What makes a good lens?
Canon L lens technology, explained simply.
The Canon L series of lenses is marked by a bright red line on the lens barrel, and it’s the lens of choice for professionals using Canon cameras. A lot goes into the making of an L lens, so that the nal image is sharp, clear, and free from optical distortions. Here are key L technologies, simply explained.
1 ASPHERICAL LENSES MINIMIZE ABERRATIONS
Spherical lenses are cheaper to produce, but because light passing through a spherical lens converges at different focal points, it results in a softer, low contrast image, like photographing through a sheer veil. This is an optical phenomena known as spherical aberration.
An aspherical lens, which has a non-spherical surface, is designed to converge light rays onto a single focal point, allowing the camera to capture high contrast images with uniform sharpness. Aspherical lenses can also reduce other optical aberrations like astigmatism and distortion.
Aspherical lenses, however, are difcult to make. Some of Canon’s aspherical lens designs require precision tolerances that are in the nanometer range, or 1/1,000,000 of a millimeter. Deviations of more than that are rejected.
2 GLASS ISN’T ALWAYS BEST
Chromatic aberration, also known as color fringing or purple fringing, is when images look blurry, or when colors appear around the edges of objects, especially in highcontrast and backlit photographs.
Chromatic aberration occurs because the wavelengths of light bend at different points within a lens, and the
lens is unable to focus all wavelengths onto the same focal plane.
To counter chromatic aberration, Canon developed lenses made from synthetic crystals, composed mainly of calcium uoride. Fluorite lenses have better light dispersion properties than regular glass, which helps to reduce chromatic aberration, and also produces clearer image delineation than conventional glass. 3 ULTRASONIC MOTORS FOR ULTRASONIC AF Almost all L lenses use Canon’s Ultrasonic Motor (USM), the company’s unique drive system. The USM converts ultrasonic vibration to rotational energy with one-thousandth of a millimeter precision, and draws minimal power from the camera while remaining quiet.
The USM has high torque, with virtually instantaneous starts and stops. All of its qualities make it ideal as a highspeed, high-precision motor for the AF system. 4 OIS HOLDS THE SHOT STEADY Optical image stabilization (OIS) is critical for getting sharp and steady images, especially at long focal lengths where small nudges translate to big movements.
Canon’s OIS technology detects camera shake with a vibration gyro sensor and shifts parts of the lens to counteract the image blur.
While Canon’s IS technology is lens-based, there are also cameras that come with in-camera body OIS. There are pros and cons to either approach, but generally speaking, lens-based OIS is more ne-tuned, as it’s tailored specically for the lens it’s housed in. Lens-based OIS can also be more effective in compensating for the more violent shakes from long telephoto lenses. 5 COATINGS REDUCE GHOSTING AND FLARE Flare or ghosting optical aberrations can occur when light reects off the lens or sensor. Canon’s L lenses contain a combination of unique coatings to combat these aberrations.
This includes the newly developed Subwavelength Structure Coating (SWC), which minimizes are and ghosting by using wedgeshaped nano structures.
Air Sphere Coating (ASC) applies a lm containing microspheres of air over the vapordeposition lens coating, resulting in exceptional anti-reectivity. Super Spectra Coating, which allows up to 99.9% of light to emerge from each lens element, delivers optimum color balance, so that colors are reproduced accurately.