The longer a scope’s ER, the smaller its field. Witness an LER handgun scope or an IER scope on a Scout rifle. Most scopes designed for receiver mounting on rifles have 3 to 4" of ER. Dialing up a variable from low to high magnification, you might well see ER shrink.

Arguably, scopes for the masses date to 1930 and Bill Weaver’s 10-oz., 2¾X Model 330. It had a 3/4" steel tube 10½" long, a 35-foot FOV and an ER of 3½". Next came the similar 4X 440, its 27-ft. FOV also limited by an eyepiece little bigger than the tube. Soon 7/8" tubes (as in Lyman’s Alaskan) appeared. After WWII, alloy replaced steel in 1" tubes and eyepieces blossomed like umbrellas behind them. FOVs grew.

At its 1953 debut, a 4X Leupold Mountaineer boasted a 35-ft. field. Six years after Leupold’s Vari-X II in 1965, Redfield had a Widefield scope, its generous rear lens blocked top and bottom to yield a TV-screen field. Low Profile Widefields followed in 1975. Their tubes were flattened too. The merits of squashing a perfectly good circle was lost on many shooters.