Have you ever wondered about the secret inner workings of modern lenses? If you’re like me, you have but are way too incompetent to ever want to actually take one apart yourself. In fact, camera brands and lens manufacturers are often quite secretive about what goes on ‘under the hood’ so to speak.
In this video, Gordon Laing from Camera Labs was lucky enough to be invited to film a Canon engineer stripping down an RF 28mm f/2.8 STM pancake lens.
Designing a Compact and Affordable Lens for Mirrorless Cameras
The Canon RF 28mm F/2.8 STM lens was designed to be a compact and affordable pancake lens for the EOS R mirrorless system. Canon chose to make a 28mm focal length because it is a versatile option for both still photography and video on full-frame and cropped APS-C systems. However, designing wide-angle lenses for mirrorless cameras presents a unique challenge due to their shorter back focus distance than DSLRs.
If not addressed correctly, the shorter back focus distance of mirrorless cameras results in greater softening and darkening towards the corners. Canon developed a new optical design with unconventional-looking elements to overcome this challenge. The RF 28mm lens incorporates three large aspherical elements towards the rear, which deliver shallower light ray angles at the extremes, reducing corner softness and darkening.
Innovative Manufacturing Techniques for Aspherical Elements
The RF 28mm F/2.8 lens employs three aspherical elements in its design. Aspherical elements have a shape that is not a section of a simple sphere or cylinder, and they help reduce issues like spherical aberrations. Canon utilizes different manufacturing techniques to produce these elements based on target quality, weight, price, and shape complexity.
The traditional method involves grinding the shape from fluorite crystals, producing high-quality but heavy and expensive elements typically reserved for the flagship L series lenses.
Another technique is replica spherical, where a normal spherical element is bonded with a resin layer to achieve the desired shape. The remaining two methods involve moulding techniques using either glass or plastic resin. Plastic moulded elements, known as PMOs, offer a cost-effective solution for complex shapes and have been employed by Canon in many of its lenses, including the RF lineup.
Disassembling the RF 28mm F/2.8 STM Lens
In a step-by-step disassembly process, Canon’s engineer, Shinsuke Ito, guided the exploration of the RF 28mm F/2.8 STM lens. The lens’s construction and use of space become apparent by removing various components. The lens mount, circuit boards, focusing motor, switch, and barrels are dismantled, revealing the arrangement and placement of different elements.
The Optical Construction Revealed
The lens consists of six groups of elements, totalling seven individual elements. Each group plays a specific role in achieving the desired optical performance. The design exploits the space created by the rectangular PMO aspherical elements, allowing for a more compact lens barrel. Plastic moulding enables complex shapes and reduces weight and production costs.
Canon’s RF 28mm F/2.8 STM lens shows the innovative use of optical technology in manufacturing. Along with other design considerations, PMO aspherical elements allow for a compact, affordable lens that delivers impressive optical performance. Additionally, plastic moulding has proven to be a practical and cost-effective method for producing elements with complex shapes.
As Laing states, the newer, cheaper plastic moulded lenses are capable of giving the more expensive L series lenses a run for their money. It might be worth considering all the available options before splashing out on a new L series lens.
If you’re interested in camera and lens manufacturing, definitely take a look at this video. It’s fascinating and shows just how complex and amazing the construction of these lenses is.