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UltravioletPhotography

Why is this? (1)


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I've noticed that when photographing flowers which are yellow in the visible spectrum (c. 560-590 nm) they very often come out yellowish in my UV shots (both on converted EOS M and Sony a6000, using U340+S8612, flash, white balanced against PTFE): I understand that yellow in the UV shots indicates wavelengths of around 350-365 nm). Examples of this are buttercup, dandelion, forsythia, geranium, japonica, laburnum, nasturtium, welsh poppy, sunflower.

 

And flowers that are blueish in visible (400-500nm) often look blueish in UV (380-400nm). Examples are periwinkle, toadflax, aquilegia, campanula, bluebell, delphinium. forget-me-not, hyacinth, iris.

 

Why is this? Just co-incidence?

 

I guess in the case of blue, the two bands are next to each other, so perhaps not so surprising. But yellow?

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Andy Perrin
I think it IS just coincidence. The false colors depend on the sensitivities of the dyes and camera in the UV, but those dyes could have had any response at all in UV since they are not designed for that band. The exact reason for the false colors is a subject of ongoing and endless debate in these parts (which maybe you know if you were lurking previously). But conceivably a different set of dyes could have been used to produce a "true" UV false color, where by "true" I mean each channel would respond to one sub-band of UV light in the same way that the visible light response is designed. (Say, 300-340nm = blue, 340-370 = green, 370-400 = red.) And which channel to assign to which sub-band would be up to the designer.
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Andrea B.

The yellow Iris pseudacorus produces both yellow and blue false colours when white balanced against the Labsphere white standard or some PTFE. Some yellow sunflowers Rudbeckias produce such a pale yellow false colour that you could almost call it white.

 

I will try to find the links for these examples.

 

Note: All my examples have been white balanced against a Labsphere Spectralon 99% reflective white standard.

 

yellow -> yellow & blue in the Iris pseudacorus

https://www.ultravio...nother-example/

 

yellow -> almost white with pale tints of blue, yellow in a Rudbeckia hirta cultivar

https://www.ultravio...gloriosa-daisy/

 

Other yellow examples

 

yellow -> yellow & blue in Jersusalem Sage

https://www.ultravio...jerusalem-sage/

 

yellow -> all UV-black in Coreopsis verticillate

https://www.ultravio...dleaf-tickseed/

 

yellow -> all UV-black in Daisphora fruticosa

https://www.ultravio...bby-cinquefoil/

 

yellow tips -> blue tips in a Zinnia bicolor cultivar

https://www.ultravio...bicolor-zinnia/

 


As for blue, I think that, yes, it is mostly true (so far!!) that a bluish flower will have either a blue false colour or a UV-black false colour. I thought I remembered a bluish campanula which became false white, but I couldn't find it. So I might be mis-remembering that.

 

 

.

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Thanks, Andrea.

 

Beautiful photographs - really illustrate the difference between specialist UV lenses like yours and the vintage UV-tolerant lenses that I use. I spent a few idle moments yesterday Googling around for specialist UV lenses - and concluded that I would just have to keep on trying to squeeze what I can out of my Cassar S and El-Nikkor 105mm.

 

Still, my Baader U should be arriving tomorrow. Perhapsthat will help me up my game.

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Andrea B.

Bernard, you will do just fine with a UV-capable lens while you learn to get the best out of your UV gear. Those of us who have the specialist lenses like the UV-Nikkor or Rayfact, for example, are not always using those lenses to their full capability. Your UV-capable lenses will work well between 350/365 - 400 nm which is where we typically shoot anyway for landscapes or portraits.

 

Is the EL-Nikkor 105 an old or new version? The old EL 105 is quite nice for UV.

 

I think that having good UV light is quite important so do look into getting a UV-flash of some type. And perhaps an auxiliary UV-LED torch to aid in focusing on Live View.

 


 

I am still musing over the blue --> false blue observation. :D High UV (370-400 nm) apparently passes through the Bayer filter in the same way reflected visible violet or low blue-violet does. So it would be recorded raw as a magenta/violet which would white balance to the blue/violet area. But if you had a blue flower which was reflecting UV in the region (300-340 nm, say), then what would we get after white balancing? I don't know. I don't even know if there are any blue flowers which reflect low UV.

 

People are probably tired of hearing me say not to make too much out of the false colours of UV. There are just too many factors at play in their production. :lol:

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Andrea - I got an old-style El-Nikkor 105mm for UV. Tested it against the Cassar S which I had been using, and it seemed identical in UV transmission. I did some testing on the Cassar which seemed to support data I had got from UVP somewhere that it could just about record at 320nm. I'm in the process of getting some narrow band filters to see whether this is true and if there is enough sensitivity to get images.

 

I've also got a 2.8/50mm El-Nikkor for visible macro work but went for the new type on that: it's excellent when used with bellows. I gave it a test for UV, but it was pretty poor - about 2 stops worse than the Cassar S and a definite colour cast which I assume means it was cutting off the shorter wavelengths.

 

I had my first real failure today. Tried to get a tulip, but the flare from the outside of the flower nearest to the lens/flashguns just washed out the detail in the interior. I guess the problem is the U340+S8612 filter sandwich. Can't wait to see whether I have more luck with the Baader U which should be arriving tomorrow.

 

On flashes, I could do with some advice/suggestions. I got a secondhand Mecablitz CT45, which was one of the recommended ones - but that died on me after about 100 flashes. I now use three cheap* but powerful WS-560 (with lenses removed), but that's not practical for out of the "studio". The SB-140 seems highly rated, but too expensive for my taste. Any suggestions?

 

* Someone is selling these new on ebay for about $20 each!

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Andrea B.

The problem at 320 nm is having enough 320 nmm light with which to make the photograph. UV-LED torches are usually at 365 nm peak. So you need a UV-flash. The dropoff in sunlight is huge between 400 -> 300 nm. Choose a high summer day with strong sunlight if you are not using a UV-flash boost.

 


 

Tried to get a tulip, but the flare from the outside of the flower nearest to the lens/flashguns just washed out the detail in the interior.

 

I usually use a 15 - 30" exposure during which I flash 2 to 3 times around or in the flower, but not so close as to cause washouts. This can only be done indoors.

 

For a jonquil or tulip you can "peel back" a petal for a view of the interior. Not a natural effect, but still interesting.

My old King Alfred cultivar>

For the record I did not use my flash multiple times in that series as is obvious from the shadows. Intentional though. :)

 

Bulb flowers like tulips, narcissus, crocus and the like are often iridescent in UV. So again, controling the flash angle and firing flash more than once helps may help.

 

The propensity for flare is of course increased with stacked filters, but I've never really seen it much with the typical 2-4 mm stack unless photographing directly into the light.

EDIT: This sentence was restructured for clarity.

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dabateman

I have only used the Canon 199A flashes. They are good for UVA, greater than 360nm. In all my tests for lower wavelengths, they haven't been good. Others seem to use modified Godox flashes. There are many posts about them.

 

Here is my post on mounting small bandpass filters:

https://www.ultravioletphotography.com/content/index.php/topic/3125-mounting-25mm-filters/page__hl__%2Bsmall+%2Bfilter__fromsearch__1

 

I have a bunch of them, the best ones are 390bp25 and the 330Wb80 improved filter. But when I got a second 330WB80 improved filter, it wasn't as good, So I returned it.

 

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Thanks for your advice, Andrea/Da. I've used multiple flashes a few times, but it's not really practicable for most of my shots where I use focus stacking and create stereo pairs. So it takes 100-200 shots to get one final image. If I multiple that up a couple of times for multiple shots then first of all I'll die of old age because of the time it would take, but also my flashguns would exhaust their batteries before I complete the sequence.
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A typical call for a studio flash.... run them off A/C mains and there is all the power in the world for the entire stack.
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Andrea B.

Agreed. Studio flash is the only way to go for deep stacking needs. And the studio work.

Or perhaps a couple of Blak-Ray lamps?

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Thanks for the ideas. I just don't have the space for studio flash kit, but the Blak-Ray might be an idea - there are a few on ebay.

 

Da - re. mounting small 25mm-ish filters, I've been in discussion with Bob at Omega Optical about getting some suitable filters to try out the idea of making 3 colour-separation images in the UV spectrum to create full-false-colour images. He's going to put a special set of filters together to give me a fighting chance of getting somewhere, and has offered to put them in 28mm mounts for $8 more. So that's solved that problem.

 

I also found a solution to the problem discussed above about failing to get a satisfactory image of a tulip. The answer was to choose the right tulip! I tried one that had opened a lot more, and was dark in UV so that there wasn't any flare off the petals and not too much contrast between the inside and outside. The result was some excellent images. Below are a couple of 3D stereo shots, and 2D versions for those that don't have red/cyan glasses - but to be honest they only come to life in 3D. The 3D versions work very well at full quality, letting you zoom in and wander round the flower. Included is a visible light shot for comparison.

 

post-245-0-30142300-1557436329.jpg

 

post-245-0-12676900-1557436334.jpg

 

post-245-0-43836700-1557436330.jpg

 

post-245-0-91668400-1557436329.jpg

 

post-245-0-74289900-1557436334.jpg

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Andy Perrin
Very nifty, one day I will get myself some of those glasses! The focus stacking must work very well -- those are all quite sharp everywhere.
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Thanks, Col - yes, the daffodil came out really well in 3D.

 

Andy - yes focus stacking works really well - the sharpness across the whole image is at the best level that the lens can give. In these shots I'm using an El-Nikkor 105mm outside of its comfort zone, so the image quality is OK but not superb. Using focus stacking in visible with a proper macro lens can give stunning results. I bet using a dedicated UV lens would give fabulous results. It's not always the best approach through - in 2D you might want differential focussing to make the main feature stand out. But in 3D you want everything in focus so that the eye can look around and settle on whatever grabs your interest.

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I am jealous. I have a nice collection of stereo views for the old time stereopticons, but a few years ago I suffered retina detachments, and now have only one good eye, so 3D has become lost to me. :( :angry: :(

 

What software are you using for focus stacking, if I may ask?

 

Steve

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Sorry to hear that you are down to 2D now, Steve. Fortunately most people take only 2D anyway, so most of the time you aren't missing anything.

 

I use Zerene for focus stacking. It's the only photo processing software I have ever paid for - otherwise I've always found freeware I can use (I have an old Photoshop Elements, but even that was a gift!). I tried some of the free focus stacking software like Picolay, but found I had to play around with settings, and always ended up with some undesirable artefacts in the final result.

 

I tried Zerene and Helicon (I think the name is) on their 30-day free trials. Helicon ran faster, but I found that Zerene gave better quality results. I've been very happy with it - hardly ever get any failures, and never have to play around with settings. It has a synthetic 3D capability, which gives an impression of depth from a single image stack. I haven't used it much, and my results have been mixed.

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Marianne Oelund

Below are a couple of 3D stereo shots, and 2D versions for those that don't have red/cyan glasses - but to be honest they only come to life in 3D. The 3D versions work very well at full quality, letting you zoom in and wander round the flower. Included is a visible light shot for comparison.

 

post-245-0-30142300-1557436329.jpg

 

Bernard: I love these! But I don't have red-cyan glasses, and am accustomed to cross-eyed 3D viewing. Do you have a left/right image pair you could post?

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Marianne Oelund

I've noticed that when photographing flowers which are yellow in the visible spectrum (c. 560-590 nm) they very often come out yellowish in my UV shots (both on converted EOS M and Sony a6000, using U340+S8612, flash, white balanced against PTFE): I understand that yellow in the UV shots indicates wavelengths of around 350-365 nm).

 

And flowers that are blueish in visible (400-500nm) often look blueish in UV (380-400nm).

 

Why is this? Just co-incidence?

 

With regard to the blue channel, it's not coincidence. It's just the short-wavelength tail of the blue dye spectral response.

 

For the red and green channels, though, it is entirely coincidence. They just happen to have some response in the UV band which is not likely by design.

 

I'm missing quite a bit of equipment yet, but I put together a quick spectral response for my stock Nikon D50. I don't have a PTFE target (I used HDPE, with an unknown short cutoff), I don't have a UV grating (I used a plastic grating) and I don't have a short-UV light source (I used sunlight), but in spite of all those shortcomings, I was able to get response just below 340nm. I was a little surprised at that.

 

The D50 is probably fairly typical of a number of cameras. Here you can see how the color channels respond in the UV-A band and visible. I used the UV-Nikkor 105/4.5 and a Baader U 2" filter for the UV portion, 30 sec exposure at f/4.5 and ISO 1600 (plus ~1 stop push in post). Exposure for the visible portion was 1/8 sec at ISO 800, f/4.5. The UV band was pasted into the visible image to produce the composite, with some care to preserve alignment. The wide band is the camera's image via the diffraction grating, and I've split out the RGB channel bands above that so you can see the tails. Some key wavelengths in nm are listed across the bottom.

 

post-248-0-00700500-1557550423.jpg

 

I'm working on response plots also, if anyone would like to see the data graphically.

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Marianne - re. posting the stereo pairs for crossed-eye viewing, I don't now what the requirements are in terms of size, separation, left/right orientation. So I'll just post the pairs for the tulip photos so you can copy them to your PC and do whatever you need. Let me know how you get on.

 

Left:

post-245-0-02202300-1557559877.jpg

 

 

Right:

post-245-0-58719500-1557559877.jpg

 

 

Left:

post-245-0-62344000-1557559878.jpg

 

Right:

post-245-0-61168200-1557559879.jpg

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Marianne Oelund

Marianne - re. posting the stereo pairs for crossed-eye viewing, I don't now what the requirements are in terms of size, separation, left/right orientation. So I'll just post the pairs for the tulip photos so you can copy them to your PC and do whatever you need. Let me know how you get on.

 

 

That is simply stunning - I have giant stamens projecting out of my screen! And the full-flower image has so much depth, more than I expected.

 

The 3D view seems to multiply the detail you can perceive.

 

Beautiful work, Bernard. Thanks very much for the left/right pairs. I really enjoyed the views.

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Thanks, Marianne. Yes, that tulip shot worked well in 3D. Not every one is as good - some of those pesky flowers will insist on moving while I'm shooting the sequence, which gives a pretty unpleasant viewing experience.
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Just made this one today - and I like it. It's a common or garden Dead Nettle (weed). Quite an interesting structure if you look from below. It's beautiful in visible light, but if you prefer dramatic, go for UV!

 

Stereo anaglyph:

post-245-0-45282200-1557594712.jpg

 

Left:

post-245-0-40394100-1557594710.jpg

 

Right:

post-245-0-93585300-1557594710.jpg

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