This time around I am going to be designing a lens based upon a traditional design. The previous designs I have created have been a combination of intuition, luck, and time consuming trial and error working almost completely in lens design software (ZeMAX). With limited time and a clear specification of what I want, I have opted to create a variant of the famous Cooke Triplet. The design is the work of H. Dennis Taylor who conceived it while he was working as an optics manager at an English telescope manufacturing company. Taylor and Hobson became the builders of the official Cooke Triplet Portrait lens around the turn of the 1900's. Coincidentally, Taylor and Hobson were based in Leicester, the city where I study at university.
The design looks simple, but is actually the first design to offer enough degrees of freedom to correct all first order aberrations. As a result, Cooke lenses offer a good performance versus manufacturing complexity trade off. This is why I picked it for this quick prototyping phase.
The design consists of 3 elements: 2 positive double convex lenses made of one material, sandwiching a negative double concave lens made of another. Also featured, between either the 1st and 2nd, or 2nd and 3rd elements, is an aperture stop. The surface radii, airspace thickness, stop location and material selection offer multiple degrees of freedom giving me the opportunity to design a lens with minimal aberrations like coma, spherical, axial colour etc. Using my lens design bible, I worked through the equations from scratch, starting with my base specification of 50mm @ f/4. The workings are shown in order on the scans below.
To design the lens I actually followed the design method for a different type of lens, a 4 element Celor. This lens has a lot of similarities and can be used to design the entirety of the Cooke Triplet if you know which bits to ignore. Note that I only designed the rear 2 elements on paper, as this particular design benefits from symmetry during the design process. That means that I can design the back half at 100mm f/4 (double the EFL), and then mirror the system to get 4 elements (with the centre two joined with no airspace, effectively forming 1 element and creating a triplet). This can then be entered into ZeMAX on the computer and altered to be asymmetric and then optimised.
