"The M IV/1T Carl Zeiss 7x50 binoculars, without case, weighed two pounds, eight ounces, as compared with the other best 7x50's in the Atlantic at the time-the British Barr and Stroud Pattern 1900A, which weighed a full pound more. The Zeiss was seven inches long, the Barr and Stroud nine. The Zeiss internal glass surfaces were bloomed to reduce reflection losses, not so the Barr and Stroud. The diameter of the objective for both was 50. The Zeiss glass was striking for its high light transmission, 80 percent compared to 66 percent for the un-bloomed Pattern 1900A. The Zeiss provided excellent eye freedom in that it was not necessary for the observer's eye to hold rigidly to one position in order to maintain the best optical performance. Movement of the observer's eye across the axis had little or no effect on the quality of definition in the various regions of the field of view, whereas in the Pattern 1900A any slight movement of the observer's eyes across the axis resulted in serious optical deterioration. The distance between the eye point and the eye lens, which determined the ease and comfort with which the instrument could be used, was preferable in the Zeiss, providing better eye relief. in comparisons of watertightness, the Zeiss again was clearly superior: a Zeiss immersed in water for ten minutes showed no bubbling, whereas a Stroud and Barr bubbled within two minutes. A pair of loose didymium glass filters with a strong absorption band in the yellow of the spectrum could be fitted to the Zeiss eyepieces enabling nearby boats to communicate with each other at night by yellow lights."
OPERATION DRUMBEAT by Michael Gannon Harper & Row 1990