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UltravioletPhotography

Test: Pentax-110


enricosavazzi

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enricosavazzi

The Pentax-110 18 mm f/2.8 is a very small lens, see http://www.savazzi.n...x110lenses.html . It is so small that there is no real reason for leaving it at home when going out with a UV camera. It is also peculiar in that it does not have an aperture diaphragm (the aperture was built into the camera body, just at the rear of the lens). The simplest way to change aperture is to glue a black washer at the rear of the lens. Focus is manual (there is also a fixed-focus, called "pan-focus" or PF, version of this lens, to be avoided because it is really necessary to focus the lens accurately).

 

There are plenty of Pentax-110 to Micro 4/3 adapters available on eBay. There may be at least ten adapters available for each Pentax-110 lens on the second-hand market. They are all the same, with a strong light leak through the cheaply made bayonet locking mechanism. I light-sealed my lens to its adapter with black silicone.

 

Image resolution and contrast are decidedly better than in the Lumix 14 mm I tested recently, and the image remains good in the corners. There is some vignetting in the corners visible in the test image, but it is caused by the 1.25" Baader U filter. There is no vignetting with a 2" Baader U, but this combination is more sensitive to flare and I don't have a suitable lens shade. UV transmission is likely restricted to 380-400 nm (I am not quite ready yet to start making spectral transmission charts), but there is a hint of false color, unlike the Lumix 14 mm.

 

Whole image, reduced

post-60-0-86882700-1488036695.jpg

 

Near center crop

post-60-0-88527200-1488036715.jpg

 

near corner crop

post-60-0-18922800-1488036729.jpg

 

and an image shot through a triple glazed window, just for the fun of it. All the window frames and windowsills are white in VIS, so what we are seeing is real UV, not a VIS leak. The outer window surface I shot through was a bit dirty, which reduces contrast.

post-60-0-28922500-1488036750.jpg

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I have also been comsidering these pentex lens for my nikon 1 camera, i think the good transmission comes from having fewer and thinner glass elements. If can remove the coating layers then it could be even better.
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I have also been considering these Pentax lens for my Nikon 1 camera, i think the good transmission comes from having fewer and thinner glass elements. If can remove the coating layers then it could be even better.

 

For the record, I have done such a thing. I have used an optical-grade polishing creme (with very small micron-diameter abrasion particulates, which is essential to not scratch the glass but only to strip away thin coatings and cleaning marks), and it hasn't really improved UV transmission all that much. Perhaps another 5-8nm of transmission-depth improvement, at best. However, with the stripping away of the front element's coatings, it has dramatically reduced contrast (to the point of rendering very flat-looking, ho-hum images), flaring also significantly increased, and I also started noticing other optical anomalies (e.i. internal reflections, hot spots) with the removal of the coating.

 

My own opinion, hence, is that stripping away the coating introduces far more additional issues than is worth the trouble.

 

Granted, it was a messy job, and I may not have removed the coating evenly across the entire surface of the front element ... but ... from what I am able to gather, the coating is there for a very good reason, in this particular optical build. My initial assessment is that this coating on the Pentax-110 18mm F/2.8 lens doesn't appear to be too UV-suppressive, but it was apparently designed to improve optical performance in other areas by reducing image-degrading optical anomalies.

 

Again, to be fair: My initial attempt was not very well-controlled (I may have failed to strip away adequate coating from some regions of the front element near its edges). It is a very tiny lens, so it is difficult to work with, to begin with. Still, if stripping away only part of the coating (especially the region in the center of the front element) results in such a marked decrease in optical quality, I doubt that removing the remaining coating would make things better; it would only make things worse (my assumption, based on observation).

 

If my initial test is to "prove" anything (if it is indeed reliable enough), it is two points:

 

1. There seem to be more physical factors involved in what governs UV transmission through glass elements, besides just the presence/absence of lens cementing and/or number of glass elements. I suspect that the glass itself (based on how it is smelted and what ratio of minerals are used) could also play a role. Also, the actual geometric curvature of each element, as well as the optical configuration itself, may also affect UV transmission.

 

2. Not all coatings inhibit UV transmission. Some coatings are applied to improve optical quality, without significantly suppressing UV transmission. Therefore, removal of some types of coatings will dramatically degrade the optical quality of the lens, but not significantly improve upon UV transmission. This seems to be a great variable, from one lens to the next.

 

EDIT: Upon further contemplation, I also have to consider the limitations of my insight based on the fact that I only stripped away the coatings from the front element, but not taking apart the lens and stripping away the coatings from each and every element, and from both sides of each element. Perhaps, only then, will there be a fairer assessment. It is possible that UV transmission may indeed significantly improve in such a case, and perhaps some of my observed optical anomalies may not even exist if all element surfaces are identically and uniformly stripped (rather than leaving one element stripped of its coatings, while leaving the coatings intact on the surfaces of all of the other elements). What a tricky game, this is. And how cumbersome of an endeavor, when dealing with such a tiny lens, at that. :unsure:

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enricosavazzi

As Iggy points out, removing the coatings can have serious side effects, most of them undesirable. The outermost coating of the front element, at least in some lenses, is specifically designed to reflect UV, so this coating is the foremost candidate for removal, if one wants to try. However, it is easier to just choose one of the known-good lenses for UV imaging.

 

AR coatings are optimized for a specific wavelength interval, and can also be optimized for a broad or narrow interval. This involves tradeoffs. For example, the AR coatings of the CoastalOpt 60 mm Apo are optimized for an unusually broad wavelength range. The tradeoff is that they do so at the expense of efficiency. They cut reflections to a lower overall extent than the VIS-optimized AR multicoatings used in VIS-only lenses. This might be one of the factors that give rise to the well-documented "hotspot" displayed by the CoastalOpt 60 mm in the presence of off-axis illumination and at certain magnifications.

 

AR coatings can be specifically optimized for UV or NIR, instead of VIS. Some of the optical windows sold by Thorlabs, for instance, can be ordered with either UV-optimized of VIS-optimized coatings.

 

In general, the type of glass and the length of the optical path through the glass determine UV absorption. Lens geometry affects the length of the optical path through the lens, but overall lens thickness is easier to take into account by just looking at the optical scheme. Some wideangles intentionally use one or two very thick elements to correct some aberrations, and these lenses are generally poor candidates for UV imaging.

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This involves tradeoffs. For example, the AR coatings of the CoastalOpt 60 mm Apo are optimized for an unusually broad wavelength range. The tradeoff is that they do so at the expense of efficiency. They cut reflections to a lower overall extent than the VIS-optimized AR multicoatings used in VIS-only lenses. This might be one of the factors that give rise to the well-documented "hotspot" displayed by the CoastalOpt 60 mm in the presence of off-axis illumination and at certain magnifications.

 

I think there is indeed a tradeoff, my quartz cmount 25mm lens seems to have very minimal coating, as the glass elements give colorless reflection, and the lens indeed suffers from hot spots because of multiple internal reflection. When shooting into a extreme bright spot of light, it would appear as multiple images all across the picture. Maybe due to the nature of its short wavelength, there is no effective coating solution.

 

For the record, I have done such a thing. I have used an optical-grade polishing creme (with very small micron-diameter abrasion particulates, which is essential to not scratch the glass but only to strip away thin coatings and cleaning marks), and it hasn't really improved UV transmission all that much. Perhaps another 5-8nm of transmission-depth improvement, at best. However, with the stripping away of the front element's coatings, it has dramatically reduced contrast (to the point of rendering very flat-looking, ho-hum images), flaring also significantly increased, and I also started noticing other optical anomalies (e.i. internal reflections, hot spots) with the removal of the coating.

 

My own opinion, hence, is that stripping away the coating introduces far more additional issues than is worth the trouble.

Thanks for your infomation Iggy, but still I am tempted to try it myself and see what happens when all coatings are removed, maybe with some cctv lens first when I can get them cheap.

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Thanks for your infomation Iggy, but still I am tempted to try it myself and see what happens when all coatings are removed, maybe with some cctv lens first when I can get them cheap.

 

By all means, certainly, such experiments should be repeated and compared by as many people as possible. The more peer-review insights, the better. Because, like I stated earlier, my own coating-removal process was rather messy and non-uniform. Who knows, you may yield better results than my own, if you find a more controlled, reliable, and uniform way to strip away coatings.

 

(My own test should not be the final word, certainly.)

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