Schneider Xenoplan 26 mm f/5.6 prototype
lens characteristics and adapting

This page describes an unusual Schneider industrial lens, marked Xenoplan 26/5.6-0001. Typically for Schneider lenses, this means that the lens speed is f/5.6 and the focal length 26 mm, with the last four digits usually indicating a model number (albeit evidently not in this case).

I was unable to find any technical information about this lens online, or in Schneider literature in my possession. Schneider did reply to my inquiry about this lens and confirmed that these are indeed prototypes, but the representative who came back to me pointed out that he would not provide any information about the technical specifications, purpose or original purchaser of these lenses, and sounded a little curious about these prototypes turning up on the second-hand market. Thus, it is possible that only a few prototypes were made, before the design was shelved and never went into production.

I am only aware of five specimens of this lens, all advertised on eBay by the same Israel-based second-hand seller. Three of them are now in my possession. A fourth specimen was sold some time before I bought mine. All specimens carry a black plastic sticker saying "PROTOTYPE". In the specimen of Figure 1, this sticker had mostly broken off, and I removed what was left of it. As of February 2023, the same seller is advertising on eBay a fifth specimen (or possibly more than one) but the ad images show one of the specimens in my possession, not the actual specimen for sale. The eBay item number of this last eBay ad is 275349222529.

In addition to the company logo, "MADE IN GERMANY" and "Xenoplan 26/5.6-0001" engravings, these lenses have additional markings and stickers. On the specimen in Figure 1, the engravings say "SN 15267979 # 1085196". My other specimens of this lens have serial number 15267986 and 15274659, respectively. The number at the right of the # symbol remains the same in all my specimens, and might be a batch or order number.

The above picture shows the two other specimens, with original stickers. The one at the left has a different thread mount (M45 x 1) than the other two (see below), and came from a different eBay sales batch of just one specimen. One of its stickers says "0", while the other batch carries a "1" sticker. There is no red-dot marking the optimal orientation of the lens relative to a line-scan sensor, nor any other marking with the same function (except possibly the yellow round sticker, present on two of my specimens). The "OH2002" sticker might point to a 2002 year of manufacture or purchase. The lens on the left also had a large sticker around its front that was evidently peeled off years ago, leaving abundant dry adhesive. It took some effort to clean it up after taking this picture.

All specimens of this lens that I am aware of carry an unmounted 26 mm dielectric filter, 2.0 mm thick (bottom of Figure 1, and within the rear lens mounts in Figure 3), held by a proprietary M29 x 0.5 threaded retaining ring within the rear mount of the lens. This filter transmits blue light and NIR, and reflects golden yellow light when seen on-axis. It completely blocks NUV. Perhaps it is just a coincidence, but the same seller that sold the lenses also advertises a large number of mounted 37 mm blue filters marked "Schneider 37,0 BP 465 70", which likely means that 37 is the diameter of the filter mount, BP means bandpass, 465 is the center of the transmission band in nm, and 70 is the width of the transmitted band in nm, i.e. a transmission range between 490 and 430 nm, which includes both cyan and blue.

A filter that transmits blue and NIR has uses in agricultural and forestry monitoring, for instance to collect data on leaf cover (see e.g. Valle et al. 2017, PYM: a new, affordable, image-based method using a Raspberry Pi to phenotype plant leaf area in a wide diversity of environments. Plant Methods 13: 98). A lens for these applications works best if it provides a good correction of chromatic aberration (both transversal and axial) across the VIS spectrum and into NIR, or at a minimum a lack of chromatic aberrations in both blue and NIR.

Night-time natural illumination consists mostly of blue and NIR, so in principle the same lens and filter might also be used for night-time surveillance in the absence of artificial illumination. This finds uses either in long-range surveillance, or applications where one must avoid the tell-tale emission of the NIR illumination sources ubiquitous in industrial and commercial surveillance. An actively cooled sensor or a light intensifier may be necessary for these uses.

The barrel is heavy and entirely made of metal (except for a thin rubber strip on the aperture ring). The aperture ring can be locked with 3 hex-inset screws, a common feature of industrial lenses designed for use in industrial environments subjected to vibrations. The aperture scale, ranging from 5.6 to 16, is engraved three times around the lens barrel, on an adjustable ring also locked in position by 3 hex-inset screws. Releasing these screws allows the ring carrying the aperture scales to freely rotate for alignment to the aperture index, as well as to be removed from the barrel. Afterwards, the aperture ring can also be slid forward and removed, exposing the lever that transmits the ring's rotation to the diaphragm.

The diaphragm, as seen from the front and rear of the lens, does not fully open to the width of the surrounding barrel. It is possible that the optics are designed to be faster than 5.6, but intentionally limited to the latter speed to limit the amount of aberrations, also a common feature in industrial lenses. The ring that carries the aperture scales has no stops that limit the aperture range. The slit of the diaphragm lever in the barrel stops the diaphragm lever at either end of the scale, so increasing the aperture range would require, at a minimum, the removal of all optical groups mounted in front of the diaphragm. I have no way of knowing whether the diaphragm itself could open to a wider aperture than allowed by the slit in the barrel.

Curiously, in all three specimens of the lens one of the three aperture index marks is very slightly offset with respect to the other two. There are no aperture click-stops. The diaphragm has 12 blades and is fairly well rounded.

The optical design is obviously asymmetric, with diameters of the front and rear pupils, respectively, of 5.0 and 12.7 mm (Figure 4). This yields a pupil ratio (defined as P_out / P_in ) of 2.54, quite high for a general-purpose lens and more typical of a retrofocus wideangle. This makes the lens potentially useful reversed in photomacrography, where the high pupil ratio results in a faster nominal lens speed (f/2.2), compared with a lens of unity pupil ratio. For example, at a 2x magnification, a lens with nominal aperture set to f/5.6 and unity pupil ratio has an effective aperture of f/16.8, while a lens with pupil ratio of 2.54, also set to f/5.6 and 2x, has an effective aperture of f/10. This is likely to make a visible difference in the amount of diffraction produced by the lens. A likely problem with using this lens in reversed orientation is the close distance from the subject to the end of the lens barrel.

The front filter mount is non-standard. A 35 mm step-up filter adapter is too small and does not engage the thread of the filter mount. A 35.5 mm step-up adapter was difficult to find, took weeks to arrive (and two out of three of the shipped adapters were damaged in the mail), but when one finally arrived in usable conditions, it turned out to be too large to screw in.

The filter mount is machined in a thick metal washer attached to the barrel by six small screws (Figure 1). Removing the screws (Figure 5) reveals that the washer is not just a filter mount, but a machined ring that applies pressure onto a copper-colored sheet spring that, in turn, transmits the pressure to internal parts, presumably to prevent them from shifting and/or turning with respect to each other. Without the screws, the filter mount lifts up from the lens barrel by about half a mm. If for any reason you remove this type of filter mount, you will do well not to disturb the optical components underneath (e.g. by turning the lens upside down), since the optical groups and their mounts very likely have been tested multiple times during assembly of the lens to place them at optimal reciprocal distances and orientations. It therefore appears that this lens prototype is one of those high-end machine vision lenses that require an individual, slow manual assembly and testing. Any tampering with the internal components is therefore very likely to spoil the performance of the lens.

All barrel markings are laser-engraved, rather than silk-screened like in other Schneider industrial lenses. With a total length of 76 mm and a maximum diameter of 52 mm, this lens is much longer than one would expect for a 26 mm wideangle with an f/5.6 speed. The front and rear optical surfaces are spaced roughly 64 mm from each other. The weight (295 g including the rear filter) is also remarkably high for a lens of this size. In comparison, the Makro-Symmar 5.9/120 shown in Figure 1, which is also a machine vision lens in metal barrel, at 166 g is just a little over one-half the weight of the Xenoplan 5.6/26-0001.

All AR coatings but one give amber-colored reflections. They look similar to the broadband coatings of special-purpose lenses for multi-spectral photography (e.g. the CoastalOpt 65 mm). This is consistent with the dual-bandpass filter mounted at the rear of the lens. This type of broadband coating, unavoidably, is less effective than the multicoatings used for VIS lenses, which are just optimized for VIS. Therefore, lenses with these broadband coatings often require a judicious use of lens shades and rear baffles.

I have no information on the optical scheme. The elements at the front of the diaphragm produce at least seven reflections (one of those deep within the lens stands out in bright mirror-like white), with at least five more reflections seen from the rear of the lens. The optical elements seem to be tightly packed together along the length of the barrel, without the ample spacings sometimes present between the front group of a wideangle and the following groups. The front optical surface of the first element is concave.

All the above features point to an expensive lens to produce. Only testing, however, can tell whether such a lens, apparently developed for a specialty application, is useful for imaging on a medium- to high-end consumer camera.

Adapting the Xenoplan 26 mm f/5.6 for infinity focus

A simple test of the location of the lens focal plane can be done by looking into the front element and placing a test target (e.g. a printed page of text) in focus at the rear of the lens. This shows that the lens must be mounted with the rear end of the barrel just 15 millimeters from the sensor. Therefore, the lens can only achieve focus at infinity when mounted on a mirrorless camera. The threaded lens attachment is too close to the rear of the lens to allow the latter to be mounted at the end of even a very short focusing helicoid.Also, a relatively long focusing helicoid (extending by at least 10-15 mm) is necessary for comfortably testing/using the lens at a range of focusing distances.

Short of modifying the lens barrel by machining off a substantial amount of metal, which I am not equipped to do and is difficult to do without damaging the optics, it is necessary to use a wide focusing helicoid with 65 mm threaded attachments at its front and rear and an extension range of 17 to 31 mm. The lens must be mounted partly sunk within the helicoid. Multiple adapters between lens and helicoid, and helicoid and camera, are also necessary. In particular, I inserted the lens barrel into the helicoid via an adapter equipped with a cylindrical sleeve and three fastening screws. Two stacked step-down filter adapter rings were also necessary to mount this adapter deep enough within the mouth of the helicoid. Ordering the various adapters by mail order, testing them, then ordering different adapters once I discovered problems with those already in my possession required three iterations and a few months. Multiple parts were lost or damaged in the mail, further delaying the project.

After finding the correct depth of the lens barrel within its sleeve and tightening the retaining screws, I sealed the sleeve around the lens barrel with black silicone to prevent light leaks. I also used one turn of teflon tape around the lens barrel to limit the damage by the retaining screws and better fit the 51.5 mm lens barrel into the 52 mm sleeve.

Mounting a 1¹/₄" Baader U filter as close as possible to the front element of the lens and a sufficiently large lens shade to avoid vignetting also required a custom combination of adapters, in addition to epoxying a 35 to 37 mm step-up adapter into the non-standard front filter mount of the lens.

The result is a large and ungainly assembly of parts that, after a few modifications and adjustments, eventually did what it was supposed to do (above figure). The camera in the figure is a full-spectrum converted Olympus Pen Lite E-PL6 equipped with a grip for more comfortable handling and an Arca-compatible plate. The widest scalloped ring operates the focusing helicoid. Because of its large diameter, focusing is quite precise in spite of the limited amount of turning of the focus ring (a little less than half a turn). The 13.7 mm extension of the helicoid is more than sufficient to range from infinity focus to 0.5x and to allow a rear-mounted filter to be used if desired.

The list of parts used in the configuration shown in the above figure, from front to rear:

• 72-67 mm step-down filter adapter
• widely conical lens shade with 52 mm rear male thread and front 72 mm female thread
• custom filter holder, built with:
  • 43-52 mm step-up adapter, modified by removing the rear male thread and attached with MA2.5 screws to the rest of the filter holder (see here)
  • 37-52 mm step up adapter
  • 37-30.5 mm step down adapter
  • 1¹/₄" Baader U filter re-mounted in 30.5 mm filter ring, screwed into the rear of the preceding adapter. Note the purple (="copper") side facing to the front.
• 35-37 mm step up adapter epoxied into front filter mount of lens
• Xenoplan 5.6/26-0001 (original rear filter and its retaining ring removed)
• 58 mm male thread to 52 mm sleeve adapter, fastened around lens barrel
• 58-62 mm step down shoulderless ("flat") adapter
• 65-62 mm step down shoulderless ("flat") adapter
• 17-31 mm focus helicoid with 65 mm mounts
• 42x1 to 65 mm step-up adapter
• Micro 4/3 to 42x1 adapter, or Sony E to 42x1 adapter (as flat as possible)

Not included in the above list: thread sealant, adhesive flocking, matte black paint, black silicone sealant, cyanoacrylate glue and teflon adhesive tape.

Summary

The Schneider Xenoplan 5.6/23-0001 was manufactured, probably around 2002, in a small number of prototypes (at least five, as far as I know). It is probably a very expensive lens to manufacture, assembled and optimized by hand, and developed for special applications. The short distance between rear of the barrel and focal plane means it can only be used on mirrorless cameras. Adapting it for this use and adding a focusing helicoid results in a massive assembly of numerous adapters.