Magnifiers: a closer look (XI)
Professional focal length measurement for amateurs - first results!

In the July issue we had presented the theory behind an advanced focal length measurement method - making up an almost universal micro-optics magnification measurement method!.

The respective practical procedure had been explained in August.

Focal length measurement results can be considered as the crucial, most interesting information in the basic study of micro-optical components like magnifiers or microscope objectives. Once the focal length f has been established, a simple calculation will reveal the magnifying power V of a magnifying component, e.g. a microscope objective, on the basis of the formula V = 250mm/f[in mm].

You will be able to check to correctness of magnifier and microscope objective inscriptions. Moreover you will be able to reveal the magnifying power of many historical magnifiers (ages 100 and plus) which as a rule do not bear any inscription.

Let's give it a go and begin with a check of two worldclass company magnifiers (Zeiss and Leitz magnifiers):

Fig 1: Leitz 10x magnifier with interchangeable optics' head

Technical annotations (magnifier shown in fig. 1): This instrument might be used as a stand-alone mobile magnifier, or, alternatively, as an objective in one of those old "simple", i.e. one step dissecting microscopes [Giordano 2006] which basically consisted of a stand with adjustable height bearing a magnifier. The optical head shown here is a classical "Steinheil triplet" component made up of three lenses with two different glass types cemented together. Whereas the optical head appears to be thoroughly threaded and blackened against straylight, the handle looks like a rather primitive wire-plus-soft-soldering product. The weight of the optical head alone amounts to 33.8 g, the handle is adding some more 11.6 g. We measured a focal length of 25.5 mm resulting in a magnification value V of V = 250 mm / 25.5 mm = 9.8x. We think that this magnifier was produced in the first half of the 20th century. Please note that the inscription is on the understatement side, just naming the magnification value and the company name.
User evaluation: as to be expected from a classical Steinheil system of moderate magnification power the image is free of color fringes, planar and clear. But it has rather modest field of view, ca. 15 mm. When compared with more modern magnifiers it will become obvious that those tend to offer a wider angle of view, see e.g. the widely used modern 10x 6LED folding magnifier, which is offering a field of view with ca. 20 mm. Nevertheless the older Leitz magnifier is a solid, perfectly usable instrument - though lacking any glamour or proud gadget impression.
The specified "10x" magnification is in excellent accordance with our measurement value of 9.8x. You can trust in the precision of those old instruments which were constructed with advanced university and industry work in mind.

Fig. 2: Classic, small 10x folding magnifier by Zeiss

Technical annotations: the optical element is a cemented two-lens system (a so-called doublet) with 13.6 mm in diameter. The housing is narrowing this diameter down by one millimeter to ca. 12.5 mm. Weight 11.7 g.
Measured magnification: 9.9x - bravo!
Appoximate date of production: see Zeiss Mikroskopie-Katalog Mikro 1 from 1939, p. 97.
User evaluation: With its length ot merely 3.6 cm this is a truly pocketable device. Its field of view is measuring 15 mm in diameter which is acceptable when keeping in mind the small size of the instrument. As with most vintage Zeiss folding magnifiers the folding mechanism is impeccable - not to tight and not too loose, even at this age coming near to 100 years. As to be expected from a cemented doublet the image is achromatic and crystal clear. No stray light problems thanks to the black housing. Perfect design and usability. Moreover, a black "Bakelite" housing like this one will provide a superior feeling when used outside in cold weather.

At this moment one might ask whether there is any need to check those inscripted magnification values on reknowned company products at all. But we will see in the upcoming magazine issues that nowadays many magnification inscriptions are dramatically wrong, erroneous at least, sometines even fraudulent. Furthermore there are many vintage and antique magnifiers with no inscription at all and of course we would like to discern whether they should be considered as mice or elephants. Just have a look:

Fig. 3: Very old two lens folding magnifier, with strong magnification and no inscription at all.

Technical annotations: this is a folding magnifier with two separate lenses and a small iris diaphragma (with 7 mm diameter) in between. The iris is quite a clever add-on, as it can be folded in for the combined (high) magnification and left out during the use of one of the single lenses. The lens fittings are solid classic craftswork in brass, fixing the lenses by means of threads and screws. Please note that there are no threaded rings which became more common in later times. The handle scales are made of tortoise shell, the spacers of brass, the rivets of iron. Weight 13.8 g. The field of view of the combined optical system is close to 5 mm, quite respectable when keeping in mind the high sum magnification. We were not able to find this magnifier depicted somewhere in literature, so we can only guess its age. In any case it should be well above 100 years.
The measured magnifications of the individual lenses are 11.5x and 12.9x (similar but not identical). The measured sum magnification is 21.8x.
If you should ponder why the overall magnification is not the mathematical sum of the magnification values of the components you might have a look back at our July issue with the respective formula.
The two individual lenses differ in various respects: firstly they are not identical in magnification as already declared above, secondly they differ with respect to fluorescence under UV light: the less magnifying lens is delivering a strong green fluorescence effect whereas the stronger lens doesn't show any effect at all. This finding might be interpreted as an indicator for pre-industrial manufacturing.
User evaluation: clearly, this is an extremely fine instrument capable of strong and field usable magnification (close to 22x!). But only a terrible fool would try to use such a delicate instrument outside, under rough conditions!

Fig. 4: Tiny antique folding magnifier - again without inscription or hallmark

Technical annotations: the optical element is made up of a so-called Coddington lens (as simple cylinder lens with with a groove, kind of iris, in mid-height). According to [Giordano 2006, p. 51] this type of magnifier was produced in the first half of the 19th century. The material looks like silver but as there is no hallmark it might be as well some of those other contemporary alloys mimicking silver. Measured magnification: 25.6x! Weight 9.2 g.
User evaluation: a nicely built, perfectly working tiny folding magnifier. The field of view is very small, approximately 2 mm in diameter. Nevertheless this might be a fine instrument for those who know exactly where to look close! But keep in mind that typical modern 10x magnifiers might have a 10fold larger field of view, measuring 20 mm and more (see fig. 6).

Fig. 5: "Winkler&Wagner" magnifier (ca. 1913). Measured magnification 33.8x!

Technical annotations: this magnifier was already presented in one of our previous issues and praised as an exceptional instrument - not only due to its extreme magnification. It houses a tiny cemented Steinheil triplet. Its only inscription is a number "6" which might be as well deciphered as a "9". The problem is that our magnification measurement result doesn't fit to either of those two alternatives: 6 mm focal length would indicate a 42x magnification, 9 mm focal length a 28x magnification.
Our own focal length measurement provided a magnification value of 33.8 x (!). This is the highest value measured by us up to this date.
Usable field of view: ca. 2 mm. Weight  13.5 g. This instrument is depicted in catalogue No. 9 (1913) distributed by the Vienna entomology provider Winkler&Wagner, indicating a seller but not the actual producer.
User evaluation: in spite of its extreme magnification this instrument is ingeniously designed and thoroughly usable even outside. Straylight is perfectly blocked due to the black viewing port and a deeply recessed lens. Moreover, the lens head is featuring an ideal, conical geometry which is still allowing some natural light to reach the object under investigation from aside. And yes, also this instrument is a collectible rarity - too precious for field work.

There are some cases in which the inscriptions are differing from the actual magnification - but not dramatically. One shouldn't be too pedantic in those cases, in particular as long as the deviation is not exceeding 10%. E.g. one of our favorite tardigrade finding magnifiers, the "10x 6LED" magnifier is in fact yielding a 9x magnification in our measurements - but we still love it!

Fig. 6: 6 LED "10x" folding magnifier with an actual magnification of 9x. This is an acceptable deviation and will not be noticed in practical field work.

Technical annotations: this is a well-known magnifier, sold worldwide at moderate prices (sometimes relabeled and sold at less moderate prices!). It is by no means an inscription fraud, as it houses a true cemented triplet. You might have a look at its image quality in comparison to a basic cylinder lens magnifier here. Though you might end up to understand that the optical quality difference between the triplet and the cylinder lens is noticeable we have to concede that sometimes the more primitive magnifier might perform a basic task as well.
Measured magnification: 9x. Field of view ca. 20 mm. Weight 53.9 g.
User evaluation: this is a bargain magnifier with impressive image quality, a true triplet and more importantly, with a very advanced 6 LED soft illumination which will help tremendously under dim light conditions. Moreover, the LED light is helpful in order to color-detect tardigrade 'tuns' (i.e. tardigrades in the dry state) within dry moss samples: Echiniscus tuns appear with a markedly blue tinge. As a consequence this magnifier is extremely useful in order to check in the field whether a given moss sample might be densely inhabited or not:

Fig. 7: Dry moss leaf, illuminated by means of the 6 LEDs of the magnifier shown above. The tiny Echiniscus tuns (each measuring merely 50 µm) will appear as blue spots under this LED light!

Nevertheless we were rather disappointed that a closely related product with almost identical housing and design, namely the "30x 6LED" folding magnifier, yielded a measured magnification value result of 15x (!). So it appears to be clearly mislabeled. On the other hand practical experience has taught us that the housing with its wide opening wouldn't make sense in combination with a 30x magnification (simply too wide). As a consequence the actual magnification might be seen as a white lie, with a still working 15x magnification and the glamour of a 30x magnification ...

We will continue our measurements in the issues to come. Meanwhile we would like to warn you that a 30x magnifier with more than 10 mm lens diameter is too good to be true. Therefore it is advisable to do without those mislabeled instruments. In any case it would be wise not to boast with their properties. Otherwise the inscription awkwardness might kick back to you as a personality, outing you as someone who can be betrayed easily.

Literature

Raymond V. Giordano: Singular Beauty: Simple Microscopes from the Giordano Collection. Cambridge, MA. 2006.

© Text, images and video clips by  Martin Mach  (webmaster@baertierchen.de).
The 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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