Next, taking an ultra-compact 35-105 mm zoom lens as an example, we will describe the actual design process flow while looking at a few computer screens and their output. This zoom lens has a convex-concave-convex-concave 4-group construction, with the movement of the 1st, 2nd and 3rd groups linked to the zooming action and the 1st group used for focusing. The optimum optical design for this lens type is determined by analysis software. At this stage it is possible to estimate various specifications such as the track of the zoom cam, the focus extension amount, the total length of the lens, the diameter of the front lens element and the back focus distance. The shape of each lens was selected from the optimum solution determined from the specified conditions. At this stage, a simulation of light passing through the lens is performed and the minimum number of elements required for each group is estimated from the way the light rays bend and from the various aberration algorithms. From this it can be seen that the convergence of the light rays has improved greatly. Next, with this lens it is necessary to eliminate the aberration fluctuations caused by the focusing movement of the 1st group. To do this, one element is added to the 1st group and an element is also added to the 2nd group to balance out the residual aberration amount of the 1st group. To reduce the residual aberration in the 3rd and 4th groups, an element is added to each of these groups as well. Once the final lens construction is determined, all desired specifications such as shooting distance, aperture and focal length are added into the equation and the automated design cycle is repeated many times while slightly varying design factors such as glass material and optical design, eventually leading to the final "ideal" design values. Lens Barrel Design Now that design of the optical system is completed, the process moves to the manufacturing of lens prototypes, with each lens element processed and polished with hundredth of a micronorder precision. At the same time, designers start designing the lens barrel which must hold the lens elements in precise position according to the optical design values and must move the various lens groups with high precision during zooming and focusing. Several basic conditions are required of a lens barrel, as follows: 1   The lens barrel must, in every conceivable situation, hold the lens elements in precise position according to the optical design values in order to maintain optimum optical performance at all times. 2   Mechanisms must be positioned for superior operability. 3   The construction must achieve both superior autofocuisng and manual focusing operation. 4   The size and weight should be appropriate for superior portability. 5   The construction should be designed to ensure maximum mass production stability. 6   The inner walls of the lens barrel should prevent harmful reflections. 7   The barrel should be provided with sufficient mechanical strength, durability and weather ability. The new technology factors listed below were not required with manual focusing FD lenses but must be taken into consideration when designing the lens barrels of autofocusing EF lenses. An electronic mount and various electrical circuitry must be built into the lens. Incorporation of new actuators such as USM (Ultrasonic Motor), AFD (Arc Form Drive) and EMD (Electromagnetic Diaphragm) Increase in demand for compact zoom lenses Great increase in use of rear-focusing lens design Increased use of compact designs and multi-group zoom and rear-focus systems has introduced many difficult factors such as increased sensitivity to errors and the necessity for complex processing of non-linear precision cam grooves. Even with the increased complexity, however, optimum designs are obtained using CAD (computer-aided-design) and various computer simulation techniques for design analysis. Also, in response to user demand for increasingly compact and lightweight lenses, engineering plastic materials are liberally used in popular-class zoom lenses. Thorough prototype performance checks and reliability evaluations After the optical lens elements precision-processed according to the design specifications and the prototype lens barrel are brought together and assembled to form a prototype photographic lens, the lens is rigorously tested by the quality analysis department to see if its performance satisfies the initial design goals. Many different tests are carried out, including comparison with existing products of the same class; precision measurement of specifications such as focal length, aperture ratio, aberration correction level, aperture efficiency, resolving power, MTF performance and colour balance; field tests using colour and black and white film under various shooting conditions; ghost/flare spot tests; operability tests; temperature and humidity weather resistance tests; vibration resistance tests; operation durability tests and shock tests. If any result from these tests fail to satisfy Canon's standards even by a slight margin, that information is fed back to the design group and the lens is redesigned. At present, even lenses in the highly-reputed EF lens group go through two or three "design -- prototype production -- performance evaluation" cycles, and mass production does not begin until all initial goals are achieved to Canon's full satisfaction.