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GUNS Magazine June 2012 Digital Edition - Page 28
STORY: Jacob Gottfredson Understanding this tool is important. soMe basiC riFLesCoPe theory O ne source of irritation for this author is the lack of technical data published for consumers about binoculars, riflescopes and spotting scopes. Whereas the camera bugs are given such information, sporting optics manufacturers rarely do it so their consumers can make informed purchasing decisions. Some of the data essential to scope design simply not published for public consumption include the following: Modulation Transfer Function (MTF) determines the contrast ratio, or reproduction sharpness of a lens, called spatial frequency and is measured as MTF. MTF is an important consideration in lens design, defining the ability of the lens to provide contrast and resolution. Camera makers provide MTF curves that allow the consumer to know what the ability of the lens is, where 1 is considered the highest quality. A lens system is considered to be of very high quality when the contrast curve is above .9, and where the resolution curve is above .6. However, many inexpensive lenses will maintain these MTF numbers at aperture settings at or above F-16. That is, if you want excellent resolution, contrast and depth of field from your $400 lenses, have enough light available to operate at high aperture settings. Many modern riflescopes have stops built into their mechanisms that prevent the aperture from opening all the way for the same reason that superior MTF curves can be kept high by keeping the F-stop high. On a variable scope, as you decrease the power, the exit pupil increases. For example, on a 3-10x50mm scope at 3X, the exit pupil is theoretically 50÷3 = 16.6mm and at 10X it is 5mm. 16.6mm is more than the eye can absorb, and the resolution and contrast degrade at such a high opening. Thus, some manufacturers introduce a mechanical stop that will not allow the exit pupil to drop below some predetermined size. Although MTF is one of the most important determiners of image quality, it is not generally used for the optics under consideration here because it is difficult to characterize by the human eye. DIN stanLeupold’s Mark 4 scopes are superb instruments. This one has been dards are used with on this .50 BMG for years and on other hard kicking rifles as well. It film. No such stankeeps on going. Like the Nightforce, the optics are excellent. dards are used for the eye. However, some manufacturers are working to perfect MTF curves for sporting optics. It will be a boon to all of us when they do. Color correction is an attribute in modern scope design often taken for granted. It is important to note that in riflescope design, image quality is Jacob has been using Nightforce NXS scopes in competition on heavy kicking rifles for many years. In tactical competition, they are dialed constantly for different ranges. Jacob has yet to have any failures or misses as a result of these scopes. The internals are hard to beat. optimized at the center of the field with your head in the shooting position. Although movement around the exit pupil may reveal some uncorrected coloration, the shooter should find the image to be well color balanced, crisp and bright with the eye in the shooting position. Color correction is an ongoing challenge in the process of designing a balanced optical system. A lens made of a single glass type will separate colors, just as a prism does, into the colors of a rainbow. Because of this, different colors will focus at different depths. Those colors not well focused at the image will be absent from the shooter’s view, allowing other colors to take over and tint the image. Combining glasses of different types the designer can, to some degree, cancel out this effect in the primary spectrum, bringing the blue- and red-light focal points together; however, the secondary spectrum will generally have some residual, uncorrected effects. secondary spectrum You’ll often see this secondary spectrum, where some residual defocus between blue light and green light exists. A great deal of effort is expended during an optical design to minimize this phenomenon along with minimizing the primary spectrum. The amount of secondary spectrum is often determined by the glass types and radii. The glass types and radii are determined by some basic requirements like length, zoom ratios, field of view, 28 W W W. G U N S M A G A Z I N E . C O M • J U N E 2 0 1 2