Title: The Binocular Handbook: Function, Performance and Evaluation of Binoculars.

Foreword by Neil English, author of Choosing & Using Binoculars

Publisher: Springer Nature

Author: Holger Merlitz

ISBN: 978-3031444074

218 Pages

Price UK: £39.99(Hardback)

It was with a great sense of anticipation that I finally got my hard-back copy of Holger Merlitz’s new book, freshly and expertly translated into the English language. Anyone who has expressed even a cursory interest in binoculars will be familiar with Holger’s accumulated writings on his website(http://holgermerlitz.de), where he has built up a formidable portfolio of work covering all aspects of binocular optics, reviews and theoretical speculations. His new book, The Binocular Handbook: Function: Performance and Evaluation of Binoculars, represents the culmination of many years of work.

Holger presents a full treatise on binocular optics in this book, covering everything from the design and execution of theoretical optics and their applications to building binoculars, a detailed overview of how the eye-brain interfaces with the binocular before launching into some fascinating chapters covering the testing and evaluation of binoculars during field use. In total, 9 chapters arranged in 3 distinct parts, walk the reader through pretty much everything you need to know about the fascinating world of binocular optics.

Instead of exhaustively covering the material, I would like to highlight just a few interesting topics covered in the book. I was most impressed with Holger’s use of an aeroplane’s ride from the North Pole round the world as a way of explaining why phase coatings are needed in roof prism binoculars. I also enjoyed his inclusion of discussions on unusual, cemented prism formats, such as the Uppendahl and the Perger (page 51-54) arrangements, the latter of which doesn’t require a phase correction coating and is used in the design of the third-generation Leica Geovid. Holger believes there are no binoculars containing Uppendahl prisms, but I have heard it on good authority that the little Leica Ultravid 8x and 10 x 25 may still be using such prisms. 

I found section 4.8 to be particularly amusing when Merlitz discusses sealing and gas purging. The use of the noble gas, argon, in particular, has been touted as being superior to molecular nitrogen owing to its greater molar mass. But as Holger astutely points out on page 97, the very low ambient concentration of argon in air(less than 1 per cent) would create a powerful concentration gradient causing the argon to diffuse out faster than if it were filled with nitrogen under the same circumstances. But it’s worse than that: a binocular filled with argon will cause nitrogen to diffuse inward and increase the pressure enough to eventually damage the seals! It neatly explains why all the big European binocular manufacturers have stuck with nitrogen purging.

Chapter 5 is written by Gerhard Eller, a fellow binocular enthusiast and engineering veteran, who describes the construction of a fascinating 12 x 62mm binocular using twin Leica Apo objectives and Porro II prisms serving up an impressive field of view of 113m@1000m.

The discussion on depth perception in chapter 7 and 8 proved to be fascinating entries, especially since I’ve cultivated a particularly strong fondness for compact Porro prism binoculars over their roof prism counterparts. Merlitz discusses the Japanese made RISO-1 7 x 40 instrument which were employed by the US navy during the Korean War. Indeed, he further informs us that specialised stereoscopic binoculars used in precision range finding measurements had enormous separations in their objective lenses of the order of several metres! That said, while the enhanced stereoscopic effects of Porro prism binoculars are greatly appreciated by yours truly, Holger also discusses some disadvantages of this design, such as inferior close focus performance and an optical illusion called the ‘cardboard effect,’ which was previously unknown to me.

The human eye takes centre stage in the final few chapters. With its 3-megapixel colour camera(cones) and 120 megapixel light detectors(rods), it can respond to changing ambient light levels and even alter the spectral response of the human eye. I’ve always wondered why, for example, many older glasses I’ve viewed through have a yellow tint. In discussing the differences between regular BaK4 and BK7 glass versus their HT equivalents, for example, companies like Zeiss have been able to increase the transmittance at blue wavelengths which helps in low light observations when the human eye becomes more responsive to shorter wavelengths (so called scotopic vision).

Sections 8.2 through 8.4 discuss the interesting topic of binocular efficiency and dim target detection as well as the factors – magnification, aperture, exit pupil and eye pupil size – that determine the outcomes in broad daylight, twilight and under darkness. Magnification alone determines efficiency in daylight, but the situation becomes far more complicated during twilight and full darkness, enabling binocular enthusiasts to experimentally determine the relative importance of these factors in target resolution and detection. Indeed, I’m currently busy comparing and contrasting my two main binoculars – the 8 x 30 and 10 x 35 Nikon EII – with their similar light transmission and exit pupil size – under twilight and true darkness based on Holger’s analysis, to determine if these predictions are validated.

Section 8.7 offers an excellent overview of atmospheric scattering and I really like how the author brings some basic physics such as Rayleigh scattering into the mix. Later in chapter 9, he discusses colour bias in binocular images, explaining why many binoculars having a so-called warm tone exhibit better contrast by (Rayleigh)scattering shorter wavelengths of visible light. On the contrary, he also explains why instruments delivering a cool colour tone are often better for low light work, when the eye becomes increasingly sensitive to shorter wavebands.

These are but a few invaluable nuggets of information presented in this book. The reader will note that much of this surmising is not just based in optical theory but derives from the rich storehouse of practical experience with many fine binoculars he has amassed over the decades. In short, The Binocular Handbook will prove invaluable to keen binocular enthusiasts eager to determine the best instruments to use in their arsenal, with the author gently encouraging active experimentation under real life conditions. Like most good books, it raises more interesting questions than it answers, but rest assured, there is enough content in this timely volume that will keep you thinking and looking for years to come.

Highly recommended!

De Fideli.