Schneider Xenoplan 23 mm f/1.4
|Schneider #||current name||F||M range||M||DM||D∞||Notes|
|SR 5.6/120-0058||PYRITE 5.6/120/1.0x V38||120.68 mm||0.88-1.13x||1.0x||236.5 mm*||115.8 mm*||Do not reverse|
|SR 5.6/120-0059||PYRITE 5.6/120/0.75x V38||120.24 mm||0.63-0.88x
|205.6 mm||115.4 mm||Tested here|
|SR 5.6/120-0060||PYRITE 5.6/120/0.5x V38||119.75 mm||0.38-0.63x
|174.8 mm*||115.0 mm*|
|SR 5.6/120-0061||PYRITE 5.6/120/0.33x V38||118.92 mm||0.26-0.35x
|153.3 mm*||114.1 mm*|
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 (DM) and focused at infinity (D∞). The latter, of course, corresponds to M = 0. Incidentally, the DM I measured agrees very well with the value I computed from the parameters provided by Schneider.
DM 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.
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.
|Schneider #||current name||M range||M||Notes|
|23-0902||CITRINE 1.4/23 C||0-0.1x||?|
|23-0942||CITRINE 1.4/23 C-R||0-0.1x||?|
|23-1902||CITRINE 1.4/23 C-MI||0-0.1x||?|
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).
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):
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.
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.
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