Canon EF Lens Technology

Ultra-Low Dispersion (UD) Glass Elements

The refraction of light differs depending on the wavelength. The point of focus therefore differs depending on the different wavelengths or colors. When the different wavelengths are focused at different points, the colors look smeared. This is called chromatic aberration. The longer the focal length, the more pronounced chromatic aberration becomes. Usually, an achromatic element is used in a lens to correct chromatic aberration. However, normal optical glass can only be corrected for two primary spectral colors.

Avoiding color fringing, or chromatic aberration has been one of the great challenges in the design of telephoto lenses. L-series telephoto lenses like the EF 70-200mm f/2.8L IS USM and EF 300mm f/4.0L IS USM utilize Canon's Ultra-low Dispersion glass to minimize this effect. The design of UD glass provides outstanding contrast and sharpness and minimizes the splitting of colors as they pass through the elements, resulting in cleaner color throughout the image.

EF 70-200mm f/2.8L IS USM

Circular Aperture

Certain Canon lenses feature a new Circular Aperture diaphragm unit, which uses curved aperture blades to provide for a more rounded opening as the lens is stopped down. It's especially effective at rendering out-of-focus background highlights as natural rounded shapes. In lenses such as the EF 70-200mm f/2.8L IS lens, the lens opening is virtually circular from f/2.8 to f/5.6. These lenses retain all the benefits previously available with Canon's Electromagnetic Diaphragm -- smooth and consistent stop-down operation (even at up to 10 fps with the EOS-1v), near-silent aperture control, and total absence of mechanical levers or switches in the lens mount.

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