Photographic lenses are becoming more complex and sophisticated with every new generation. While glass elements are certainly taking up most of the space inside a camera lens, modern lenses are packed with internal support structures, several types of gears and motors, electronic circuit boards and wires, as well as sensors and switches. At the same time, photographers expect these lenses to maintain an extremely high level of performance under a multitude of environmental conditions.

The development of a camera lens involves a large number of designers and engineers. In the early stages of development, optical designers primarily focus on the imaging performance of an optical system. Design considerations at this stage mainly include the choice of glass materials and surface curvatures, calculations of precise lens element positions in relation to the image sensor, elimination of optical aberrations, and definition of lens movements for focusing or zooming. The objective of optical lens design is to achieve the highest level of excellence in image quality from the center of an image to the corners. Once an optical design is completed, it is optomechanical engineers who need to create a structure that holds all the optical components in proper axial and radial alignment. This structure – the lens barrel – must meet all requirements listed below while still be producible at a reasonable price.

• Structural support for all optical, mechanical and electronic components.
• Mechanical and electronical interface with the camera system.
• Changing lens positions for focusing or zooming.
• Protection of all components from corrosion and other damage.
• Protection of the inner workings from overheating when exposed to the sun.
• Shielding of unwanted stray light from reaching the image sensor.
• A high degree of usability by implementation of control rings, switches and info displays.
• Use of durable, functional and comfortable materials for secure grip while keeping weight low.
• Compliance with the overall design of the lenses product line.

The focus preset feature can store any chosen focusing distance into the lenses memory, and the lens will quickly and precisely recall that focus distance if required. This feature is available on most EF super telephoto lenses, and it offers three controls: The preset switch, the set button, and the focus playback ring directly next to the focusing ring. The preset switch has three positions: OFF, ON and ON with sound. Pressing the set button on the lens saves the current focused distance into memory. Once the playback ring is turned left or right, the lens quickly and precisely recalls the set focusing distance. To shoot at the preset focusing distance, the camera's shutter button is pressed while the playback ring is turned. Focus preset eliminates the requirement to re-focus through the camera's autofocus system as this always involves a slight risk of focusing errors. This feature can be particularly useful for sports and wildlife photography, or in any situation where a photographer expects a subject to appear in a precise spot.

Power focus is a mode that allows the user to drive the autofocus motor electronically via the lenses playback ring instead of the focusing ring. It is more designed for videography as it can change focus distance very smoothly. This can be a very useful feature when shooting video sequences with slowly moving objects.

A very unique feature of Canon's RF lenses is the control ring. That ring can be assigned to control either aperture values, shutter speed, ISO speed, or exposure compensation, some of these only during manual mode. Due to limited space on the barrel, some lenses such as the Canon RF 50mm F1.8 STM have a combined manual focus and control ring. A 'control ring switch' is then used to select one of the two functions. The control ring is implemented in a very flexible way. A repetitive black and white pattern is applied to the inner surface of the ring. A tiny sensor (light source and photodetector) is placed just below the inner surface of the control ring and is connected to the lens electronics. This allows the lens to detect both direction and speed of movement of the control ring. The overall design allows for a very flexible placement around the lens barrel because the entire control ring system including the sensor is relatively thin and can easily be integrated into a part of the lens barrel. As the control ring is not mechanically connected to controlled elements by gears or levers, this concept is also called 'control-by-wire' or 'focus-by-wire' in case the ring acts as the focusing ring.

A camera lens is made of numerous different materials. Here is a general summary of the most common materials used for photographic lenses:

• Outer Barrel: The choice of material used for outer barrel parts depends on the type, size and price category of a camera lens. Canon most commonly uses polycarbonate or other high-quality plastic material for the enclosure of a lens. Some larger telephoto lenses that require additional rigidity and Canon's L-lenses often have their outer barrel made of aluminum.
• Inner Barrels and Lens Cells: These are typically made of polycarbonate or other high-quality plastic material. Some functional inner barrels used for zooming or focusing are typically made of aluminum.
• Lens Mount: The bayonet mount part of Canon's lenses is almost always made of metal, but there are some lenses like the Canon EF 10-18mm F4.5-5.6 IS STM that uses a plastic mount.
• Zoom and Focusing Rings: These are usually made from the same material as the outer barrel, with a serrated rubber ring pushed over it to increase grip.
• Lens Electronics: Depending on the type of electrical component, common materials used are fiber composites like FR-4 (used for printed circuit boards), copper (used for wires, wire traces, and coil windings in motors) as well as countless other materials used for semiconductors and other electronic parts.
• Seals: Lenses may have O-rings and additional seals to protect the insides from dust and moisture. These seals are most commonly produced from natural rubber.

Uncoated black lens barrels can get hot when used in the sun. This is both uncomfortable for the photographer and can lead to thermal expansion of the barrel material. This thermal expansion may lead to warping (when one side is heated more than the other) or to uniform expansion of the lens barrel (if all sides are heated equally). Either way, thermal expansion is very problematic for the optical performance of a lens as it can change the intended positions of lens elements. This is particularly a concern for longer telephoto and super-telephoto lenses with large surface areas and long barrels.

High temperatures beyond 70° C (160° F) are not only a concern for the barrel itself, but some optical materials can get seriously damaged in the heat: Fluorite lens elements are particularly susceptible to heat and might crack if subject to rapid temperature changes, and therefore should be protected. Last but not least high temperatures may also affect the autofocus drive system.

To protect the lens from these effects, Canon applies an infrared reflective heat shield coating to the outer barrel of longer telephoto and super-telephoto lenses. Infrared is the invisible heat radiation that is part of the natural sunlight. It was 1976 when Canon's first approach to anti-reflective coatings was to paint the lens barrel in a pure white color. A lens covered in white is particularly reflective to all wavelengths including infrared light, and therefore remains cooler than black barrels. While covering the barrel in a pure white was effective from a thermal point of view, it was soon discovered at sports events that the bright white actually caused stray light to nearby photographers, negatively affecting their image quality. Taking this effect into consideration, Canon has since adjusted their color of heat shield coating over the years, giving it a slightly softer tone of light-gray that doesn't cause so much glare. This is the reason why Canon's larger lenses have that very distinctive color applied to their barrel. In addition, Canon has also applied major improvements to their heat shield coatings in terms of scratch resistance and reduced discoloration when exposed to sunlight for long periods of time. In summary, heat shield coatings substantially reduce thermal expansion of the lens barrel and therefore ensure a high level of optical performance by keeping lens elements in correct positions and by protecting fluorite lens elements from degradation.

Every Canon lens comes with two protective dust caps - one for the front lens element and one for the rear lens element. These not only protect the lens elements from contamination like splashes or fingerprints but also from physical damage like scratches and impacts.

• Rear Dust Cap: The rear dust cap is always screwed on a lens when it is detached from the camera body. The rear dust cap is placed on the bayonet mount of the lens and is turned clockwise approximately a quarter of a turn until it is securely attached. What's very nice about rear dust caps is that they fit on all other lenses of the same mount type.
• Front Dust Cap: It is recommended to place a front dust cap on the lens every time the lens is not used for a while. To attach the front dust cap to the lens, the two tabs on the cap are pinched together so that the serrated side brackets contract. When the cap has been placed on the front element of the lens, the tabs are released, and the brackets firmly clings to the lens. The front caps always come in the diameter of the respective lens, so they cannot be used interchangeably throughout different sized lenses.

• Front Filter Diameter: This piece of information – also referred to as the filter size – is usually printed on the front of the lens barrel next to the front lens element. It tells which size the mounting thread of a screw-on filter (such as a polarizing filter or a neutral density filter) must have in order to fit on the front element. For example, when a lens indicates 'Ø 77mm' on its front element, only filters with a thread diameter of 77 millimeters can be directly attached to the lens. It is possible though to use an adapter that accommodates larger diameter filters to a smaller sized lens, but not the other way around.
• Minimum Focusing Distance: The barrel of a lens sometimes has another detail printed on its side – An indication of the minimum focusing distance, also called the closest focusing distance. For example, if a Canon lens has '0.25m/0.8ft' printed on the barrel, that means that the lens can produce focused images even if the distance between an object and the camera's image plane (location of the image sensor) becomes as small as 0.25 meters (or 0.8 feet).
• Lens Mount Index: The lens mount index (there is one on the rear of the lens and a similar index symbol on the camera mount) is an alignment symbol that shows at which position a lens needs to be inserted into the camera's mount when attaching it to the camera body.
• Lens Hood Attachment Index: This is an alignment dot on the front element of a lens barrel that indicates at which position a lens hood needs to be positioned when attaching it to the lens so that it can be turned into a fixed position.

The outer barrel of a camera lens is the main container for all interior components. Its main task is to hold all optical elements in proper alignment axially and radially. Depending on the intended purpose of a lens, an optomechanical engineer has to make a decision about the barrel design. There are two major concepts:

1. Installing all lens elements at completely fixed positions

When a specific lens doesn't require to change its focal length or plane of focus, all lens elements can be permanently fixed in place. This results in a lens with a fixed field of view and a fixed focal distance. With this design, focusing is often achieved by moving the entire lens barrel relative to the focal plane (unit focusing). With all lens elements at a fixed position, a lens can be designed with one main barrel where all lenses are directly placed inside.

A straight barrel design can be chosen if all lens elements have the same diameter. Spacer rings are used to keep all lens elements at the correct distance from each other, and a retainer ring is either threaded or glued into the barrel to keep all components in place. A stepped barrel design is preferred when lenses of different diameters need to be placed in one barrel. The stepped geometry doesn't require spacer rings to keep the lenses at the proper separation, but all lens elements might need to be tightened individually with an own retainer ring.

The sub-cell design is slightly different. Using this barrel construction, lens elements are placed in individual sockets and are secured in place. These lens sockets - also referred to as lens cells - are then inserted into the parent barrel. The sub-cell design of lenses typically increases mounting accuracy because if adjustments have to be made it is easier to remove material from a metal or plastic socket than from a sensitive glass lens element. Therefore, sub-cell mounting is a preferred method for the assembly of highly specialized lenses as used in security cameras, microscopes (objective lenses and eyepieces), laser cutters, medical equipment, and others. Note that all of these barrel designs with fixed lens elements are not used for Canon lenses. Photographic camera lenses require many of their optical components to change their position for focus adjustments, zoom and image stabilization, and therefore a different approach in barrel design must be taken.

Zoom barrels are sub-barrels that are used to change the focal length of a zoom lens. These zoom barrels can have various helical slots cut into their wall from which two or more lens cells are suspended via pegs. The pegs reach through linear slots of another zoom barrel. When one of these zoom barrels is turned (and the other one is stationary), the cells are pushed along the optical axis. The turning zoom barrel usually has a connector handle that joins with the zoom ring on the outer barrel of the lens. This is how turning the zoom ring effectively changes the position of two or more lens cells deep inside the lens. The illustration shows two inner barrels of the Canon EF-S 17-85mm F4-5.6 IS USM lens. The cyan barrel (black in reality) has mostly linear slots (and only few helical slots) while the black barrel has mainly helical slots (and only few linear slots).

A focus barrel is another sub-barrel that is used for adjusting the focal plane by moving the focusing group of lenses. It uses a similar mechanism as the zoom barrel, in this case called helical focusing mechanism. However, the focus barrel is not only connected to the focusing ring on the outer barrel - it must also be driven by the autofocus motor of the lens. Therefore, Canon lenses often use a differential gear to allow both manual input and motor input to drive the focusing group of lenses. The chapter about autofocus systems goes into some detail about this differential mechanism. Modern RF and RF-S lenses do not have a differential gear but the electronic focus-by-wire solution.

Lens element mounting describes the technique used to attach optical elements to their cell. Every mounting method must meet certain requirements to maintain the integrity of the optical system:

• It should offer an adequate resistance to shock and vibrations so that lens elements do not easily come loose from their sockets.
• It should reduce stress at the contact surface between the lens cells and the edges of each lens element that might result from thermal expansion of the different materials.
• If used, adhesives should not attack the optical material of lens elements, and should not be water soluble. In addition, to avoid stress on lens elements, adhesives should not shrink significantly when curing.
• Finally, a mounting method must ensure ultra-precise positioning of lens elements. This involves placing lens elements at their exact axial position but also centering lens elements so that they have the optical axis of the system pass exactly through the center of the lens element.

In the design and construction of a camera lens, stray light is always a concern. Light can enter the barrel from unintended angles and will degrade image quality when reaching the camera's image sensor. It is a best practice to design all inner surfaces of a lens with light-absorbing surfaces or light-blocking structures.

It is very typical that lens cells are made from black polycarbonate so that they absorb light instead of reflecting it. The same applies to inner sub-barrels that are either made of black polycarbonate or from aluminum that is painted or coated in black. Another practice almost always used is to cover mechanical structures and electronics with black interior linings so that no metal parts could ever be exposed to stray light as these would reflect light particularly well. It is also possible that interior surfaces are coated with special powders that give inner parts a velvet-like surface for improved light absorption. In addition to black colors and coatings, it is very commonly seen in camera lenses that interior surfaces are not just flat but rather have grooved structures or other shapes. Some camera lenses even have baffles installed at various positions between lens cells so that stray light can be caught in different places. All these installations are precautions so that stray light is either absorbed or blocked to prevent it from reducing the image quality.

The L that some Canon lenses have in their name stands for luxury. The series of L-lenses is a professional line of camera lenses, and it meets higher quality standards than other lenses not marked with that letter. They are known for superior build quality and optical performance. L-lenses are easily recognized by their red ring around the front of the lens barrel. In addition, these lenses have a red 'L' written in their name, directly next to the F-number, as for example in the Canon RF 24-70mm F2.8 L IS USM. But what is it exactly that users can expect from Canon's L-series of camera lenses? Here is an overview:

• Superior Imaging Performance: Canon applies a number of optimizations to their L-lenses which ultimately leads to higher image quality and less aberrations. One factor is that Canon sets extremely high requirements to the initial computer-aided design of an L-lens, putting a lot of effort into the correction of various aberrations not just for certain focal lengths but throughout the entire zoom range. Therefore, an L-lens typically shows less image defects and achieves higher results for contrast and resolution. Another factor is that Canon uses higher grade optical glasses for their L-lenses and adheres to smaller tolerances during the production of glass elements. This makes the individual glass elements themselves less prone to errors, and lets the complete lens behave more like in the computer design.
• Rugged Barrel Materials: One of the most important aspects of Canon's L-lenses is their increased robustness and durability compared to other series of lenses. Designed for the most demanding tasks, the outer barrels of L-lenses are typically made of metal to better withstand possible impacts. In addition, as these lenses are designed for professional photographers who use their lenses in any weather, Canon's L-lenses offer some additional protection against the elements. Weather sealing is installed at the lens mount, focusing and zoom rings (and the control ring on RF lenses), as well as the buttons and switches integrated into the lens barrel. It should be noted, however, that weather sealing doesn't make a camera lens watertight, but it increases protection against splash water and dust.
• Fixed Aperture Lenses: Canon designs almost all of their L-lenses with a fixed / constant maximum aperture throughout the entire zoom range. This is why there is almost always just one F-number on L-lenses. There are only very rare exceptions to this rule, like the Canon RF 100-500mm F4.5-7.1 L IS USM.
• Large Aperture Lenses (Fast Lenses): Another aperture-related advantage of L-lenses is that these generally offer larger maximum apertures than other series of lenses, so there are more Canon L-lenses with maximum apertures of F2.8, F2.0. or F1.4 compared to standard lenses. If someone should look for an even larger maximum aperture of F1.2 there is only one choice but to get an L-lens because no other series of Canon lenses offers such a fast lens.
• Aperture Blades: There is also a difference in the geometry of aperture blades. The diaphragm (which is the mechanical component forming the aperture) of an L-lens typically has more blades than similar components in standard lenses. While this doesn't seem like much of a difference, having more aperture blades allows the unit to form a more circular opening, creating a softer out-of-focus blur (bokeh).
• Focus: There aren't many resources about focus-related differences between L-series and standard lenses. Some sources claim that L-lenses are equipped with faster focusing abilities.

In summary, L-lenses offer very significant and noticeable advantages, especially in the areas of imaging performance, build quality, and weather sealing. Unfortunately, this is also reflected in the price. L-lenses are rarely below USD 1,000 and the larger super-telephoto versions of L-lenses can cost up to USD 20,000 (Canon RF 1200mm F8 L IS USM, price checked on 04/29/2024).

It is worth mentioning that Canon has some standard (non-L) lenses that offer an equal level of optical performance than their luxury role models. For example, the Canon EF-S 17-55mm F2.8 is considered an insider tip among photographers due to its stunning image quality. For that reason, professionals not always rely on specifications but tend to check the actual performance of a lens. If aspects like barrel materials and weather sealing are not so important, standard lenses are sometimes an equally good choice.