Our Answers to ten common questions will ease your mind.
I know that Pop Photo uses the SQF system to test lens sharpness. I even know that SQF stands for Subjective Quality Factor. But how does it work?
SQF was developed by Edward Granger, a professor at the Rochester Institute of Technology and senior scientist at the Eastman Kodak Company during the 1970s, as a better way of measuring lens sharpness than the prevailing system at the time, known as Modulation Transfer Function (MTF). MTF measures a lens' ability to reproduce discrete bars (or lines) in increasingly smaller arrays.
Sharp lenses can reproduce more "line pairs per millimeter" (lpm) than can unsharp lenses. MTF numbers, however, are meaningful only to optical engineers, so Granger set about designing tests that could have real-world meaning to most photographers.
The actual SQF tests are performed on an optical bench: The tested lens is mounted on a movable, computer-controlled stage and focused at infinity. Light from a laser is used to illuminate a metal plate into which very fine crosshairs have been etched-2.5 microns wide for a normal lens, or much smaller than the diameter of a human hair. (Larger crosshairs are used for short focal length lenses; smaller crosshairs, for long lenses.)
The crosshairs are optically altered by a device called a collimator to appear to be at infinity. The collimated light passes through the crosshairs, through the center of the test lens, ultimately striking a high-resolution CCD array where the "light spread" (blur) of the image is measured.
The greater the light spread, the less sharp the lens. We measure the spread from the full range of lens apertures.
After testing the center of the lens, we continue with off-center and edge measurements. The results are compiled, compared, and weighted: Image center readings comprise 50 percent of a lens' SQF score; off-center readings, 30 percent; and image edge readings, 20 percent. (The percentages are slightly different for digital lenses.)
By running the readings through a series of computerized calculations, SQF can quantify lens sharpness and interpolate the rate at which sharpness will deteriorate as image size increases.
What is ultra-low dispersion glass?
A clear, optical-quality glass that goes by many names, it's especially effective at controlling color fringing in tele lenses. Lenses that use it are usually designated "apochromatic." Color fringing, also known as chromatic aberration, is what happens when colors don't focus to the same plane.
In very large blowups, if you see a small halo of color-red, say-along an object in a photo, your lens isn't focusing red to the same plane as the other colors that make up white light. Canon and Tamron's Low dispersion (LD), Nikon's Extra-Low dispersion (ED), Sigma's Extraordinarily Low dispersion (ELD), and Tokina's Super-Low dispersion (SD) glass promise to control the problem.