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UltravioletPhotography

Suggestions for narrow-pass filter


Bill De Jager

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Bill De Jager

I have one lens that seems to have (and has been suggested on nikongear.com to have) poor correction of wavelengths across the usable UV range, and I expect I'll find more of that sort once I start testing my other acquisitions. The suggested solution was a narrowpass filter. Well, the new Edmund Optics catalog is burning in my hands, with narrowpass filters beckoning. Plus, I now have the correct adapters for my filter box, so I can use a modestly sized filter behind a lens instead of being frustrated that I can't obtain (or afford) a huge UV filter in front of the lens.

 

So, the question is what UV range should I shoot for and how wide of a band? This filter would be initially for general use testing lenses under solar UV. I'd like to be able to record floral patterns though that's not the goal per se. I was thinking of a filter with a center at 350nm, 355nm, 360nm, or 365nm and a 10nm 50% pass width, keeping in mind that some UV beyond the stated band will still pass.

 

At this point I need advice on which of these CWL filters might be best for use as described above while offering the best long-term flexibility for later use on my better UV lenses.

 

Thanks,

Bill

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I'm not sure about the answers to your questions, but I did wonder about the choice of Edmund. Aren't their filters kind of pricy? I thought Thorlabs was a bit cheaper.
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Bill De Jager

I'm not sure about the answers to your questions, but I did wonder about the choice of Edmund. Aren't their filters kind of pricy? I thought Thorlabs was a bit cheaper.

 

Thanks, I'll have to check them out.

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Bill, the key thing about the Edmund narrowpass filters is that some are extremely shiny. This is not good for photography. (I think that the primary customer for such filters is an industrial one.)

 

FWIW, the Edmund filters are excellently made and of high quality.

 

Below are two links to my testing of the hard-coated Edmund Optics 340FWHM10 (OD4) which is mirrored. This filter is passing what might be considered monochrome UV light, but the light is recorded in the R and G channels so the raw look is orange. It is the hard coating which gives the Edmund 340/10 such high UV transmission of between 95-98% around its 340nm peak.

 

The hard-coated, shiny filter creates lots of flare and reflection rings in the photograph. (Something we have seen with other UV-pass filter + lens combinations.) I tried to work around the rings by making huge crops from the middle of the photo, using a contrast slider & a multitude of other editing trickery and attempting to block rays from entering the lens from the side (lens hood or tube). These efforts produced a fair UV photo but not at all a good one.

 

Link showing ring & flare (scroll down):

Trying again with the 340/10

And from an earlier test, link:

[Filter Test 340/10] Gotta work out a problem or two, but...

 

NOTE: I have not yet tried a rear mount of the 340FWHM10.

 

Anyway, all this brings me to a key point when selecting narrowband UV-pass filters --- what lens will you be using and what will your UV illumination be? Like, don't pick a narrowband UV-pass around a peak of 320nm if you don't have a lens which will transmit there and if you don't have a way to illuminate your subject there.

 

That is an obvious statement, of course. But you'd be surprised how many folks forget the part about the lighting. The UV-in-sunlight chart drops rapidly between 400nm down to 300nm.

 

Edmund makes some narrowband UV-pass filters which are not hard-coated. But they have much lower UV transmision. IIRC, transmission peaks are between 20-40% as compared to the hard-coated versions with transmission peaks of 95-98%. The non-hard-coated filters also have a much broader "bleed" outside the band. They seem more like medium-band to me.

 

We can shoot approximately between 300-400 nm in UV. A given object may reflect UV, or not, in different sub-bands of that small 100 nm band just like we can get green, yellow or orange in the band from 500-600 nm. However, at this point, I don't think I've noticed a whole lot of differences between narrowband and broadband UV shots. Shooting carefully set-up narrowband UV exposures remains largely unexplored here on UVP. And I don't know of any narrowband UV-reflecting standards to help control such experiments.

 

FWIW, a filter can have a lot of flaws before it seriously affects a photograph. The primary thing to look for in filter "seconds" sold on Ebay is any dig or scratch which might affect transmission of non-UV light. Serious scratches can also induce flare. Minor scratches, probably not. Tiny bubbles or pits never cause a problem.

 

 

In Summary:

 

* It's probably best to skip hard-coated narrowband filters.

 

* Figure out your lighting before selecting a narrowband peak.

 

* Get a transmission chart and verify that there's no IR-leak.

Just because a filter is narrowband, don't assume all Vis or IR is blocked.

 

* I think that for an initial experiment, choosing a narrowband around 365 nm makes sense

because you could always use an easily obtained UV-LED 365nm flashlight for illumination.

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Bill De Jager

Andrea, thanks so much for your detailed and informative response! I had missed your hard-coated 340nm experiments but they are very informative.

 

Here's how I see the lens issue; let me know if this makes sense. If the lens doesn't go far into UV-A, and if I'm using a Baader Venus filter, then I'll be getting a fairly narrow band anyway which ought to minimize chromatic aberration issues. Based on past experience and discussions here, the unprocessed image should look violet or purplish if the lens doesn't transmit far into the UV-A range.

 

If I'm using the Baader Venus filter and I'm getting a blurry image with a fair amount of red in the unprocessed image, then it's very possible that the lens is seeing well into the UV-A range but is not focusing different wavelengths together. This is what I seem to get with the Nikkor 400/4.5 P lens head when holding the filter in front of the large front element. In this case, a narrowband filter well into the UV-A range (365nm or perhaps lower) should be able to capture a narrow band of wavelengths and perhaps allow a clearer image.

 

Thanks for the warning about the shininess of the hard-coated Edmund filters; I had been looking at the more conventional ones due to their somewhat lower cost. As I stated, I'm thinking mainly of sunlight at this point, and that's still a viable illumination source at 350-365nm or thereabouts at my latitude. I like the idea of the narrowpass filter being lined up with the illumination from a 365nm LED. Eventually LEDs that are practical for UV photography will go to lower wavelengths, but by that point the 365nm ones will just be cheaper.

 

I'm itching to go to smaller wavelengths at some point but that will just have to wait. I do have a couple of monolights with uncoated tubes cooling their heels. I got an amazing deal on used ones and grabbed them while I could, knowing they'd sit around for a while unused. I will also need a safe place to use them without damaging others' eyes or household goods, and that space will be created in the garage after I get some other things done first around the house in the next few months.

 

Despite my lack of UV work for quite a while I've been doing some photography, just in the visible range (nikongear.com). With the drought ended in my part of California and the hills downright soggy, I really need to gear up to photograph wildflowers in visible and UV starting about 6 weeks from now. It's going to be a great spring here!

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In Summary:

 

* It's probably best to skip hard-coated narrowband filters.

 

* Figure out your lighting before selecting a narrowband peak.

 

* Get a transmission chart and verify that there's no IR-leak.

Just because a filter is narrowband, don't assume all Vis or IR is blocked.

 

* I think that for an initial experiment, choosing a narrowband around 365 nm makes sense

because you could always use an easily obtained UV-LED 365nm flashlight for illumination.

Can I add:

 

* rear-mounting of shiny filters behind the lens can avoid the reflection ring problem. I was seeing the same effect you were, Andrea, with my Omega 330WB80, but rear-mounting it solved most of that issue. Try it with the poster putty! It takes only five minutes or so with that method.

 

 

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Forgive me if I have not read everything in this topic, and perhaps my question was already answered, but do you have this problem with only one lens? Have you tried other lenses that have good results?

What camera and where did you have it converted and how long ago was it converted?

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Bill De Jager

Forgive me if I have not read everything in this topic, and perhaps my question was already answered, but do you have this problem with only one lens? Have you tried other lenses that have good results?

What camera and where did you have it converted and how long ago was it converted?

 

The blatant lens is the Nikkor 400/4.5 P. The Pentax 1000/8 seems to have some limitations on how well it can focus in UV but it's not so bad. I suspect the latter lens may have some sort of non-chromatic aberration in the UV range. But in any case I'd like to have a narrowband filter as an option going forward.

 

I actually have several cameras converted to broadband, but I'm pretty sure I used the Nikon D5100 for my informal test of the Nikkor lens. I didn't actually take a photo, but held up the filter in front of the large front element while using live view. The difference from other lenses was dramatic.

 

That camera was converted about 4 years ago by Lifepixel. It does great with a UV-Nikkor so it's not a problem with the camera.

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enricosavazzi
There are a couple of eBay sellers (or possibly a seller with multiple IDs) offering Omega bandpass filter seconds at a fraction of the original prices. They are often advertised as filters for spectrography because many of them have optical defects (e.g. the vitally important interference coatings may be damaged or are starting to peel off). I got some quite usable ones from these sellers, as well as a few (including a couple with multi-100 USD prices) far too damaged and useless for anything (coatings lifted off from 1/4 of the surface, air-corroded surfaces), as well as a few lost in the mail, so you never quite know what you will get. Just make sure you can return them if necessary (which is practical only if you live in the US).
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To test for chromatic aberration in UV: crumple up some aluminum foil, illuminate strongly & shoot. In the editor, zoom in and you will see obvious fringing around the peaks and creases of the crumpled foil from CA in uncorrected lenses.

 

...the unprocessed image should look violet or purplish if the lens doesn't transmit far into the UV-A range.

 

Bill, the false color appearance can be somewhat deceptive because different converters interpret the raw colours differently. Also, the basic raw false colour can vary by filter. Should you become more interested in the "true" false colour in a raw image, then I recommend the Raw Digger app. Here's a link where I looked at the raw false colours from several filters: [Filter Test] Raw Colour Differences in 6 UV-Pass Filters

 

Omega Bob on Ebay should be willing to answer questions about the seconds filters they have for sale. As Enrico has noted, sometimes the digs, scratches and whatnot are too much for a photographic filter.

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The blatant lens is the Nikkor 400/4.5 P. The Pentax 1000/8 seems to have some limitations on how well it can focus in UV but it's not so bad. I suspect the latter lens may have some sort of non-chromatic aberration in the UV range. But in any case I'd like to have a narrowband filter as an option going forward.

 

I actually have several cameras converted to broadband, but I'm pretty sure I used the Nikon D5100 for my informal test of the Nikkor lens. I didn't actually take a photo, but held up the filter in front of the large front element while using live view. The difference from other lenses was dramatic.

 

That camera was converted about 4 years ago by Lifepixel. It does great with a UV-Nikkor so it's not a problem with the camera.

 

Thanks Bill. This is off topic, but the reason I asked in such a way is because I had a D7200 converted full spectrum by LifePixel almost a year back.

I have always been very happy with LifePixel.

I have not used the D7200 much, because I was having poor focus with UV and IR shots. Visual and full spectrum focus shots look OK.

I focus in live view, and have tried various lenses, filters, and all photos with the D7200 look poorly focused.

I have compared to my D7000 full spectrum (also LifePixel), and it gives sharp focus in UV and IR.

So I can't make any sense of it. I have not called LifePixel about this, I need to, and should have a long time ago, but I put the camera on the shelf, so to speak, of things "to do", and never got around to doing it.

I was trying this out again the other day, just before you posted this topic, and I was getting the same blurry results with UV and IR shots.

The camera should focus quite well in IR with my kits lens, as is the case with my other UV/IR cameras. I have no clue, never had this problem before.

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Bill De Jager

Thanks Bill. This is off topic, but the reason I asked in such a way is because I had a D7200 converted full spectrum by LifePixel almost a year back.

I have always been very happy with LifePixel.

I have not used the D7200 much, because I was having poor focus with UV and IR shots. Visual and full spectrum focus shots look OK.

I focus in live view, and have tried various lenses, filters, and all photos with the D7200 look poorly focused.

I have compared to my D7000 full spectrum (also LifePixel), and it gives sharp focus in UV and IR.

So I can't make any sense of it. I have not called LifePixel about this, I need to, and should have a long time ago, but I put the camera on the shelf, so to speak, of things "to do", and never got around to doing it.

I was trying this out again the other day, just before you posted this topic, and I was getting the same blurry results with UV and IR shots.

The camera should focus quite well in IR with my kits lens, as is the case with my other UV/IR cameras. I have no clue, never had this problem before.

 

Cadmium, that's really weird. The usual reason for poor focus after conversion is errors in remounting the sensor, but if visible is doing fine that would seem to not be the problem. I wonder if the new filter sandwich is defective in a way that only affects some wavelengths?

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Do they use a sandwich for full spectrum? I just thought it was one clear filter.

Yes, the D7200 and the D7000 look pretty much the same sharpness with visual and full spectrum, but the D7200 looks very un-sharp with UV or IR,

therefore I have not made any use of it, because most of what I do UV and IR.

I need to resolve that, especially before I have any other cameras converted.

I keep thinking it might be the clear filter..., but I really can't figure it out.

The problem seemed similar to your problem, in a way, and your problem I have not heard of before either myself.

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Bill De Jager
I used the term sandwich without thinking. I suspect it's just a single piece of glass since there is no low-pass filter included.
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.....the D7200 and the D7000 look pretty much the same sharpness with visual and full spectrum,

but the D7200 looks very un-sharp with UV or IR.

 

The D7200 moves up to 24 MP from the 16 in the D7000. My experience with 24 - 36 MP is that you must take the precautions to avoid mirror slap vibrations and hand-holding shakiness. On tripod and using Mirror Up should be the rule.

If you post some examples, we might be able to determine the problem.

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