Schneider Xenoplan 23 mm f/1.4
stacked on
Schneider Makro-Symmar 120 mm f/5.9

This page was prompted, in part, by the earlier finding by Robert O'Toole that the Schneider Makro-Symmar 120 mm f/5.9 in V38 mount is an excellent close-up and macro lens, as well as tube lens when coupled with several types of reversed lenses of shorter focal lengths, including the Schneider Xenon 28 mm f/2. For a few years, I have had a Schneider Xenoplan 23 mm f/1.4 rattling in one of my drawers, as well as a couple of Schneider Makro-Symmar 120 mm f/5.9. It seemed like a good idea to test the Xenoplan 23 stacked onto a Makro-Symmar 120, to see whether this combination can be of any use.

It is first necessary to mention that many of the Schneider close-up and macro lenses of fixed focal lengths are made in multiple versions, each optimized for a given magnification range, virtually impossible to distinguish by visual inspection except for specifications printed on a label attached to the lens barrel. It is also worth mentioning that Schneider is somewhat old-fashioned in indicating lens magnification as β (lowercase beta) and expressed as a negative number (because the image projected by the lens is inverted with respect to the subject). Finally, the Makro-Symmar 120 mm in V38 mount is marked as f/5.9, while the apparently same lens in M39 mount is marked as f/5.6. According to Robert O'Toole, the actual aperture is closer to f/5.9. The actual aperture may vary slightly across the different versions.

Specification sheets for some of these lenses have become a little difficult to find online, for multiple reasons:

• Several of the old Schneider web sites are still online but seem to have been silently abandoned, and are no longer maintained. Perhaps Schneider finds this easier to do, than taking these sites offline.
• Many of the old PDF lens datasheets are no longer available on Schneider web sites. Copies of these datasheets, however, are often still available on resellers' web sites.
• As of January 2022, Schneider finished renaming all its industrial lenses with names as informative as Charoite, Jasper and Sylvine. See this document for the full list, complete of new and old names. If you cannot find an old datasheet, Google the new name of the lens. Several resellers still advertise Schneider lenses with their old names.

All lenses discussed on this page are described as vibration resistant and mounted in barrels equipped with a proprietary Schneider V38 mount at either side of a "Makro-Iris", which is a section of barrel carrying the diaphragm and aperture ring, where the front and rear cells of the lens are screwed in. All parts of the lens barrel are made of metal, except for thin rubber rings for better handling, and fixed in position by set-screws. Small thumbscrews are available to reversibly lock the adjustment rings without needing a hex key. A good range of fixed extension rings, helicoids and camera adapters (not all of them compatible with each other and with all lenses) are or were available for V38 lenses. See this report for currently available products (dead link).

Schneider Makro-Symmar 120 mm f/5.9

I own two specimens of this exact lens and version, one a little battered and worn out (shown in Figure 1 and Figure 2) and a second in much better physical conditions. The specimen illustrated above also has a scuff on the coating of the front element that cannot be removed by cleaning the lens, visible as a small light-colored area in the picture, evident damage to the coatings caused by molds, and other minor damage.

One purpose of my test was comparing the performance of the two specimens when used as a tube lens, or as a close-up lens. To make a long story short, I can see no difference whatsoever. The conditions of this specimen do warrant a significantly lower second-hand price (on the order of 50%), but have no effect whatsoever on image quality. Perhaps the damage may result in a very slight veiling glare in extremely harsh conditions (e.g. sunlight directly striking the front element), or perhaps it may produce a very hard-to-see unevenness of bright bokeh disks on a dark background, but in reasonable shooting conditions the two lens specimens produce indistinguishable results. A misaligned element, on the other hand, may be totally undetectable on visual inspection, but most certainly would prevent a good performance of the lens.

All sample images on this page were shot with the "bad" copy of Makro-Symmar 120 mm used as tube lens.

The main specifications of the different lens versions of Makro- Symmar 120 mm f/5.9 (collected from different Schneider sources or measured/computed by myself) are as follows:

where F is actual focal length (which slightly differs from the nominal 120 mm and among versions) and M is the nominal optimal magnification. The above table shows the older model and version denomination of these lenses in the leftmost column, and the current names of these versions in the next column.

The table also reports the total distance from sensor to mount flange of the lens with the lens focused at its optimal nominal magnification (D_M) and focused at infinity (D_∞). The latter, of course, corresponds to M = 0. Incidentally, the D_M I measured agrees very well with the value I computed from the parameters provided by Schneider.

D_M and D_∞ are reasonable starting points for tests of this lens when used as a tube lens in a stacked configuration. The first of them places the lens at its optimal design distance from the sensor. The second focuses the lens at infinity, which is the normal focus for a tube lens in an infinity-corrected system. So far, I only measured these distances in the two specimens of this lens in my possession, and the other values (indicated with *) are only calculated. The measured D_∞ is a good basis for computing the corresponding values for the other versions of this lens, with a minor uncertainty due to the position of the rear pupil being slightly different. In my lenses, the optical distance between rear pupil and V38 rear mount flange of the lens is 4.8 mm.

All SR 5.6/120-00xx versions are specified for 12,000 pixel line sensors with a width of 60 mm (which in practice is the diameter of the image circle). A red dot is embossed on the barrel in some specimens. This indicates the best orientation of the lens, which is mainly relevant if the lens is used with a line sensor. Note that the red dot orientation is perpendicular to the optimal orientation of the line sensor.

The optical scheme uses 8 elements in 4 groups and is only slightly asymmetric. While the two cemented groups are mounted directly against each other in the front cell, they are separated by a space in the rear cell. This spacing seems to change among the different versions, but only by a little. Possibly, the spacing between rear and front cell is also tweaked among the different versions. Because of this, one can assume that the pupil ratio of this lens is very close to 1. In practice, what Schneider did is conceptually similar to the floating group/groups routinely used to correct aberrations in most modern macro lenses, albeit placed at a fixed position in each lens version in order to avoid the costly mechanical system required by adjustable floating groups. The Schneider Macro Varon (now called PYRITE 4.5/85/0.5x-2.0x) is an example of a lens equipped with such a mechanically adjustable floating group.

I have not been able to examine an SR 5.6/120-0058, but judging from its magnification range, this model is optically symmetric (except perhaps for a small difference in diameter of the front and rear elements) and it would be pointless to reverse it.

The physical length of the rear cell in the SR 5.6/120-0059 (and probably also 0060 and 0061) is greater than the front cell, in order to accommodate the greater spacing between groups. The threaded mount of the front and rear cells differs, so it is not possible to accidentally swap the two cells when reassembling the lens.

Xenoplan 23 mm f/1.4

The Xenoplan 23 mm f/1.4 is currently called CITRINE 1.4/23 C. This lens is physically small and unassuming, but by no means cheap if purchased new. Its recommended price online varies between 676 € and 820 € plus sales tax, but used specimens sell for as little as 22 € (in a version with fixed aperture) because these lenses are poorly known and not "fashionable" on the second-hand market. My specimen has a long and fine-pitched thread at the rear of the barrel that fits a lockable focusing mount ending in a C thread, and lacks a variable aperture. I removed the focusing mount in preparation for reverse-mounting the lens.

The pupil ratio of this lens is remarkably high, roughly measured as between 2 and 2.5. The rear pupil is vignetted by the rear optical element and not visible all at once if one looks into the rear of the lens, which makes a precise measurement difficult because of the fixed aperture. Schneider specifies a pupil ratio of 2.25 for the present versions of this lens. The front pupil is deeply recessed within the lens, which is a possible problem because it may cause vignetting once the lens is reversed. The maximum angle of view is specified as 28° (presumably diagonally and at infinity), which is a bit narrow, roughly equivalent to the diagonal angle of view of a lens of 85 mm focal length on full frame, according to the table on Nikonians.org.

There have been discussions on whether some Schneider lenses, including this one, are interchangeably marked as Xenoplan or Apo-Xenoplan but are optically the same. I have no first-hand information on this. The most recent discussion of these Schneider optical designs is in this thread on photomacrography.net.

In general, I am not impressed with the quality of several Schneider Apo Componon lenses, which display evident chromatic aberration in spite of their Apo denomination, or that are apochromatically corrected only for lateral, but not axial, chromatic aberration. This example does suggest that Schneider applies the "Apo" denomination in a somewhat "flexible" way. Sometimes there may be acceptable explanations for this, e.g. an enlarger lens (most old Apo Componons were designed in the film era) needs to correct lateral chromatic aberration only in the focal plane but not outside the latter, and does not need to correct axial chromatic aberration, because the subject (a photographic film) is expected to lie entirely within the focal plane and its DOF. Imaging a three-dimensional subject is thus not the intended application for such a lens. The argument has also been made that a photographic film, when heated, tends to buckle in a generally predictable way (at least in a glassless film carrier). The lens, therefore, by design may not provide a geometrically flat field but a curved one that is a compromise between the geometry of cool and flat versus hot and buckled film.

The lens is specified to cover a 2/3" sensor (8.8 x 6.6 mm) up to 5 Mpixel with 3.65 μm pixels and an 11 mm image circle, which adds up to a best-case theoretical resolution of 137 line-pairs per mm on-sensor (i.e. 3,480 line-pairs/inch). The high speed and high pupil ratio of this lens make it potentially capable of significantly high resolution when reversed, at least if it is diffraction-limited (which is a big "if"). Stacking on the Makro-Symmar 120 mm yields an even higher nominal lens speed, although the actual performance is often difficult to predict.

There is no new name for my particular lens version with fixed aperture, which may therefore be discontinued or only available on special order. All the present models of this lens have an aperture ring. I found no magnification data for my version in Schneider literature, but I assume it is the same as the current models.

I am told that the 1:4.0 on the lens label is not a magnification, and is instead the speed (f/4) of the fixed-aperture version. The diameter of the front pupil, 8.3 mm, is quite easy to measure (Figure 4). This is too wide for an f/4 nominal aperture, and instead close to nominal f/2.8. I don't know why the Schneider specification is seemingly incorrect. My specimen of the lens does not seem to have been altered after leaving the factory. Perhaps the Schneider specification refers to the effective aperture when focusing this lens in the close-up range? This seems somewhat far-fetched, but Schneider is not known for making its specifications easy to understand.

Based on the published pupil ratio of 2.25, the diameter of the rear pupil is 18.7 mm. By slightly inclining the lens sideways, I was able to measure the optical distance of the rear pupil from the rear lens element (35 mm, corrected for the slightly oblique measurement). At a magnification of 7.5x (see also below), the rear element allows a cone of light of 58° to enter the lens from an in-focus point of the subject. The rear pupil of this lens version, however, only allows a 22° cone, which limits the NA of the lens, at this magnification, to 0.19. In theory, the rear element could allow an NA close to 0.49. The relatively high diameter of the rear element is explained by the fact that this version is actually a modified f/1.4 lens. A wide rear element also helps to prevent darkening in the periphery of the image.

There was some discussion on photomacrography.net about removing or drilling out the fixed aperture of this lens to produce a faster lens. However, the version of this lens with variable aperture is said not to perform well, at high subject magnification, at f/1.4, and to require stopping down to f/2 or 2.8. Therefore, I did not investigate the possibility of using a wider fixed aperture.

The three currently available models differ only in mechanical specifications (C-mount with manual aperture, ruggedized C-mount, and C-mount with motorized iris).

Tests

I carried out initial tests on a Sony Alpha 7R II (42 Mpixel full-frame). For the first test, the Makro-Symmar 120 mm was mounted at infinity focus on a stack of Schneider V38 tubes that included a short helicoid, useful to focus the lens with precision on a distant landscape and afterwards to lock the focus ring to prevent accidental changes.

When used as a tube lens, the Makro-Symmar almost always needs to be used with its diaphragm fully open, to avoid vignetting. This is the case also with the Xenoplan 23 mm.

Already at this stage I had a suspicion that, in addition to the much recessed front element of the Xenoplan 23 mm, the combination of adapters and extension tubes shown in Figure 5 was likely to cause vignetting. One concern is that a full-frame sensor has a 43.3 mm diagonal, while the M42 Schneider adapter ends with an internal diameter of only 37 mm (including an added anti-reflection material) and is placed only 18 mm from the sensor. Another concern is that the Schneider helicoid has a quite narrow internal clearance, which forces it to be mounted some distance at the rear of the lens, where it does partly obscure the rear element of the Makro-Symmar as seen from the inner edge of the M42 adapter.

The magnification with this setup is approximately 5.8x and the working distance around 17 mm, which is more than enough to provide a good illumination of the subject. A large amount of vignetting occurs (Figure 6).

On the other hand, the amount of detail in the central half of the frame (Figure 7) is impressive, with no evidence of axial chromatic aberration in out-of-focus areas and no lateral chromatic aberration in the periphery. There is some loss of resolution, probably due to spheric aberration, in the outermost regions of the image circle, which is to be expected. The DOF is of course extremely limited and comparable with what we get with relatively high-NA microscope objectives. As a whole, this lens setup could be used without cropping on Micro4/3 sensors, and on APS-C if a slight darkening and loss of fine detail in the corners (or alternatively, a little cropping) are tolerable.

In order to find out where in the setup this vignetting occurs, I tried each of the following steps (after restoring the initial configuration before each new attempt):

• Removing the Xenoplan 23 mm. This largely eliminates the vignetting. Only the extreme corners of the frame remain black. This proves that the Xenoplan lens is the main cause of vignetting.
• Adding a 28 mm long M42 tube between the two lenses. Vignetting increases with respect to the minimum distance between the two lenses allowed by the reversing adapter, so this is not the way to go.
• Removing the reversing adapters and manually holding the Xenoplan lens as close as possible to the Makro-Symmar reduces the amount of vignetting, compared to mounting on the adapters. The corners still darken to a moderately higher extent than after removing the Xenoplan. Thus, the Xenoplan still has an effect on vignetting, albeit less than with a long adapter. It may be possible to cut off the front 5 to 10 mm of the lens barrel to place the two lenses even closer together, but for the moment I will not go that far.

The last attempt was the best so far but, in the absence of a suitable shorter reversing adapter, I continued my tests by replacing the Schneider helicoid and extension tubes with a set of tubes with 57 mm threads, which are more than wide enough to avoid any risk of vignetting. The stack of tubes still ends on the camera side with an M42 to Sony E adapter, which I cannot replace.

For the test in Figure 8-9, I used Zerene Stacker's DMap method to stack a set of 20 pictures, shot at focus increments of 25μm. I did tweak the DMap settings, but saw no differences. The above figures are processed with DMap default settings. With this subject, Zerene Stacker's PMax method gives visually identical results.

Focusing the Makro-Symmar 120 mm to its design magnification of 0.75x moves the lens further out by roughly 90 mm, which I did by adding another 90 mm of extension tubes with a diameter of 57 mm. Magnification becomes 7.5x, working distance about 13 mm, and I used 33 images at focus steps of 15μm to compensate for the decreased DOF.

Given the working distance of 13 mm and the diameter and position of the rear lens pupil, NA at 7.5x computes to 0.19, which is just a little less than the NA of the Mitutoyo M Plan Apo 7.5x (0.21).

Vignetting, as expected, is much reduced (Figure 10). This is approximately the same vignetting I get without the Xenoplan 32 mm, which is apparently caused by the lens adapter closest to the camera body. Image quality in the center is about as good as in the earlier tests (Figure 11). The "good" portion of the image now covers approximately an APS-C sensor. There is a little evidence of axial chromatic aberration roughly 3/4 of the way to the full-frame corners, and farther out toward the corners also lateral chromatic aberration. Curvature of field is also still present, but almost exclusively outside the "good" area.

For the moment I did not investigate whether mounting the two lenses closer together does anything useful. I will update this page if I learn more.

Summary

The Schneider Xenoplan 32 mm f/1.4, reversed onto a Schneider Makro-Symmar 120 mm f/5.9 (Schneider part # SR 5.6/120-0059) focused onto the sensor at its optimal 0.75x magnification, produces a total magnification of 7.5x and a good contrast and image resolution onto the area covered by an APS-C sensor.