If you’re looking at the Canon manufacturer lens pages, trying to decide on a lens, you may have noticed that at the bottom of the Overview tab for every lens are several icons that provide some information about the lens’s construction or feature set. I have yet to find a comprehensive guide anywhere on the internet that explains what each icon means; nor has Canon elected to make them informative in any way shape or form other than to just be icons.
So here is as best as I can decipher them, what the icons mean.
Auto Focus Stop Button
Some super-telephoto lenses have an extra control that by default will stop the camera’s auto-focus system while pressed so the lens can be manually focused without having to disable auto-focus completely. Many modern bodies also support reprogramming this button to other functions such as stoping or starting the AF system, activing the Auto-exposure lock, changing the AF point being used, changing the auto-focus mode, or starting the lens’s stabilization system.
Aspherical Lens Elements
Traditional camera lenses were made strictly from spherically ground glass elements, that means that the profile the lens was ground and polished to conformed to a section of a very large sphere. Spherical only designs pose problems in that they will not converge parallel rays exactly to the same point and thus induce distortion. This impacts the lens design by limiting aperture sizes, the ability to compensate for distortion in super-wide angle lenses and force lens designs to be larger. Aspherical lenses provide a means to help counter those problems resulting in smaller lenses with larger possible maximum apertures and less distortion.
The number under the letters AL indicates the number of aspherical lens elements used in the lens’s design.
Bokeh, the quality of the out of focus areas in an image is effected most dramaticly by the shape of the aperture. Early lens designs attempted to counter this by increasing the number of aperture blades to smooth out the shape of out of focus highlights produced by the lens. But obviously an infinate number of aperture blades can not be mounted in a lens. The solution is to cuvre the aperture blades, to form a circular or nearly circular aperture over the majority of the lens’s aperture range.
Flourite Glass Element
Traditional lens designs made use of only a very clear blend of glass generally referred to as Optical glass. The problem this raises is that due to the way light bends (refracts) though glass, different color light-rays will bend a different amount, resulting in a lens that produces chromatic aberations or color fringing, often visiable in high contrast areas as color fringes that sround the contrasty edge like a halo. Changing the material used though, also changes the way that light bends when it passes though the lens element. By using the differing characteristics of Florite in combination with traditional optical glass, designs can be produce that reduce the ammount of chromatic aberration.
The number under the letters CaF2 indicates the number of Flourite elements used in the construction of the lens.
Traditional optical designs rely on refraction or the way light bends when it passes though a medium with a different index of refraction (like optical glass).
Dust and Weather Resistant
Dust and weather resistant lenses are designed and gasketed so that when used with an appropriately sealed camera, the whole combination can be exposed to harsher conditions (like blowing sand or spray from surf) with a reduced chance of damage being caused.
In a lens with a floating element, or a floating design, the position of multiple groups is shifted when the lens is focused. This allows the lens to correct for many optical aberrations though out the complete focusing range and not just at one or two optimal shooting distances.
Some lenses, like ultra-wide angle lenses, where even the thinnest screw on filters would cause vignetting, and large super-telephoto lenses, where the front lens diameter is far too big for a screw-on filter, provide alternative ways to attach color correcting and special effect filters. In some instances, this is done by providing a mechanism to secure a gelatin filter (a flexible plastic like material) at the rear of the lens. In the case of super telephoto lenses, this is done by providing an removable holder that will accept standard sized screw-on filters near the rear of the lens.
Full Time Manual Focus
Most auto-focus lens designs couple the auto-focus motor, weather in the body or in the lens (as all Canon lenses are designed) with the focusing ring and the mechanism that moves the lens via a gearbox and drive train. In this type of system, turning the focus ring with out disenguaging the auto-focus gearbox can damage the precision mechanism. In some designs though, the gear box or the focusing motor its self (in the case of a ring type Ultrasonic Motor) can be decoupled from the focus ring, so that the photographer can override the focus position at any time by turning simply turning the focus ring, as opposed to having to disengauge the AF motor via the AF/MF switch.
Inner or Rear Focusing
Traditional lens designs focused by moving all of the lens groups or by moving the front group. Neither method is ideal as that both require moving more mass than necessary; the front elements are the largest and heaviest in the lens, and moving all of the elements is even less ideal. Internal and rear focusing designs strive to minimise the mass of the lens elements that are used for focusing thus allowing incrased focusing speeds. This is done by moving smaller, and therefor lighter elements closer to the rear of the lens. Inner and rear focusing designs are differentiated by where the moving lens groups are. Inner designs move a smaller, lighter lens group towards the front of the lens, while rear focusing designs move either the rear lens group or one adjcent to it.
Camera shake is a constant factor in how sharp an image can be made, to combat this traditionally a mono-pod of tripod would be used when shutter speeds above a certain range (the hand holding rule of thumb of 1/focal lenght of the lens, for 35mm cameras) weren’t possible. Image stabilization, adds a new tool to the sharp image tool box, a virtual monopod of sorts. Lenses with image stabilizers use gyroscopes and high speed actuators to shift the position of one of the lens groups, effectively compensting for small camera movements while the stabilizer is active. Modern stabilization systems claim the ability to offer a 4-5 stop reduction in camera shake, or the ability to hold the camera with shutter speeds upto 8 to 10 times slower than the hand-holding rule of thumb would sugesest is necessary.
Super Ultra-low Dispersion Glass
Similar to Ultra-low dispersion glass and Flourite, Super Ultra-low Dispersion glass refines the formula for low-dispersion glass further yealding even better performance.
The number below the letters S-UD indicate the number of Super Ultra-low Dispersion elements used in the lens’s design.
Unlike traditional electric motors which use a magnetic field to convert electrical energy into rotational energy ultra sonic motors use a different approach. Ultrasonic motors work by using high frequency vibrations to create the rotational energy. These vibrations are so high in frequency that they can’t be heard, rendering a perfectly silent auto focus motor.
Canon lenses can have one of two types of USM motors, ring or micro. In a ring-USM lens, the focus motor is wrapped around the circumference of the lens and drives the focusing system directly. In this way it can also support full time manual focus.
The second type, micro-USM, is a USM motor that’s packaged in a format similar to a traditional micro motor. Lenses with these motors don’t generally support full time manual focusing (the EF 50mm f/1.4 USM) being a noteable exception. Micro-USM really only offers the benefits of a quieter focus motor in a more traditional designed lens.
Ultra-low Dispersion Glass
While Flourite elements help yield designs that perform better than optical glass only designs, the artificial nature of Flourite crystals causes such elements to be expensive to produce. In order to reduce lens costs while still maintaining the same level of quality Canon developed Ultra-low dispersion glass, an optical formula with characteristics similar to flourite but that is easier and more inexpensive to produce. Further, when combined with Flourite elements the lenght of the lens can be further shortened.
The number below the letters UD indicates the number of Ultra-low Dispersion elements used inthe lens’s design.