The Nippon Kogaku and Nikon EL-Nikkor older series in metal barrels, as well as the newer Nikon EL-Nikkor N series, contain 63 mm models (discussed here). The next focal length in these series is the 75 mm f/4. Oddly, there is also an EL-Nikkor 68 mm f/3.5 in a metal barrel different from the common EL-Nikkor lenses and similar to specialty lenses like the Apo EL-Nikkor, some of the Ultra-Micro-Nikkor and the Micro-Nikkor 70 mm f/5.
EL-Nikkor 68 mm f/3.5
The EL-Nikkor 68 mm f/3.5 (above figures) has a rather large barrel projecting quite a bit behind its rear mount. The aperture ring ranges from F/3.5 to f/16 with click-stops at each full aperture (including the unmarked f/4 very close to the 3.5 mark) and the aperture scale is repeated twice around the lens barrel. I have no literature on this lens (it is not mentioned in any of the brochures available here), and therefore I have no idea about its optimal magnification and optical design. Judging from a partial disassembly of the lens, it seems to be an asymmetric double-Gauss with 6 elements in 4 groups, i.e., not the same design as the EL-Nikkors 63 mm (in which the rear doublet is located in the rearmost position).
The lens mount is the standard 39 mm thread, and the filter mount is 43 mm, like the EL-Nikkor 135 mm f/5.6 of the older series. Therefore, a 43-52mm step-up adapter ring and a Nikon BR-2A can be used for reversing this lens. The barrel is mechanically quite similar to the the Micro-Nikkor 70 mm f/5, albeit not made of brass like the latter, but more conventionally of aluminum alloy. The lens coatings have also a similar deep red-purple colour. Like in some Ultra-Micro-Nikkors, the lens specifications are engraved around the outer rim of the filter mount. The interesting thing in these markings is that they say Nikon, not Nippon Kogaku. Therefore, the lens should not be as old as the Micro-Nikkor 70 mm and other specialty lenses, and might date from the 1970s or 1980s. The serial number begins with 740 and continues with three digits counting progressively. Mine is 250, which tells us that at least 250 specimens were made. My searches turned up only one other confirmed specimen (i.e., with serial number visible) illustrated on the web.
This lens is so scarce that I saw it only once on eBay in the last few years (and of course I gladly gave it a new home). Such a rare and poorly known lens does not have a definite value, and its price depends on how badly one or more collectors will want it at the specific time a specimen shows up for sale. Judging from past experience, a rare but obscure lens usually does not fetch much. In my case, I got it for about one-third of my actual bid, and much less than the far more common 63 mm f/3.5 fetches.
Knowing so little about this lens, I decided to test it in photomacrography, in the close-up range and in IR, UV and multi-spectral photography. I already knew from web postings that this lens is excellent in landscape photography, as well as in the close-up range in visible light. Although it is marked EL-Nikkor, the barrel details suggest that it might be more closely related to the Apo-EL-Nikkors or some of the technical lenses, so I started my tests under the suspicion that its performance could be better than that of more common models of EL-Nikkor.
Lens performance in UV photography
Assuming that this lens is corrected in the near-UV to an extent comparable to other EL-Nikkors (which are tested here), I expected it to be a good performer down to approximately 365 nm. The test camera is a UV- and IR-enhanced D70s. As a light source, I used an UV-modified Vivitar 286HV electronic flash, and a Schuler filter on the lens. The latter is necessary because the filter used for the modified flash lets through a rather high amount of IR. As a comparison lens I used the UV Rodagon 60 mm f/5.6, which is designed for use in the near-UV but displays no focus shift between the near-IR, visible and near-UV ranges.
There is a focus-shift of the EL-Nikkor in the near-UV, toward the rear of the subject. This is evident at all subject magnifications, but especially in the macro range. The EL-Nikkor is much easier to focus than the UV Rodagon, because of its wider maximum aperture, but the amount of focus-shift is sufficient to create problems with DOF, even at f/11. As a result, if focus-shift is not compensated for manually, pictures with the EL-Nikkor are visibly out of focus even when substantially reduced. Therefore, it appears that this EL-Nikkor is not designed for colour-correction in the near-UV, unlike the more common EL-Nikkor models.
The luminance histogram of the two lenses is virtually identical. However, the colour balance is different, with the UV Rodagon producing redder tones and the EL-Nikkor more magenta ones. This indicates a different spectral distribution of transmitted light in the UV range. As a whole, however, the total UV transmittance of the two lenses is comparable, as indicated by the picture luminance.
The EL-Nikkor appears to have a better resolution than the UV-Rodagon in the near-UV, if the focus setting is compensated for focus-shift.
Lens performance in IR photography
Also in this case I used the UV Rodagon 60 mm as comparison. The test shots were taken with an 820 nm IR-pass filter, which is deeper into the IR than ordinary for this type of photography (in most cases, a 720 nm filter is used). The purpose of this choice is making any focus-shift more easily detectable. This filter also removes any colour information from the test pictures, while a shorter cut-off wavelength may still record some visible red. An incandescent 40 W bulb was used as light source, and focusing was carried out in the visible range before mounting the IR-pass filter.
The two lenses perform in very similar ways in the near-IR. The EL-Nikkor displays a small amount of focus-shift at >820 nm, which, however, is sufficiently compensated for by closing the aperture at f/11, even in the macro range. In this respect, it performs better than common EL-Nikkor models. This lens displays no problems with internal reflections or central spot with the tested subject.
Lens performance in close-up visible photography
The Micro-Nikkor 70 mm f/5, Micro Nikkor 60 mm f/2.8 D and AF-S Nikkor 18-70 f/3.5-4.5 G ED (at 70 mm) are the other choices for this comparison. All lenses were tested at f/11. Unlike the tests on this page that involve UV and/or IR, a D200 was used as test camera. This test was carried out with the camera-to-subject distance kept constant. I did not bother to change the latter because the focal lengths of these lenses are fairly similar, and the expected difference in magnification not very high (which turned out as expected, with one exception).
The Micro-Nikkor 70 mm produces the best resolution among the tested lenses. It easily lifts out and renders small detail invisible with any of the other lenses tested here. This is what this lens is designed for, and expected from earlier tests. The EL-Nikkor 68 mm is quite good, but not extraordinary. In fact, it is about equal to the Micro Nikkor 60 mm. The latter shows a magnification a bit lower than expected, most likely because the rear compensating element reduces the actual focal length when the lens focuses at a close distance. This is quite common in modern macro lenses, and in this case the actual focal length is probably around 55 mm at this distance. The real (and negative) surprise of this test is the AF-S Nikkor 18-70 mm f/3.5-4.5 G ED, which, in spite of being set at its maximum focal length of 70 mm, behaves roughly like a 48 mm. The used distance is still within the focusing range of the lens (actually with a few cm to spare). This lens also provides the lowest resolution in this test.
Lens performance in close-up multi-spectral photography
Although I initially planned this test, the substantial amount of focus-shift of the EL-Nikkor 68 mm in the near-UV makes this lens clearly unsuitable for multi-spectral photography. Therefore, this test was not carried out.
Lens performance in photomacrography
For this test, I compared the reversed EL-Nikkor 68 mm with the Macro-Nikkor 65 mm f/4.5 and the reversed Micro-Nikkor 70 mm f/5. All lenses were tested at f/8. Since the Macro-Nikkor 65 mm is designed for magnifications higher than 1x, it must not be reversed. The magnification in this test is approximately 4x.
The EL-Nikkor 68 mm performs brilliantly in photomacrography. Although the Micro-Nikkor 70 mm is the sharpest among all the lenses not designed for photomacrography that I tested for this application, the EL-Nikkor performs on par with this lens, perhaps very slightly better in some details and very slightly worse in others (or it simply might be a matter of slightly different aliasing by the sensor, because we are already at or beyond the practical maximum resolution of the camera). We are also dangerously close to the point where diffraction affects resolution (in fact, it already slightly does so according to theoretical calculations).
The EL-Nikkor 68 mm and Micro-Nikkor 70 mm also perform virtually indistinguishably from the Macro-Nikkor 65 mm (which is arguably the best photomacrographic lens ever made, and slightly better than the Zeiss Luminar 63 mm). At this magnification, focusing is difficult even with a microscope focusing rack, and the image in the viewfinder is quite dark (especially with the Macro-Nikkor, which explains the different placement of the focus plane in the test picture). The one-stop difference in maximum aperture between the EL-Nikkor and the Macro-Nikkor make the former much easier to focus, and preferable for this application.
The EL-Nikkor 68 mm f/3.5 is good but not exciting in close-up photography in visible light, and is instead extraordinarily good in photomacrography. At 4x, it is equal to the Macro-Nikkor 65 mm in resolution and contrast, and much easier to focus than the latter lens because of the wider maximum aperture. The EL-Nikkor 68 mm is not designed for colour-correction in the near-UV, and dsplays a substantial amount of focus-shift in this application. Although the EL-Nikkor transmits near-UV very well and its resolution in the near-UV is better than with the UV-Rodagon 60 mm, the EL-Nikkor 68 mm is not suitable for near-UV photography, especially since better alternatives are available among more common EL-Nikkors. The EL-Nikkor 68 mm, on the other hand, performs well in the near-IR, with little focus shift even at wavelengths above 820 nm.