Accidental lenses for UV photography:
Movie and small format lenses
Lenses for UV photography fit into two main categories:
- Lenses designed for UV photography. These lenses may be called intentional UV lenses.
- Accidental UV lenses, on the other hand, were designed for other purposes (usually, imaging in the visible range), but unintentionally perform well enough in the near-UV to be usable in UV photography with modified digital cameras.
The second category includes several legacy enlarger lenses, as well as lenses designed for general photography with system cameras. Most of the latter, albeit not all, are legacy film SLR lenses. Several pages on this site deal with both categories of UV lenses, and are reachable from the photography index page. An extensive and updated list of UV lenses in both categories, including numerous accidental UV lenses discovered over the years through the testing by several UV photographers, is available on the Ultraviolet Photography web site.
Among focal lengths of 50 mm and higher, several enlarger and camera lenses perform well enough and are still easy to obtain at reasonable prices on the second-hand market. Among legacy 35 mm f/3.5 SLR lenses, several good choices are also available. There is a scarcity, instead, of lenses with shorter focal lengths usable for UV photography. Therefore, in this test I concentrated on lenses of focal lengths 25 mm and lower. Even within this interval of focal lengths, I did not test all lenses reported as usable in UV photography. For instance, I did not include in this test the Nikon 18 mm f/4 for a number of reasons, mainly because it has been reported as usable but not excellent in UV, and because this lens is quite large, heavy, and probably more expensive than it is worth for UV photography. Since I am mainly interested in UV photography with Micro 4/3 cameras, I concentrated instead on relatively small and lightweight lenses that might be expected to work satisfactorily on this format.
This page discusses a few accidental UV lenses originally designed for small-format film imaging, as well as a few native Micro 4/3 lenses. A few of these lenses have already been tested by other photographers and found to be capable of variable degrees of UV imaging, while others probably have not been previously tested for this purpose, or at least are not yet included in the list of UV lenses mentioned above. Some of the legacy lenses discussed on this page were designed for small-format film system cameras (Pentax 110 and Olympus Pen "half-frame"). Others were instead meant for movie cameras of different formats.
All lenses discussed on this page were tested with an Olympus E-PM2 converted for multi-spectral imaging. None of these lenses is designed for UV imaging, thus I tested them only in an interval between approximately 325 and 380 nm, with illumination by UV-enabled electronic flash (Bowens 1500Pro) and the following UV-pass filters:
- Baader U (approx. 335-380 nm)
- Asahi Spectra XRR0340 (approx. 295-375 nm)
- Omega 325BP10 (approx. 320-330 nm)
The last of these filters yielded completely black frames with all lenses. These results are included for completeness, and because it may be useful to have a sample of true black for visual comparison with the very dark frames produced by some lenses with other filters.
Three types of test were performed:
- Vignetting, with the lens focused at infinity and lens aperture set at f/5.6. This aperture is a compromise between allowing a relatively short exposure and a good optical performance, and is available on all lenses tested on this occasion (albeit, see the description of the Pentax-110 lenses). The illustration show reduced versions of the whole test images. The subject is a featureless PTFE sheet, unfocused. Vignetting is only evaluated qualitatively, i.e., a lens is regarded as acceptable in this respect if it does not produce a marked darkening in the corners. Since the test subject was illuminated obliquely by a single off-center flash unit, the illumination slightly grades across the width of the test images.
- NUV transmission test, relative to a reference lens (CoastalOpt 60 mm f/4 Apo). This lens has a virtually flat transmission curve in the VIS and in the used NUV range, and is one of very few multispectral lenses covering up to 24 x 36 mm sensors currently produced. It is a natural choice for this type of test. This test consisted of photographing a PTFE 4 mm thick sheet as UV and VIS white target. This material has an approximately constant reflectance in this range of wavelengths. This target was first imaged with the reference lens, and with each filter an appropriate exposure was manually found by bracketing the flash intensity at a constant lens aperture of f/5.6. The same filter, flash intensity and lens aperture were subsequently used for test shots with each of the lenses discussed on this page. This allows the images to be compared with the reference in order to assess the amount of UV transmission of each lens. The test images also allow the detection of any significant vignetting.
- NUV imaging performance. Because of the varying focal lengths and minimum focusing distance of the tested lenses, the subject magnification varies among the test images. To allow a meaningful comparison, exposure in this test was adjusted to produce consistent results among lenses. Only the results in the center of the frame are shown, but image resolution in the peripheral regions is discussed when relevant.
The criteria for an accidental lens to perform well in NUV photography, according to these tests, are:
- Good image resolution and contrast, and lack of detectable hotspots and internal reflections in NUV.
- Ideally, a lens for multispectral photography should display no focus shift across NUV, VIS and NIR. However, this criterion is matched by just a few of the very best lenses designed for this application. A few accidental lenses of medium-long focal lengths approach this characteristic. Since none of the tested lenses of short focal lengths meet this criterion, I decided to relax this requirement to include only the relevant NUV band (roughly 330-390 nm). The amount of focus shift between VIS and NUV was not accurately measured in all lenses, but is discussed in a few cases.
- Moderately good transmission at the NUV wavelengths of interest. This criterion is important, but must be evaluated together with other criteria. A lens may attenuate NUV by 2-3 stops compared to VIS, but still give good NUV images. On the other hand, a loss of 4-5 stops or more is excessive, especially if a sharp cutoff in NUV transmission takes place within the tested range of wavelengths.
Tested lenses and test results in brief
- Olympus G.Zuiko Auto-W 20 mm f/3.5
- No vignetting
- Slightly lower center NUV resolution and contrast than other lenses. Expensive on the second-hand market.
- NUV attenuation: 2-3 stops
- Pentax-110 18 mm and 24 mm f/2.8
- No vignetting
- Good NUV resolution and contrast with the 18 mm. No adjustable lens aperture (must use a washer as fixed aperture, or mount a small diaphragm in a lens adapter). Rather low resolution and massive focus shift with the 24 mm.
- NUVattenuation: 1 stop
- Sigma 19 mm f/2.8 EX DN for Micro 4/3
- No vignetting
- Good NUV resolution and contrast. Rather high NUV attenuation but could still be usable. AF available.
- NUV attenuation: over 3 stops
- Olympus 12 mm f/2 for Micro 4/3
- No vignetting
- Slightly lower center NUV resolution than other lenses, excessive attenuation. AF available.
- NUV attenuation: over 5 stops
- Mir 11 and Vega 7 in a variety of mounts
- High vignetting in Krasnogosk mount, lower vignetting in other mounts
- Good NUV resolution, contrast and (in some versions) transmission, but vignetting is a problem. It can be corrected only with substantial cropping.
- NUV attenuation: 2-3 stops
See the above links for more details. Some of these lenses display a lower resolution and/or field curvature in the corners, not examined in this test. Since these are accidental UV lenses, you should not expect results comparable with those of true UV imaging lenses. On the other hand, no wideangle lenses designed for UV photography are available.
None of the tested lenses display a usable UV transmission at 330 nm. Some of them are usable at 360 nm, but most are only useful in the 370-400 nm range.
Several other lens models of short focal lengths are comparable to the ones tested on this page. For example, it would be interesting to test the Olympus 17 mm f/2.8 because of its short focal length and relatively simple optical formula. On the other hand, it is very likely a waste of time to test modern vidocamera lenses in C and CS mounts, because their small image circles, low resolution and UV-absorbing multicoatings make them unsuitable for NUV imaging on Micro 4/3 cameras.
Nine small-format camera and movie lenses of short focal lengths were tested for NUV photography on Micro 4/3. Most of them give a good image resolution and contrast in NUV photography, but some suffer from vignetting, or from excessive NUV attenuation. The Pentax-110 18 mm f/2.8 is perhaps the most interesting, although it lacks an adjustable lens aperture.