• Ultraviolet Photography
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This experiment didn't work out.

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#1 Andrea B.

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Posted 05 October 2017 - 16:56

I set up a crumpled aluminum foil experiment yesterday to illustrate chromatic aberration in UV-capable lenses. It did not work out well because the little yellow "target" flower in the center of the foil and the surrounding grass created masses of colourful reflections on the foil which naturally confused the search for purple or cyan/magenta fringing.

Oh well. I can always try again. I'm thinking that I'll put the crumpled foil in a small light box with white walls and background. That should lesses stray colour reflections.

Here are a few shots to show you what I was trying to do. :D :rolleyes:

D610 + Coastal Optics 60/4.0

This lens is a UV/Vis/IR colour corrected lens. We would expect little chromatic aberration. However, there are so many colours floating around on the foil, that it might be difficult to decide what should not be there. There is also a grating effect from the foil surface Still, I think I see no obvious signs of fringing.

Visible [f/5.6 for 1/3200" @ ISO-400 with Baader UV/IR-Cut Filter in Sun]
Attached Image: coastal60-4_visSun_20171004wf_6961pn01.jpg

Visible [unresized excerpt from full-sized version of preceding photo]
Note grating effect from foil surface causing a kind of "iridescence".
Attached Image: coastal60-4_visSun_20171004wf_6961pn0101.jpg

Visible [unresized excerpt from full-sized version of preceding photo]
What is this blue from? Is foil slightly blue so that is shadows become darker blue?? Or maybe there are blue sky reflections.
Attached Image: coastal60-4_visSun_20171004wf_6961pn0102.jpg

Ultraviolet [f/5.6 for 1/6" @ ISO-400 with Kolari UV-Pass Filter in Sun]
I could not really get a firm white balance in the UV shot(1*). There are tinges of false cyan and false purple as well as the expected false yellow and false blue.
Attached Image: coastal60-4_uvKolariSun_20171004wf_6966pn01.jpg

Ultraviolet [unresized excerpt from full-sized version of preceding photo]
Attached Image: coastal60-4_uvKolariSun_20171004wf_6966pn0102.jpg

Ultraviolet [unresized excerpt full-sized version of preceding photo]
Attached Image: coastal60-4_uvKolariSun_20171004wf_6966pn0101.jpg

(1*) ADDED LATER:
The fix for that missed white balance is to shoot a white card with the given camera + lens + filter + illumination for use in your converter. Here I was relying on an existing D610 colour profile together and random stabs on neutral areas with Photo Ninja's white dropper. That works 98 times out of 100. Here it was less successful.


D610 + Micro-Nikkor 105/2.8D

This lens, an older version of the current Micro-Nikkor 105/2.8G AFS, is somewhat UV-capable. Again there are so many colours present in the foil that it could be difficult to say what colours are fringing and what are simply from reflection.

Visible [f/5.6 for 1/3200" @ ISO-400 with Baader UV/IR-Cut Filter in Sun]
There seems to be some purple/magenta fringing to the right of the flower.
Attached Image: nikonUVnikkor105-4.5_visSun_20171004wf_6882pn01.jpg

Visible [unresized excerpt from full-sized version of preceding photo]
Attached Image: nikonMicroNikkor105-4.8D_visSun_20171004wf_6920pn03.jpg

Visible [unresized excerpt from full-sized version of preceding photo]
Attached Image: nikonMicroNikkor105-4.8D_visSun_20171004wf_6920pn04.jpg

Ultraviolet [f/5.6 for 1/6" @ ISO-400 with Kolari UV-Pass Filter in Sun]
Because this lens would not be corrected in the UV, we should expect to see some fringing and perhaps we do here. But because the typical white balanced UV shot is expected to show false yellow and false blue, can we be sure? I need to try a more controlled experiment before saying 'yes'.
Also note that this lens did not produce quite the bi-coloured false yellow/blue flower that the CO60 did above. That suggests to me that its UV transmission range is not very wide.
Attached Image: nikonMicroNikkor105-4.8D_uvBaaderSun_20171004wf_6923pn01.jpg

Ultraviolet [unresized excerpt from full-sized version of preceding photo]
I'm pretty sure there is obvious purple fringing here. But a better experimental set-up might remove the remaining bits of doubt. "-)
Attached Image: nikonMicroNikkor105-4.8D_uvBaaderSun_20171004wf_6923pn0102.jpg

Ultraviolet [unresized excerpt full-sized version of preceding photo]
Attached Image: nikonMicroNikkor105-4.8D_uvBaaderSun_20171004wf_6923pn0101.jpg
Andrea G. Blum
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#2 Andrea B.

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Posted 05 October 2017 - 20:40

Edit: added small comment about white balance.
Andrea G. Blum
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#3 OlDoinyo

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Posted 05 October 2017 - 22:54

Try roughing up the foil rather than crumpling it--either by manual means such as steel wool, or by etching briefly by spraying with drain cleaner or lye solutions and rinsing off with plain water.

#4 Andrea B.

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Posted 06 October 2017 - 02:11

Will do !! Thanks for the suggestion.
Andrea G. Blum
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#5 Mark

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Posted 28 October 2017 - 23:06

Hi Andrea,

I find what I believe is a chromatic aberration problem when I shoot with a certain combination of lens/filter. The problem may also present with other lenses (I'll have to specifically test sometime), but I think it is due to the particular filter - which is a 340 nm uv bandpass filter from Asahi.

Here's an example (from an upcoming post of mine :)). This overview shows the subject propped up in a small glass which I covered/wrapped in aluminum foil. I also hung sheets of foil around the subject, to box it in for the sake of both keeping out stray/contamination light, and to help contain & concentrate the UV source. As you can see, the environment is understandably bright - just have a look at the rim of the glass though, in the center just under the petals in the foreground:
Attached Image: 10-01-2017_19-24-52_0340_glare1.jpg

Here's a closer look, with a dropper sample of the aberrant color:
Attached Image: 10-01-2017_19-24-52_0340_glare2.jpg

This glare/flare is completely unrecoverable (aside from manually recoloring the area(s)), even from RAW files. And I've only seen this problem with this particular UV filter (I have several others, none of which have given me this problem).

Its not usually an issue in most of the subjects I shoot - as it usually only presents in/around the background. Although, there have been times where parts of my subjects happen to be very UV reflective, and that becomes a problem I can't just crop out.

#6 Andrea B.

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Posted 28 October 2017 - 23:31

Reflection off a glass rim like that is specular ("mirror") reflection. Such a blown highlight quite often cannot be recovered because all three of the R, G and B Channels are oversaturated, i.e. have "hit the wall". Depending on the filter/lens combo you are using you can get more or fewer specular reflections. So that you get it with one particular 340 filter and not other filters is not surprising. [Added: Of course the foil hung around your floral subject is going to aid in the production of specular reflection if you are using a UV LED or flash or lamp.]

Over the years I have observed that cyan-colored specular reflections in UV photographs happen in some converter apps but not others. I typically get cyan specular reflections when using a Nikon app like Capture NX2 to convert a UV photo. In other converter apps I have had some success in keeping the specular blowouts looking white. Photo Ninja, for example, handles UV specular reflections quite well - although I have seen a hint of greenish-cyan or cyan creep in on occasion.

In PN you can neutralize non-white specular reflections by just desaturating the cyan patch in the Color Enhancement tool after you have made your UV false-white balance. In other apps, as you have already observed, you can just "paint" over a cyan specular reflection with a desaturating brush or a grey brush.

The kind of fringing caused by chromatic aberration is typically found along strongly lit (or backlit) outlines and along edges. That is to say, it is literally a "fringe" and not a wide patch like in your photo. Chromatic aberration can blur detail but usually does contain detail. A specular reflection has no detail at all.

Hope this helps?
Andrea G. Blum
Often found hanging out with flowers & bees.

#7 Mark

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Posted 29 October 2017 - 00:44

Thanks Andrea. Perhaps this could be called 'specular aberration' then, on account of it being a defect/deviation, rather than a well behaved reflection? Either way, its a tough one to deal with - less so when using diffuse sources (like fluorescent BL-B's), but certainly more so when using intense/point sources (like some of the UV torches we use).

Also, just to note - as you've seen this happen when using Nikon apps for conversion - it so happens that I use Nikon's Capture NX-D for all my RAW conversions these days (including the above image).

#8 Andy Perrin

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Posted 29 October 2017 - 01:05

Mark, why not just underexpose the image a bit and bring up the shadows afterwards?

#9 Mark

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Posted 29 October 2017 - 01:16

@Andy: Sure, that is certainly an approach - but in the midst of shooting full multispectral sets I can't always tell (simply from the camera viewfinder/histogram) when there are these spectral hot spots. I could err on the side of expecting them, but then I would be generally underexposing when its not always necessary. Using in-camera white balance settings for each shot (i.e., UV, VIS, etc) helps a lot with exposure setting, but over the course of shooting a full set that requires a lot of button pushing and touching of the camera I really don't like to do (for the sake of maintaining image alignment). If only Nikon would make a mobile app that is actually useful - for example, to change camera settings like white balance pre-set...

#10 Andy Perrin

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Posted 29 October 2017 - 01:49

I always exposure-bracket everything. As you say, you can't really tell, but if you exposure bracket then you can just pick the good one (usually).

#11 Andrea B.

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Posted 29 October 2017 - 03:21

Or you could display your flower in something besides a shiny glass container? Place it on a rock or on some cloth placed over a book or other prop. Unless you work really really slowly, the flower should hold up well enough for a few shots.

Or, here's a trick...... Get a nice piece of fairly thick paper or maybe thin cardboard, carefully poke a small stem-sized hole in it, put the flower stem through the paper and then into the shiny glass container with water. Tilt the flower-in-paper to face the camera so that the stem remains watered. Now you have a plain paper background for the flower with little chance of specular reflection because the paper covers the shiny container. [I hope I described this well enough to made sense!]

In my photos above, the small yellow flower is sitting in a green plastic "test tube" (with water) used by florists. Those tubes are easily disguised by various props when setting up a still life scene for photography.
Andrea G. Blum
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#12 Andrea B.

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Posted 29 October 2017 - 03:35

Ebay Floral Tube search link: https://www.ebay.com...+tubes&_sacat=0
Andrea G. Blum
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#13 Andrea B.

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Posted 29 October 2017 - 04:27

Sorry for rattling on here. But because you mentioned that during a multispectral set you don't want to engage in so much button pushing, etc, may I suggest that you try a session using the Nikon Monochrome Picture Control for all shots? Mono enables better usage of the brightness histogram so that you can expose in such a way as to prevent brightness from "hitting the wall" on the right. You can't prevent specular reflection entirely unless you totally control the illumination and type of props used in the still life. This is usually impossible of course. But you can try to minimize the effects of the mirroring by proper exposure -- as bright as possible to avoid excess noise, but not so bright that the specular reflection increases beyond its natural boundaries.

Nikon D750:
Turn on Live View.
Engage Live View Exposure Meter.

On left side of back LCD, press i button to see icon list pop up.

Using controller wheelie, scroll through the icon list and highlight Exp OFF icon.

Press OK.

Using controller, scroll to Exp ON icon.

Press OK, then press i button again.

Now you have the exposure meter showing in Live View on the right.

Engage Live View Brightness Histogram

On right side of back LCD, press the info button above the controller wheelie

and cycle through the choices until the brightness histogram appears.

Now you are all set to easily control your monotone exposure.
Start with the exposure meter on the metered setting.
Check the live histo. Push/pull as needed.

With this method you may very well still blow one channel (sometimes 2 channels) which will become apparent
during your conversion back to false colour and subsequent false white balancing. One channel (sometimes 2) is (are) usually easy to pull back in any Nikon app. (Use a Levels Tool R, G or B curve as needed).

************

Kindly ignore anything which you already do or know!! :D :D :D

Corrections and alternate advice always welcomed here.
Andrea G. Blum
Often found hanging out with flowers & bees.

#14 Mark

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Posted 29 October 2017 - 13:44

@Andy: Thanks - I've used exposure bracketing for other types of shooting, so I'm not sure why I haven't considered it already here. I'll admit, when I first read your suggestion I though 'no way!', because in trying to get exposures just right I often generate as many as 15+ images per shoot (for what is a 'full' multispectral set to me [five images]). But then, upon reconsidering, I believe the camera can be set (?) to automatically shoot each bracket (good for minimizing camera 'handling'), and if I'm going to end up with extra images to sort through anyway then why not automate it! It would put a hard fix on the number of shots I need to work through (as sometimes I actually get it right on the first shot :)), but all in all it could actually work out to be an even more streamlined process. I'll fiddle with the camera setting a bit and see if the bracketing is automated.

@Andrea.1: I like your hole-in-a-piece-of-paper idea. I like it a lot. Just one little problem though - in the UVIVF shot the paper/cardboard/just about anything of that nature will fluoresce and add 'contamination' to the image. I do have some black flock paper which doesn't seem to fluoresce, but its rather not cheap (special stuff from Edmund Optics), and it attracts and holds lint like nobody's business (the bane of my UVIVF images!). Pointedly, this is in part why I've chosen to use aluminum foil all around, as it is not only a good reflector for the UV images, it also it of course 100% light blocking (re: stray light), and it does not fluoresce at all. In a sense it seems like the perfect background - though of course, we see it has its down side. I'll have to do a little experimenting with materials/papers/etc to see if I can find a cheap/disposable option for this, since I'm not likely to need exactly the same hole/paper size/etc forever; and I'm sure it will get dirty/linty (!)/old rather quickly. Still, I think this will be a good solution in the end. Thanks for the idea!

@Andrea.2: If only I was a little less excitable and would just take a moment sit down and read the manual (any manual really, for anything!). Thanks for the tip on the live-view histogram. I have it enabled now and will give it a try in my next shoot. In the meantime, I wonder though - is the histogram pulled from the average RGB values, or from the green channel only (I've read that some cameras pull the green channel only - for simplicity?, other?, I have no idea). That could make the histogram more or less effective, especially if the blowouts I'm seeing are in either of the other channels. We'll see.

Edited by Mark, 29 October 2017 - 13:51.


#15 Andy Perrin

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Posted 29 October 2017 - 17:23

I know my Sony has the automated bracketing. Honestly, though, I rarely get specular reflections like that that actually overexpose -- usually Photo Ninja can pull them back. You shoot RAW, right?

#16 Andrea B.

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Posted 29 October 2017 - 18:10

Again, it is OK to blow one or possible two channels and still make a good highlight recovery. But if you blow three channels, highlight recovery becomes more difficult. Specular reflections are by definition a 3-channel blowout. So the aim of using the Live brightness (lumonisity?) histogram is to keep any 3-channel blowout as small as possible, to keep it from expanding beyond its natural boundaries. The Live View histogram is not raw, but do not let that dissuade you from using it to optimize your exposure. It is very useful.

I don't recall just how much you can let the Live View histo on the D750 "hit the wall". I know that on my D810 I have sometimes 2-3 stops that I can overexpose - for a visible photo. (Quite an amazing camera is that D810. Like wow!)

With a D750 in ultraviolet, you will have to experiment with how much pull-back you can get for 3-channel blowouts. Start by simply pushing that LV histo to the right as much as possible without touching the right wall. It is OK if exposure looks too bright, because it is easy to pull back using an Exp slider in the converter. It is harder to increase with an Exp slider because partly because that reveals lots of noise. Later you can experiment to determine if it is possible to hit the wall with a D750 in UV and still pull back highlights and blowouts.

***************************

Nikon Automated Bracketing & Bracketing Burst for D750
It's very nifty!
Set top left dial to Ch (continuous high speed).
While holding BKT button down on left side front,
  • use front rollie wheel to dial in bracketing increments as seen in top LCD.
    (And see supporting settings below.)
  • use back rollie wheel to dial in number of bracketed shots.
Turn on Live View with live histo and metering strip.
Focus. (lol)
Set exposure to be 0 on meter strip.
Press shutter and hold until bracketing burst is complete.
(It is useful to have a remote control for this hold-and-shoot bracketing burst because when shots are over 1" long, you are likely to induce some motion from holding down that shutter for 3-9 shots.)


Supporting Settings for Bracketing & Bracketing Burst
Custom Setting Menu > b Metering/exposure > b2 EV steps for exposure cntrl > {1/3 step, 1/2 step} > OK

For UV bracketing I usually use 1/2 step. For me, using 1/3 step is too fine-grained in UV. Of course, YMMV, so experiment to find your desired setting.


Custom Setting Menu > e Bracketing/flash > e6 Auto bracketing set > AE only > OK
Custom Setting Menu > e Bracketing/flash > e7 Bracketing order > {your choice} > OK
Andrea G. Blum
Often found hanging out with flowers & bees.

#17 Alaun

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Posted 29 October 2017 - 18:24

Instead of crumpling the foil, I recommend sweet yourself with some fine chocolate :) and then use the foil, the chocolate was wrapped in.

It can have a very fine (tiny) pattern printed in during a last rolling step and these little imprints have a regular structure and show some specular highlights very similar to the cones on flowers.

(the picture was taken with ambient light, Nikon D4 (not modified) and a Zeiss 100mm Macro lens)

Attached Images

  • Attached Image: _DSC1956withLensProfile.jpg

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