees
Sigma 8mm f/3.5 EX DG Fisheye 180 degrees
Sigma 4.5mm f/2.8 EX DC HSM Fisheye (crop sensor) 180 degrees
Nikon 10.5mm f/2.8G AF DX Fisheye 180 degrees
Panasonic 8mm f/3.5 Fisheye 180 degrees
Rokinon 8mm T3.8 Cine Fisheye (crop sensor) 167 degrees
Canon 11-24mm f/4L 126 degrees
Sigma 12-24mm f/4 DG HSM Art 122 degrees
Sigma 8-16mm f/4.5-5.6 DC HSM (crop sensor) 114 degrees
Olympus 7-14mm f/2.8 ED PRO 114 degrees
Fuji XF 10-24mm f/4 R OIS 110 degrees
Sony E 10-18mm f/4 OSS 109 degrees
Panasonic/Leica 8-18mm f/2.8-4 ASPH 107 degrees
Rokinon 10mm f/2.8 ED AS NCS CS (crop sensor) 105 degrees

We’ve got some urgent things that are going to take up the next week, but we should have another post with images from the lens in 10 days or so. In the meantime, here is a quick teaser to give you an idea of how wide 270 degrees really is. 

C-4 Fisheye Lens

 

Roger Cicala, Aaron Closz, Brian Caldwell, and Wilfried Bittner

February, 2019

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Geek Articles
  • Brenda

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  • It wasn’t must my call. Brian is the designer, I looked more at potential markets. Part of it was we saw a unique niche for this. Part was knowing the price people were willing to pay for the Nikon 6mm fisheye. Part was what Brian wanted to tackle first.

  • Hendrik Müller

    Hallo Roger and the other 3 C’s,
    Your sense of humor is unsurpassed, not to mention the optical, engineering and manufacturing’s skills to create such a novelty even as a april’s fool. WOW
    If only I had the funds, I would pay you full costs to create another one for me.
    So, instead of meaningful money I can send you my meaningless congratulations and my best wishes, for your 2 companies and your private hobby.
    regards from Switzerland
    Hendrik

  • Vladimir Gorbunov

    Dear Roger, according to your comments under the various fisheye lenses, you’re not a huge fan of them. Then why choosing the extreme fisheye as your own project, instead of something like collapsible 24-400 mm F/2.0?

  • Brian Caldwell

    Roger pretty much covered your questions, but I wanted to add a little about the lens cap that Wilfried designed. Its so sturdy that the lens could probably survive a 10-foot drop onto concrete. Here’s a photo of Wilfried *standing* on the lens cap as it rests on a scale. His feet cover most of the cap, so its a bit hard to see, but trust me its there! Admittedly, Wilfried is only a little more than half my weight, but still . . .

    https://uploads.disquscdn.com/images/4a86a665ca546214ef3818665b4935e78e565599c71eb6298fd9842081e57269.png

  • Brian Caldwell

    I call him Willie, and his sidekick is a cast skull I call Yorick. Its an amazingly lifelike wax head I bought from a colleague, who picked it up at a Hollywood curio shop, who in turn got it when the Autry Museum of the American West got rid of a bunch of stuff.

    Really good wax figures are rare as hen’s teeth, and I’m extremely fortunate to have this one in my collection. Prior to finding Willie, I had searched for several years for a good wax head, but even items costing $thousands looked like crap.

    Willie has great facial features and skin tone, wears a variety of hats and bandanas, and will stay absolutely still for months on end if need be. He’s not perfect, and has some damage to his right side, with most of his right ear missing and a big dig on his right cheek.

    Here’s a closer view:
    https://uploads.disquscdn.com/images/ffb3c9794e06dedea29c6936c7f8064283738364999ed11011f0d1e50b42d6cb.jpg

  • Brian Caldwell

    Brandon is right – the blue band is a result of chromatic aberration of the pupil. However, this has no effect on bokeh.

    The problem with discussing things like pupil aberrations is that they are so obscure that even many lens designers won’t really know what you’re talking about. In the case at hand, it may be more useful to think of the blue band as being caused by chromatic variation of vignetting, where the vignetting is created at the metal edge of the large front retainer ring.

    If you look at a layout showing 135-degree chief rays (rays passing through the center of the iris) at three colors you will see a large separation of the colors at the first surface, but almost zero separation at the image plane:
    https://uploads.disquscdn.com/images/fc375d38f51ccce02e012cad691cf8980ef00326a0b83cdd70ff1cc46f070990.png

    All three colors make it through the first surface at 135-degrees off-axis. If you increase the semi field of view beyond 135 degrees red will get vignetted by the front lockring first, followed by green, and finally by blue. So, you wind up with a blue ring at the extreme edge of the image circle – beyond 270 degrees FFOV – that becomes more and more blue as you go further out, until finally blue gets cut off and you have black. Virtually all fisheye lenses behave this way.

    The image inside the blue ring has extremely low chromatic aberration. From deep violet through deep red, lateral color never exceeds 10 microns at the image plane for. In fact, the glass types for 10 of the 12 lens elements were chosen for their abnormal partial dispersion properties specifically to keep lateral color as small as possible.

  • Brandon Dube

    When you see “stretched” looking bokeh from a large aperture wide-angle lens, that is an artifact of the pupil being poorly imaged off-axis.

  • Ilya Zakharevich

    IIUC, here you mean “pupil” in Pickwick sense: whatever apertures obscure rays for a particular source of light. (Somehow, I think of pupil only in paraxial approximation, where it does not jump to different planes depending on the source. ;-] The latter kind of pupil is visible only on bokeh circles?—?and chromatic aberration on it would make edges of bokeh colored.)

    Very interesting! So “your” pupil at the extreme edge of view is a meniscus (intersection of two ovals). It should have a funny diffraction pattern! Are these patterns visible in photographic contexts?

  • Doesn’t have to be much different, only takes a little mind over matter :).

  • Someone

    Thanks for your reply!

  • Brian Caldwell

    I wrote an answer to this the other day, but I must have neglected to hit the “post” button.

    For objects at infinity, which is roughly 2meters from the lens and further, everything in the 270-degree field of view is sharp. So, the object “plane” would be a sphere centered on the lens with a radius equal to the object distance. As you move in closer the ideal object plane starts to move away from the lens the further you go off-axis. A handy way to model this is to use a conic surface for the object plane.

  • rocketride

    And what are this hypothetical cyclist’s cervical vertebrae made of?

  • rocketride

    I know that. In order to get Zemax to correctly draw the lens, The semidiameter of R2 would have had to been extended enough to bring the point where the flat back plane meets the curve back past the edge of R3. It looks like you were already scary close to a full hemisphere there. I’ve forwarded this blog entry to pretty much everybody I work who would be interested.

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