Soligor 300 mm f/5.5
Soligor marketed several models of 300 mm f/5.5 lenses, usually very cheap on the second-hand market. The model shown above uses an early style of barrel, aperture and focus rings. Since a comparable style is also found among some of the Soligor 35 mm f/3.5 lenses known to perform well in UV photography, I decided to test this 300 mm lens in UV imaging. Its H80175 serial number does not match the information summarized on flickr, so I have no idea which brand actually produced this lens model.
This lens uses a preset aperture ring marked from f/5.5 to f/32. I tested this lens at f/11, since image quality at f/5.5 is not quite acceptable (although better at f/8). For this test, I used a full-spectrum Sony 7 II, Baader U filter and, for comparison, Coastalopt 60 mm f/4 Apo. The subject is a sunlit car.
This lens does not have a tray or insert for internal filters. I don't have a Baader U filter that fits the 62 mm front filter thread of this lens (and I don't think it is commercially available as a standard size). For this test, I simply fit a 2" Baader U onto the front filter thread with a step-down ring. This does cause a modest darkening in the corners of the frame with the lens fully open, but no vignetting is visible at f/11.
For a meaningful comparison, I recorded both images with a custom white balance created with the Coastalopt 60 mm. Because of the different focal length of the two lenses, I cropped the image taken with the Coastalopt 60 mm to approximately match the one taken with the 300 mm, then reduced both images for web presentation. I did not otherwise post-process the two images.
Interestingly, both lenses show well the difference in false-color between the front of the car (and possibly the bonnet/engine hood) and the rest, indicating that parts of this car exterior had substantial replacements, repair and/or repainting, likely after a crash. The car is light gray in VIS, with no obvious mismatches in color.
The image recorded with the Soligor 300 mm shows a slight bluish color cast, which suggests that the lens transmits less UV in the 350-375 nm band than the Coastalopt. However, the difference is only moderate. Although the Coastalopt also displays a higher contrast, the Soligor is not bad in this respect.
The color histograms of the two pictures show the same story in a different way. Both histogram heights are normalized, so the high green peak near the left side of the Soligor histogram causes the rest of the histogram to be artificially compressed in height along the y axis. Higher color values (= lighter colors) are on the right side of the histograms. Yellow is a superposition of red and green values. Magenta is a superposition of red and blue. White is a superposition of all three primary colors.
All the green and yellow peaks in the Soligor histogram are shifted toward the left, compared to the Coastalopt histogram. This indicates an overall lower amount of false-yellow and false-green is present in the Soligor image, relative to false-blue (as expected, since these seem to be the false-colors that roughly correspond to shorter UV wavelengths). The yellow peak at the center-right in the Soligor histogram is well separated from the blue peak, while all three primary color peaks are largely superposed in the Coastalopt histogram. This peak corresponds to the large expanse of sunlit pavement in the test images, which is more neutrally colored in the Coastalopt image, and bluish in the Soligor image.
False-yellow and false-green are indeed present in the Soligor image, although to a moderately lower extent than in the Coastalopt image. Therefore, a custom white balance created with the Soligor lens can, at least in theory, compensate for the relatively lower levels of these false colors (except for their peak near the left edge of the histogram, where too many shades are squeezed together). Whether a specific camera can successfully create the required custom white balance, however, is another matter entirely. If false-yellow and false-green were instead only present at the extreme left in the Soligor histogram, there probably would be too little of these false-colors present, for a custom white balance to successfully expand their range without posterizing artifacts.
The exposure time came definitely as a surprise to me: at f/11 and ISO 1600, both lenses required the same exposure time of 1/8 s. It is perhaps the first time that I had this type of result with a legacy lens not designed for UV imaging. With legacy wideangle lenses, for example, I commonly see a loss of 2 to 4 stops in UV, compared with the Coastalopt. Even legacy lenses known to perform well in UV often display a loss of 1-2 stops.
The Soligor lens displays a detectable amount of "UV focus shift" with respect to VIS, which requires the lens to be refocused when swapping pass filters for the two bands.
It is at present unknown to me how the numerous other models of Soligor 300 mm f/5.5 perform in UV imaging. It all depends on their coatings and optical formulas, which are largely or totally undocumented.
The tested Soligor 300 mm f/5.5 performs quite well in UV imaging, and is perhaps the first legacy lens in my possession to provide a UV transmission comparable, for practical purposes, to the Coastalopt 60 mm Apo. It differs from the latter lens in a lower transmission in the shorter UV wavelengths, but a correct custom white balance (if allowed by the camera) should largely compensate for this loss.
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