CoastalOpt 60 mm f/4 Apo  

Figure 1. Jenoptik CoastalOpt 60 mm f/4 Apo.

The Jenoptik CoastalOpt 60 mm f/4 Apo is a special-purpose lens designed to correct axial (=longitudinal in the CoastalOpt brochure) chromatic aberration at three widely spread wavelengths and, in practice, to provide a good correction of this aberration between 310 and 1,100 nm, i.e. well into the near UV (NUV) and near IR (NIR) in addition to the visible range. It is usable between roughly 290 and 1,500 nm, if one is prepared to accept an increase in "focus shift" in the UV and IR and a drop in transmission of one stop at the extremes of this band. In addition, this lens is designed to provide a very high resolution and very low resolution-lowering aberrations as good as, or better than, any high-end camera lenses for the 35 mm format. As a result, the CoastalOpt 60 mm performs very well as a general-purpose macro lens. The primary application and reason of existence of the CoastalOpt 60 mm, however, is in NUV and multispectral photography, where there is simply nothing as good as this lens.

The price of the CoastalOpt 60 mm is higher than that of any current macro lens I am aware of, albeit only half to one-third of the current asking price for a Nikon UV Nikkor 105 mm f/4.5 in new condition. The latter is the closest lens the CoastalOpt 60 mm can be compared to with regard to NUV and NIR performance. In this case, the collector's item character of the UV Nikkor does contribute to its inflated price, but even the Tochigi Nikon UV Nikkor 105 mm, which is basically the same lens as the Nikon UV Nikkor 105 mm and is currently in production, is far more expensive than the CoastalOpt 60 mm, for reasons not clear to me. The UV Nikkor extends much further into the UV than the CoastalOpt 60 mm (albeit this requires photographic film or a "naked" sensor without Bayer filters and microlenses to take advantage of), but the performance of the UV Nikkor in the NIR is worse than the CoastalOpt's. In practice, it can be stated that the CoastalOpt exceeds the wavelength band that can be imaged by an ordinary Bayer sensor devoid of NUV- and NIR-cut filters at both the NUV and NIR ends.

Price-wise, the CoastalOpt, although expensive, breaks no records. Many telephoto lenses commonly used by journalists and wildlife photographers are far more expensive. Even a relatively commonplace Canon or Nikon 300 mm f/2.8 costs more than the CoastalOpt 60 mm.

Figure 2. Focusing the Jenoptik CoastalOpt 60 mm f/4 Apo.

This lens uses an optical scheme with 10 elements in 9 groups. Five of the elements are made with calcium fluoride, which is one of very few glass types that allow the correction of chromatic aberrations in the UV. Calcium fluoride is also easily scratched and more brittle than ordinary lens glass. The optical formula is described in US Patent 8289633. A fixed group of two elements is mounted at the rear of the lens barrel and is used to optimize the optical performance at close focusing range. The remaining elements move outwards together for focusing, and the length of the barrel increases accordingly by 38 mm at the closest focusing distance (Figure 2). The focusing helicoid allows a maximum magnification of 0.67x (1:1.5).

In spite of its extraordinary performance and design parameters, the CoastalOpt 60 mm has been found to suffer from a hotspot problem in the magnification range at 0.33x and above. This issue has been acknowledged by CoastalOpt and discussed on a few web sites, including Nikongear. Diglloyd published an extensive review of this lens, discussing also the hotspot issue. My experiments on a solution to the hotspot issue are available here.

The front filter mount has a 52 mm female filter thread, which is common in Nikon lenses (Canon lenses often use 55 mm filters). The diameter of the front lens element is much smaller (approximately 30 mm). This has practical consequences for the minimum diameter of filters mounted on this lens, which do not need to have a clear diameter of 52 mm but can be substantially smaller, as discussed here. The filter mount is made of metal and unusually thick (nearly 4 mm). In practice, the filter mount is unlikely to become dented even by very hard knocks (but the delicate fluorite elements may crack as a result of falls or strong impacts on the lens barrel).

This lens is mounted in an all-metal barrel with a Nikon F mount, is equipped with a CPU and electrical contacts, and can be used also on low-end Nikon DSLRs that require a "chipped" lens. It is designed to cover a full frame format (36 x 24 mm), but it is sufficiently sharp to be used also on cameras with smaller sensors, especially APS-C and Micro 4/3. While the aperture is automatic on compatible Nikon cameras, focus is only manual. As usual, the aperture ring should be set at the narrowest aperture (f/45) for automatic aperture operation on Nikon cameras. There is no lock holding the aperture ring at this setting. Although f/45 is unusable in the visual range, this aperture can still be used at the extreme end of the UV-A range, because the short wavelength correspondingly reduces the effects of diffraction. Depending on the wavelength, magnification and physical sensor size, an aperture between f/5.6 and f/16 should be used for minimizing the detectable effects of diffraction in the visible range. The serial number is engraved at the bottom of the aperture ring.

The diaphragm has seven blades and a reasonably rounded outline. The aperture ring provides click-stops only at full-stop intervals (not half-stop). It is possible to manually set the aperture ring between click-stops to achieve half-stop intervals reasonably well, but the half-stop position is not stable and easily disturbed by handling the lens. In practical use, I don't find a need for half-stop settings, since I can easily adjust the exposure time or flash intensity whenever necessary.

The focus ring is metal and covered with a relief pattern of broad rectangles. This is not very comfortable to the touch, and some users cover the focus ring with a rubber sleeve or rubber tape to make it more comfortable. Meter, feet and magnification scales are available, as well as markers for depth of field at f/11, 22 and 45. The focus ring is unusually stiff for a DSLR lens, and turning it requires a deliberate effort ad patience. It lacks a locking thumbscrew found on some technical and macro lenses, but its stiffness makes it unlikely that the ring will be accidentally turned, even when handholding. The entire focusing range is covered by turning the focus ring less than three-quarters of a turn, which is not much for a macro lens. This requires a very careful and slow turning of the focus ring near infinity. Since the correct use of the focus ring in macrophotography is not for focusing but for choosing a magnification, this is not a problem in close-range focusing. At the infinity end, the lens focuses slightly beyond infinity, which is necessary to compensate for thermal expansion of the fluorite optical elements at extreme temperatures.

The finish of the external barrel surfaces differs from typical commercial lenses. It is less deeply black and less shiny than, for instance, Nikon lenses with metal barrels. Less shiny means that there is a lesser risk of an image of the lens being reflected by the subject and becoming visible in the images, but a higher risk of soiling the exterior of the barrel. My newly-bought specimen came with a few small blemishes in the black finish, which are inconsequential for the lens performance but might warrant a complaint from users accustomed to the high cosmetic quality of brand lenses. In my specimen, some of the focus grease had spread onto the portion of the barrel that extends while focusing. Again, not a serious problem and easily cleaned, but it could have been caught by Quality Control before packaging and shipping the lens.

The lens model denomination and main specifications are not displayed within or around the filter mount as usual, but instead on a satin metal plate attached between the focus and aperture rings. In itself, this is a nice touch that lessens the risk of the engraved data showing up in the images as a reflection from a mirror-like subject or a glass pane in front of the subject (as typical of museum exhibits).

All writings on the lens barrel are engraved, rather than printed as (sadly) now common even in high-end lenses. The engraved text is filled with four different colors for visibility, like in legacy Nikon lenses. A narrow anodized blue ring is mounted near the front of the focus ring. The CoastalOpt UV-VIS 105 mm f/4 lens was similarly changed from its original garish silver-and-blue finish to the same opaque black finish with only a narrow blue ring showing. As a whole, this is a lens built with mechanical precision and durability in mind. At slightly more than 500 g, it is comparable in weight and handling to legacy macro lenses with metal barrels and manual focus.

This lens was originally designed by Brian Caldwell for Coastal Optics, announced in 2007 and initially marketed by Coastal Optics from 2008, before this company was acquired by Jenoptik and renamed CoastalOpt. This lens is currently still produced at the CoastalOpt facilities in Florida, USA, and each specimen is manually assembled and calibrated for optimum performance. There cannot be many specimens of this lens in circulation, and the majority are probably in the possession of forensic laboratories and police departments. The serial number of mine is 108255, but I am not sure how to interpret it. It seems very unlikely that this lens sold in over 100,000 specimens, so possibly the series started from 100001, which would mean that slightly more than 8,000 specimens were made until the time of writing. This would mean an average production of about 6 specimens per day (excluding weekends), which sounds entirely feasible for a small company. It seems unlikely that only 255 specimens were ever made, even considering the specialized use of this lens, because this would mean that not much profit can be made from its sales.

The CoastalOpt 60 mm, or at least its existence, seems to be well known among photographers (even by many people who don't actually use or need it), and is often quoted as an already legendary pinnacle of performance against which other lenses are compared, in spite of a documented hotspot issue at magnifications higher than approximately 0.33x, which was first discussed in public in 2009. Therefore, this lens is very likely to eventually become a collector's item like the UV Nikkor 105 mm.

This lens has been called, with good reason, "The reference lens for all other lenses" by diglloyd. Because of its excellent performance in the visible range, bulletin boards sometimes discuss whether this lens is worth using in macrophotography or as a general purpose lens (i.e., with a normal camera as opposed to a multispectral one). My opinion is that, because of its price, this lens is a bit of an overkill for use only in the visible range. There are already plenty of good macro lenses (e.g. by Nikon, Canon, Zeiss and even third-party manufacturers like Sigma and Tamron), and certainly there is no scarcity of excellent general-purpose prime lenses on the market. The primary range of use of the CoastalOpt 60 mm remains in multispectral photography. However, if you already own and/or use this lens in multispectral photography, it is certainly appropriate to use it also as a macro lens in the visible range. Although its focusing range is somewhat limited for this application, an extension ring of approximately 25-28 mm can be used to bring it to 1:1. I have noticed no performance penalty in doing this, although, from a theoretical point of view, the rear correcting optical group is optimized only for the nominal magnification range.

UV, around 360 nm.
UV, around 380 nm.
Visible.
NIR, > 850 nm.
Figure 3. Unprocessed sample pictures with Jenoptik CoastalOpt 60 mm f/4 Apo.

In practical use in UV and multispectral photography (Figure 3), the CoastalOpt is a pleasure to use because of its apochromatic correction, with essentially no focus shift throughout most of its transmission range (a little focus shift does appear in the UV-B, but its extent is limited). In the above samples, images where shot hand-held (thereby causing small changes in framing from image to image) on a multispectral-converted Panasonic G3, slightly cropped, converted to B&W by desaturating, but not otherwise post-processed. Pictorially much better results are possible by post-processing, but this is not the purpose of the above tests. Figure 4 is an example of slightly post-processing the last of the above pictures for a more dramatic impression. There is plenty of detail and tonal range for these purposes in the original image.

Figure 4. Slightly post-processed IR picture with Jenoptik CoastalOpt 60 mm f/4 Apo.

Just because the CoastalOpt 60 mm is a true apochromatic lens within a broad multispectral range does not mean, in macrophotography, that you can focus once and then swap any filters and continue to shoot without bothering to refocus. The examples in Figure 5 were shot by focusing in the visible range and then swapping filters without refocusing. A modest amount of post-processing was used to make the images comparable in tone and contrast.

NUV (Thorlabs FB340-10).
NUV (Asahi Spectra XRR0340).
NUV (Baader U).
NUV (Astrodon UVenus).
Visible (Baader UVIR Cut).
NIR (No-name > 850 nm).
Figure 5. Flower imaged with Jenoptik CoastalOpt 60 mm f/4 Apo and a variety of filters.

Note that the center of the flower in the Thorlabs FB340-10 and Astrodon UVenus images of Figure 5 is slightly unfocused (more so in the Thorlabs FB340-10 image). This change in focusing is caused by a difference in thickness among the individual filters. In particular, the Thorlabs FB340-10 is much thicker than most filters (very approximately 6 mm) and the Astrodon UVenus comes second (very roughly 4 mm). The remaining filters have thicknesses between 1.5 and 2.5 mm. The thickness of a filter alters the optical length of the path between lens and subject, and this requires an individual focus readjustment for the unusually thick filters to produce images exactly comparable in focusing with those recorded with thinner filters. Focusing without filters and then shooting with filters is bound to create the same problem. This means, in practice, that a precise focusing in multispectral macrophotography can only be attained with one of the following alternatives:

  • Using a camera capable of live view (which unfortunately is not the case of legacy DSLRs like the Nikon D70, otherwise especially suitable for multispectral photography once properly modified), and refocusing after swapping filters. When using electronic flash, refocusing requires also a continuous light source with a suitable emission spectrum, necessary to verify the focus.
  • Using a set of filters designed to be parfocal. In practice, this means that all filters (or filter stacks) in the set should have the same glass thickness. Some filter sets for astronomy are designed to do this, but unfortunately this does not apply to several of the best available UV-pass filters. In principle, it would be possible to achieve parfocality by stacking a suitably transparent medium (e.g., fused silica for UV imaging) of suitable thickness onto each filter to equalize their thickness, but in practice this is likely to introduce flare, loss of contrast and internal reflections in the filter stack.

The high transmission of the CoastalOpt 60 mm, extending into the lower UV-B, allows a better recording of wavelengths around 300 nm than any other lens I own (although the Nikon UV Nikkor 105 mm, which I don't have, is supposedly superior in this respect). Especially on a small-sensor camera, the 60 mm focal length is more practical for most applications than 105 mm. I believe at this point that I will not need a UV Nikkor for work deeper into the UV. Rather, a specialized quartz lens of short focal length and a UV videocamera with a sensor devoid of Bayer filters and microlenses would be more useful for this application than a UV Nikkor 105 mm, and perhaps also cheaper than the latter.

Summary

The CoastalOpt 60 mm is one of very few (in practice, just two) currently available lenses designed for multispectral photography. To my knowledge, it is also the only such lens optimized to match the spectral response of multispectral-enabled digital cameras. Considering its performance, this lens is well worth its relatively high price if you need the very best for multispectral photography, although it is probably overkill for photography only in the visible range.


web counter