85mm Custom Camera Lens

85mm F2 Sony FE Lens Design

I wanted to design and build a camera lens because I find that I learn more and gain a greater appreciation for the design and difficulties of something when I try to build my own. Through this project, I hope to learn about lenses and maybe get a usable lens out of it.

I targeted a lens with a focal length of between 50 and 100mm and a relatively fast aperture.

Starting Design

Before starting to attempt to design a lens, I researched classical element arrangements that I felt would be achievable with only a few elements and with lenses that I could access. The three forerunners were a Cooke Triplet, a Petzval, and a Tessar arrangement. I ended up picking a Cooke Triplet arrangement due to the ability for all elements to be double concave or double convex and still produce decent sharpness.

The primary difficulty of this project turned out to be creating a sharp lens with only very limited element choices. I used Surplus Shed for lenses which only provides the most basic information about the lenses they sell. To attempt to create the necessary inputs for Zemax, I had to calculate radiuses based on the focal length, thickness (which was unknown until I received them), and a generic refractive index. Because of this, I choose to limit myself to designs that would be easy to approximate. I do have the option to use a CMM to measure precise radiuses and thicknesses, but this just moves the problem to the coatings and glass properties so I am choosing to remove difficulty by sticking to double concave and convex.

By selecting the lenses with the most data provided and that would produce a reasonable focal length, I ended up with this initial design from Zemax.

This would produce an image at as low as F2 but the spot size and aberrations were much worse than artificially lowering the aperture. Above is showing an artificially lowered aperture of F2.5 at 85mm. This is using all preset radius lenses from Surplus Shed where I can only control the spacing between elements. The 200-micron spot radius is pretty bad and will severely hurt image sharpness, but it is the best that this selection of lenses can produce. I can only control the spacing between elements, not the radius.

Mechanical Design

This is the final configuration that is adaptable to new lens spacing. It is only two machined parts and two printed parts that I plan to print on a carbon printer to reduce internal reflections. Green parts are printed, gray are machined aluminum, and white are optical elements.

This will directly mount to a Sony FE mount with directly machined features. Focusing is achieved with a combination front cap and a focusing ring that directly moves the front element. This design does require that the lens be designed for a certain distance as any deviation will greatly affect image quality.

All these parts will be machined on 4 axis Haas Mill.

Recieving/Validating Lenses

To ensure that my hand calculations of the radius of the lenses were accurate with the real lenses, I used my school's CMM to get accurate radius and thickness measurements. Below are pictures of the validation setup. Using the CMM and micrometers, I was able to generate a very accurate radius allowing me to then also approximate a refractive index for the lenses.

I am not sure how accurate this method is as it trusts Surplus Shed's focal length and also neglects Abbe numbers. To mitigate this, I tried using Zemax's built-in glass library to find a glass that might have existed only off the refractive index. In the future, I may try to validate the focal length measurements using a test stand to remove more assumptions from the lens characteristics.

While the CMM was extremely accurate, it also added a lot of time in processing the data that probably did not signifigantly contribute to a sharper image.

This led to the above final design with a slightly different focal length and a better spot size. The aberrations also seem more controlled than before which should help improve the image quality.

This lens is 88mm at as low as F1.9. I am going to artificially lower the maximum aperture to increase the sharpness. The spot size reduces by a factor of nearly 5 going from F1.9 to F2.5.

The final spot is 200 microns in diameter which is not great but should be good enough to make an image.

Physical Construction

Main Body

The sony FE mount was directly machined using undercutting tools. The main challenge with this part was the multiple threads and the long depths requiring long tools. Machined in two CNC operations and finished on a lathe.

Front Cap/Focus Ring

Machined in one CNC operation then parted on a lathe. Treadmilled threads and threadmilled knurling using a NYC CNC technique. The knurling is done vertically on a mill using a threadmill by setting it to cut a high pitch multi-start thread. This was 20 threads in each direction with a pitch of 0.5 TPI.

Assembly

Front element is cemented onto focusing ring

Installing rear element

Installing internal rings

Final Assembly on Camera

Results

There is an image, just not sharp. The lens is only usable in the short focal range where I defined it in Zemax. The focusing ring is not useful and introduces significant aberrations and image quality problems due to only moving one element.

I learned a fantastic amount about the technical details of lenses and hope to make another lens in the future that will incorporate what I have learned. I think that being more open to using nondouble concave or convex lenses will greatly improve results as it opens up more refined lens arrangements that will produce better results. I would also like to incorporate an aperture and remove the focusing option.

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