Small take-along helpers: Loupes (I)

A few years ago, in  issue #24   we had presented some tardigrade image results produced by means of common, vintage, low-budget lab microscopes. As a rule all those instruments will be rather bulky and heavy, difficult to take along. As a consequence they will be used at home only, in a quiet, low-light environment.

But there are rumours about some strange people who do not simply go for a walk outside but instead like additional optical and microscopical equipment to take along.

We should be well aware of the psychological fact that stylish loupes and elegant travel microscopes will not merely serve as sober tools for clear sight but as well will enhance the personality of the owner, similar to those many impressive, technical gadgets known from James Bond films.
A loupe or a pocket microscope is essential for the naturalist, in a similar way as the stethoscope of the medical doctor, the rifle of the hunter or the crook of the bishop.

Besides, technical gadgets appear to be essential characteristics and amplifiers of our Western culture and society (including some dreams and some lies). Just think about those innumerable tiny MP3 players, miniature mobile phones, small notebooks etc.

In particular Sherlock Holmes' standard equipment, the ordinary magnifier and his "small but still powerful" universal crime solving microscope come very close to the instruments which we will discuss here in this and the upcoming issues.

Antoni van Leeuwenhoek (1632 - 1723), the ingenious and famous Dutch microscopy amateur, was far ahead of his time with respect to microscope miniaturization. Up the the present day it remains unclear whether he actually made all his discoveries with his tiny "travel-type" microscope or whether he might have owned more sophisticated, bigger microscopes for stationary indoor use as well.
Some British gentlemen owned funny walking sticks which inbuilt microscopes - but they are tremendously expensive and collectible today, so we don't even dare to depict them here.

Now, which equipment might be useful for mobile tardigrade investigations? No doubt that it should be small. How about a classical botanical loupe?

Let's have a short look at the optical principles that govern loupe usage: as a rule the human eye cannot focus to a closer distance than 25 cm. The loupe helps to diminish this distance thus allowing us to reach a closer look at things - in the proper sense of the word. The magnification factor of a loupe can be calculated by means of dividing the natural eye close-up focus distance (25 cm) through the loupe focal length distance (in cm as well) as illustrated in the figure below.

Magnification by means of reduced distance: loupes are devices which permit the human eye to approach objects even to a distance of less than 25 cm - with the decisive advantage of keeping the crisp image (of course we could approach the naked eye closely to the object as well, but this will result, as we all know, in a blurred image).
Though some - mostly young or short-sighted - human individuals are able to perceive crisp images within a closer distance
the minimum focus distance of the human eye has been defined arbitrarily as 25 cm. All loupe magnification readings refer to the ratio of those 25 cm to the focus distance of the loupe lense, as shown in the diagram. As a consequence short focus lengths will provide the highest magnifications.

Typical inscriptions on the loupes primarily indicate the respective magnification (e.g. 28x, 20x, 15 x, 10x, 8x, 6x), in cases of variable lens combinations the different possibilities (like "3x 6x 9x" on a loupe with a 3x and 6x lens, where the single magnifications of the invididual lenses sum up to the maximum possible magnification).
In some cases the usable lens diameter [in mm] is listed after the magnification specification.
Furthermore there might be the name of the manufacturer, its logo, the optical correction grade ("achromatic", "anastigmatic" ...), the construction type of the optical system ("Triplet" or "Triplett" i.e. three lenses permanently fixed to each other, "Doublet" or "Dublett", "Steinheil", "Coddington" etc.) or the number of lenses ("Four elements", "Five elements"). In addition you might come across the name of the producer country, city etc, and other informations like "gold-plated", silver grade indications like "900" ...
In a few cases you will find a focus length indication instead of a magnification reading (like "f = 3,5 cm"), this sounds more scientific.

Many internet trade loupe inscriptions can be considered as slightly fraudulent. Typical examples to be mentioned are most "30 x 21" inscriptions which mimic a 30fold magnification in combination with a 21 mm usable lens diameter. Due to optical restrictions high loupe magnifications require low lens diameters. A 21 mm lens diameter is by far too much for a 30fold magnification. When checking those "30x21" loupes more closely, i.e. comparing them to classical 10x loupes you will normally find that the magnification number is highly exaggerated.

The variations among commercially available loupes are immense, in particular when keeping in mind those many international products fabricated since the 18th, 19th and 20th century. We are going to present a few images in order to illustrate this cultural diversity:

Loupes in plastic mountings with intense colours. Left: a turquoise loupe from Taiwan with three identical plastic lenses, each with 5fold magnification (thus reaching a 5x, 10x and 15x magnification). A nice example for contemporary "retro" design (cf. the similar but historical loupe with many identical features in the next image!). Length 6 cm.
Image center: a yellow 10x folding loupe with fixed working distance and a square measuring window with mm reading. So-called "thread-counter" for use in textile property assessment. Glass lens in (here not visible) metal mounting ring.
Right side: a blue folding loupe from China "Waltex, Great Wall, Model 7534", with two different plastic lenses (4x and 6x, summing up to a maximum of 10x).

Top: very old folding loupe (ca. 1880 - 1900) made of horn, with three different lenses to be used in various combinations. Maximum magnification ca. 20x, length 6 cm.
Front, left: very old, small "thread-counter type" loupe made of brass, focus distance 2.5 cm (resulting in a 25/2 i.e. 12.5x magnification). Small series craftswork production with file traces, gold colour transparent lacquer coating and black colour light reflection shield.
Front, right: Very tiny but actually fully usable 20x loupe made of plastic (42 mm x 12 mm x 12 mm), two separate glass lenses with 8 mm usable open diameter.

Classical 10x loupe, ca. year 1970. Baroque pre-war design, fine chrome finish. Optical system consisting of two separate lenses (Doublet), one plan-convex, the other biconvex. 50 mm x 30 mm x 22,5 mm. This system is commonly called aplanatic because two separate lenses perform better than a single lens with respect to spherical abberation. Free lens diameter 20 mm.

Rare Cloisonné folding loupe of unknown age. Biconvex single lens with 30 mm (!) diameter,  6x magnification. Rather big and heavy for a folding loupe (6 cm x 3,5 cm, 64 g). Nice example of a convergence from fascinating oriental design arts with occidental machinery-technical properties. Note the detailed cranes with green beaks and read caps, the beautifully ornamented side-wall decorations and the overall perfect craftswork with carefully rounded edges and reliable bolts.

Carl Zeiss Jena hand loupe with slightly conical handle. Overall length 15 cm. Extremely asymmetrical strongly curved bi-convex single lens with 3.5 cm focal distance, i.e. 7fold magnification. Lens diameter ca. 32 mm. Pre-war model made of plastic (Bakelite or Bakelite-like), glass and metall. Rather rare style mixture between the common hand magnifier and stronger "scientific" loupes. Houses in a fine "coffin-style" black cardboard box with wooden fittings and textile spacers.

Small classical Zeiss folding loupe in plastic mounting (8x). At the time of its creation (1930s) in a fairly modern, straight-line design, which appears to have been kept unchanged for many decades. Length ca.  3,5 cm. Free usable lens diameter ca. 12 mm. In front the respective Doublet optical system consisting of two lenses made of different glass types thus providing excellent colour correction. A so-called  achromatic  system. Very crisp and colour fringe-free image quality until to the very edges of the field.

In contrast to mechanically mounting-linked lenses glued lens combinations are more difficult to recognise. In this case the borderline between the two glued elements becomes visible after some grinding:

Cross section through a glued loupe doublet lens systems.
Left: the green arrow marks the borderline between the two lenses.
Right: green lines mark the silhouettes of the two lenses.

One of the most successful classical loupe designs: True Triplet or Steinheil Triplet (see schematic diagram below), 10x, lens diameter 22 mm, "Made in Belarus". Don't hesitate to look out for this country on the world map! One of our favourite loupes. You will recognise two glue borderlines around the central bi-convex, slightly retracted lens. The only flaw of this product are the small fixing screws which exhibit a strong tendency to vanish, evaporate ... The good news: this screw escape is a rather slow process and your chances are good to get hold of them before they are definitely gone.

Steinheil-Triplet (schematic). Symmetric Bauplan consisting of three glued elements. In the center a biconvex lens made out of weakly refractive Crown Glass (normal glass) surrounded by two menisque type lenses made out of strongly refractive Flint Glass. Comparatively high production costs, excellent image quality.

Before switching to Ebay now in eager search for the magnificent triplet system: be warned. There are many products with "Triplet" inscription which are just  named  Triplet. This is similar to a motorcar which might have the name of a tropical wind but is by no means identical with this tropical wind. Many of those "Triplets" consist of a single biconvex glass cylinder which will magnify as well but will never be able to provide the same image quality as a true optical triplet system. Nevertheless you will be able to count the number of bug feets with such a simple system as well. This Triplet confusion might be only slightly or borderline fraudulent, in particular when keeping in mind the prices of those pseudo-triplets. Moreover, our Western term "triplet" will be totally unreadable and ununderstandable for most people working in Asian countries.
Casually you will come across Ebay loupe offers which are a little bit more on the fraud side like the one shown below:

A clearly fraudulent inscription: this loupe contains a single glass cylinder (dismantled, right), instead of the indicated "five elements" of the 20x loupe edge. Two lenses at the edges (separate 10x and 20x, "butterfly" design), length 52 mm, in plastic container.
Very cheap, Ebay ...
Note the wrong spelling of the word "Elements" ("Elememts")

Now, as you might be pre-loadad with anti-Ebay prejudice, please have a look at the almost identical misspelling on the loupe shown below, bottom-left. Just guess, how many lenses are in there? One?
Wrong. It has in fact, as indicated five lenses - two glued doublets and an additional central convex lens, everything properly positioned by means of spacers.
This is a fine lecture about prejudice. The world can be really terrible and brute but still a considerable percentage of the people are better than we think.

Left: Highly corrected (anastigmatic), chrome-finished 20x loupe with five optical elements. As to be expected for a loupe with high magnification it has a low lens diameter and is exceptionally small.
Right: Excellent three-lens system (glued doublet plus separate single lens, 15fold magnification. 32 mm x 24 mm x 24 mm).

Of course, what counts in the end is not so much the technical type of the optical system but the actually visible performance. Good testing objects are crisp black inscriptions on white paper. The best loupes will provide images which are perfectly well defined throughout the whole field of view and which show no signs of colour fringing at all (yellow edges, in very poor products even several edge zones of various colours).
When testing high magnification loupes: Take care to position the lens level parallel to the object level and apply proper illumination, everything else would result in unfair comparisons.

10x loupe "Liberty #42.02", from Ebay, with beautiful finish. Lens diameter 18 mm. Came in an onion type packaging, in nice printed cardboard box, labelled as "EXPORT QUALITY", therein fine LIBERTY-packaging paper, with gold colour seal print "Made in India" und a nice black housing. This is one of the loupes which we didn't dare to dismantle in order to look for potential triplet disappointments. It is definitely very nice already as-it-is.

One more typical Internet offer: the "ZIESS GOLD, GERMANY, 15 x, SERIES M" comes via Air Mail from Thailand (Honni soit qui mal y pense). In any case at least the etui is not a Zeiss company copy.

In case of doubt the lenses of a loupe should have no noticeable colour.
Left: Zeiss Doublet system, no colour add-on. Right: loupe lense by another maker with strong discoloration tendency.

Some salesmen of loupes do use poorly defined or empty, overdone terms in order to characterise their products. You will find "optical glass", "silicatic glass", " ground glass" and many other word vapour specifications.
Cheap plastic lenses have a tendency to be named as "high-grade, scratchproof computer-calculated high-tech synthetic resin lens systems".

Overall it is by no means just a problem of salesmen's morals. Most customers lack basic knowledge. They expect high magnification and large lens diameter at the same time - which is impossible for physical reasons. Moreover they are not aware of the fact that lower magnifications are much better for beginners and that higher magnifications (in particular those above 10x) are restricted to special situations and require much more skill on side of the user. Most true 20x, 30x and (rare) higher magnifying loupes are of little practical use under realistic outdoor conditions. When spying a little bit over an expert's shoulder you will normally see him or her using the 10x magnification only. The 20x, 28x etc. systems are very close to James Bond gadget boasting. Grandfathers and parents have an unsane tendency to buy utmost magnification loupes for their (grand)children with the idea in mind that the loupe might be some kind of pre-doctoral title for the poor kid (which is no advantage for the kid, as it will not be able to make much use of the expensive gift).

Photographers  have special requirements different from a botanist or a tardigrade microscoper. As a rule they need very expensive, low-magnifying loupes in order to check a slide as a whole (up to 6 cm x 6 cm) or to check the image definition on a ground glass screen.

For us, the tardigrade enthusiasts, low magnifications don't make any sense as the tardigrades are really, really small. A 10fold magnification is minimum for tardigrade observation. We will provide a visual impression of the tardigrade as seen under the Belarus loupe in the next issue as we have been overdoing our text and illustration budget this time already.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
Water Bear web base is a licensed and revised version of the German language monthly magazine  Bärtierchen-Journal . Style and grammar amendments by native speakers are warmly welcomed.

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