Overcoming 'Hot' Channel in UV Photos

[Jonny]

Hi,

 

I am currently struggling with an issue where my UV photographs are substantially brighter in the red channel than the blue/green (by between 3/4 EVs)

I've noticed that some of this is subjective - in some lighting conditions the balance changes, for example, in shade the difference widens.

 

My equipment is as follows

 

D3100 Full Spectrum

S8162 1.75mm + U340 Stack

Optoflex 35mm lens

 

I know the D3100 won't white balance IR or UV but I wondered if there was any issue with my setup or this is what happens to everyone. The D3100 was never meant to be used for UV, I got it for Vis-IR but then decided to try UV out on a low budget. I noticed some forum members are posting landscapes which appear to be done in one shot, while I'm either comping 2 images or applying heaving tone adjustments to each channel.

 

So really my questions are -

 

1. Is my setup adequate?

2. How can I best get around the issue of a hot channel - comping, different RAW software, some white balance 'trick'?

3 What causes it - white balance? Spectrum? A specific part of my setup?

 

Also, the 3100 not white balancing is annoying, but workable, are all the Nikons like this now or is there a list somewhere of what will and won't behave?

 

Jonny

[Alaun]

Hello Jonny,

 

assuming the optoflex transmits UV, the equipment seems ok.

 

If the lens transmits only slightly into UV, cutting the the UV with a 340nm filter might not be the optimal combination.

 

How do you do the WB?

 

Can you show a picture with a typical pattern (it is the time of the dandelions ;-) )

 

A "normal" landscape might give little light into the green and blue channel, (as your mentioning the shadow pictures might indicate), thus a dandelion would be fine.

 

WB can be tricky, depending on your software.

 

Werner

 

 

[nfoto]

Most cameras record UV mainly in the R channel with some spillover in G and B. The actual distribution of channel intensities depends on a number of factors. The camera probably is the least significant link in the chain, as there usually tends to be little difference between cameras as such whether they are CCD or (most common these days) CMOS-based. The Fujis may be an exception though. However, the lens itself contributes significantly to the outcome and likewise the filter(s) used. The final points are setting the w/b and the processing software used.

 

Like Werner I would suggest you try out a UV bandpass filter with more transmission in the near UV(A), for example, the U-360.

 

Dandelions are in general an excellent test subject as their UV appearance is well known and many filtration or w/b issues can be immediately obvious when a UV photo of them is presented.

[igoriginal]

I'd like to add to this thread, and point out that the U-340 filter thickness was not mentioned in the above stack? Could it possibly be too thin?

 

(Ex: In some cases, a 1mm-thick U-340 may barely suppress IR contamination when stacked with a 1.75mm-thick S8612 ???)

 

Thus, is some low level of IR leakage a possible contributor here to the reported "channel-flooding?"

 

What thickness of U-340 are you using with your 1.75mm-thick S8612, Jonny?

 

I believe that Steve (Cadmium) might be the best person to ask and/or consult on this matter, speaking of which.

 

In either case, I haven't used any UV-pass glass as thin as 1mm in a UV-pass/IR-block stack in years, as a matter of safety. (Better to slightly over-compensate, instead of risking under-compensation, when it comes to my own strategy of IR-suppression). I don't mind the moderate increase in exposure time, to do so. But that's just my own UV work ethic, of course.

[OlDoinyo]

I know that I use a basic WB of 2700G9 on my camera for UV, which gets things pretty close. On yours, if you select "sodium vapor" that will give you 2700K; then you might use the Fine Tune feature to dial in the correct amount of green. Your best setting will not be the same as mine because it is a different camera/filter combination.

 

Or, what happens if you just put the camera on AWB?

[nfoto]

In PhotoNinja, I usually end up with a w/b of 2000K and tint around -55 to -65 for the UV specialist lenses, whilst UV-capable optics will have tint ranging from -70 something down all the way to near -100. These values are with my standard Baader U 2" filter..

 

The raw files straight of fthe camera will show up in [reddish] orange for lenses such as Coastal Optics 60 mm f/4 APO or the UV-Nikkor 105 mm f/4.5, whilst UV-capable lenses tend to have a distinct cold hue of magenta/red or purplish.The Noflexar 35 mm f/3.5 is occupying a middle ground between these extremes.

[Andrea B.]

Jonny, it is all in the conversion. Because Nikon will not set a pre-measured white balance under a UV filter, we must perform that step in a converter. Nikon has a free utility converter called View NX and a free converter/editor named Capture NX-D. They are most certainly not the best pieces of software I've seen, but once you perform a WB and minor exposure adjustment in either, you can then export a TIFF for further development in the app of your choice.

 

Another good converter choice for UV work, not free, is Photo Ninja.

 

Unfortunately ACR has not always done so well with conversions of UV NEFs.

 

As mentioned above, making certain in-camera WB choices does help while shooting UV. Clark's suggestion of 2700K + G9 is useful.

 

Here is another way to shoot Nikon UV that I sometimes use because the UV false colours can be overwhelming. Set the D3100 to Monochrome with a little sharpening, neutral contrast & brightness (both 0) and no B&W "filter" settings. Then expose your UV photo so that the Luminance histogram spreads into the right hand side of the graph but does not hit the wall. This should give you a good convertible photograph in which the Red channel might be blown (fully saturated) before you white balance the false colours, but which becomes a typical blue/yellow/grey/black/white false colour UV photo after the white balance tool is applied. After you WB the false colours, then a Highlight Tool can usually pull back any remaining saturation of a channel.

 

****

 

Your choice of filter stack and the Optoflex 35 lens are just fine. Do note that when you stack an S8612 IR blocker over a U340 filter, you are moving the peak UV transmission rightwards towards 360nm. The thickness of the filters determines how much the peak transmission shifts. So you are not really gaining the benefit of a peak transmission at 340nm, but for casual UV photography this is not important.

[Andrea B.]

I should add that when using a Nikon for UV, setting the Picture Control to Neutral 0 (except for maybe some sharpening of +3 or +4) is quite useful for any choice of in-camera white balance setting. If not shooting UV in Monochrome mode, then it is also useful to reduce the Picture Control saturation setting.

 

****

 

Here are a few examples of different in-camera white balance settings. I don't have the suggested 2700K + G on hand right now, but these will illustrate the Red Channel problem and how to work with it.

 

 

In-camera Auto WB: The red channel hits the wall (becomes fully saturated) with typical Picture Control settings for Contrast (+1) and Saturation (+1) in the Standard Picture Control. It is difficult to see details on the Live View screen with these settings.

 

 

In-camera Auto WB with more green and less saturation: Now things look much better when some Green is dialed in on the WB grid. Next I set Saturation to -3 in the Neutral Picture Control.

 

 

In-camera Sodium Vapor WB with more green and less saturation: Also an improvment.

 

 

My In-camera WB Setting: This is a white balance preset I typically use when I'm not shooting UV in Monochrome. The Neutral Picture Control was used. I think I dialed Saturation back by -2 and had Sharpening +3. Other settings 0.

 

 

In-camera Monochrome Picture Control: Using Mono allowed me to make use of the camera's Luminance histogram and push the exposure to the right to reduce noise which is always a big problem in UV work. But I did not let the Luminance histogram hit the wall because that would have saturated all 3 channels to make recovery more difficult. Details are much clearer on Live View when using a Monochrome Picture Control.

 

 

After Conversion and Corrected White Balance: The final product with yellow and blue false colours. This look can be obtained in Photo Ninja or in NX-D or in View NX 2 with the click white tool. There are no other edits here, but typically I would work on overall sharpening and on some gentle saturation increase in the yellow areas. And perhaps push the shadows just a tiny little bit OR sometimes increase overall illumination a small amount.

[Jonny]

Hi,

 

Thanks that's both reassuring and great advice! View NX is much more capable of doing the white balance, although the actual RAW software is poor.

 

I just went back over some older NEFs and noticed though that this problem has gotten significantly worse in my latest images. Opening the old ones up in View NX - the WB was set to 2500 6.69 - as soon as I TOUCH the sliders though the colours go out of wack and the image turns overly red - even if I dial the same setting back in. My latest images come up bright pink out of camera and have an initial recorded WB of 5000. Setting them to 2500 6.69 doesn't fix them but I guess the software wasn't built for this

 

My only guess is that I got a reasonably good WB setting back then (no idea how) unfortunately you cannot set Kelvin on the 3100. Thanks for the other tips though, should make composing on the LCD much easier!

 

As for the U340, I don't know how thick the glass is, only that it was sold to me as 'slim' and it came from eBay in the USA. I don't think I'm getting IR leakage, I have black foliage, neutral buildings, white skies and tons of haze. I'll see if I can photograph an IR LED over my typical exposure time which seems to be between 2 and 4 seconds at f11 ISO 100

[OlDoinyo]

Although you have no direct way to set color temperature, page 88 of your manual lists some presets you can choose as a starting point. Sodium vapor is probably closest to what you want; you can take it from there with the fine-tuning function and a bit of trial and error.

[Andrea B.]

Jonny, I should have been clearer.

• Open View NX-2, find your folder of UV photos and double-click on a NEF to bring it up in the app.
  
• On the right side of the app, look for Adjustments > White Balance.
  
• From the White Balance drop-down menu, select Use Gray Point.
  
• From the sub-menu, select 5x5 Average for your Gray Point sampler.
  
• Click Start.
  
• Now, hover the cursor over the photograph and select a pink/magenta area on which to click.
  
• You should see your photo turn into the typical blue/yellow/grey/black/white tones.
  
• Return to the White Balance panel and click Finish.
  

To summarize: Adjustments > White Balance > Use Gray Point > 5x5 Average > Start > [click photo] > Finish

 

 

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

 

Opening the old ones up in View NX ....... as soon as I TOUCH the sliders though the colours go out of wack and the image turns overly red - even if I dial the same setting back in.

 

Yes, I know exactly what you mean here. The white balance - as it came from the camera - cannot be refined in View NX 2 by means of the Temperature or Tint sliders. For some reason it disturbs the colour relationships. I do not know why that happens!! But no matter because what you really want to do is reset the WB as I described above.

[Cadmium]

Hi Jonny, Shoot in RAW (NEF), use NX2, or the new free NX-D (not View NX).

White balance using full frame marquee in NX2, if you want, or small area marquee, or single point click.

With NX-D marquee is limited to a very small square, but it will work.

These are my favorite methods of getting the best white balance from my Nikon cameras.

 

Added:

After you white balance in NX2 or NX-D, then try using the shadows adjust in those programs.

Exposure compensation, and saturation adjustments can also work well in those programs.

There is a levels adjustment in NX2 and NX-D, but I like to use levels in Photoshop, so I export the NX2 product to PS as a TIFF, then levels or whatever else.

[Andrea B.]

But View NX 2 works just fine??

[Cadmium]

It does? I don't think View NX does white balance the same as NX2 or NX-D

[Andrea B.]

Oh, now I get where you are coming from. Had to think for a second. :D

 

In Capture NX-2 you can use the White Balance Marquee Tool to select a very large portion of the UV image or even the entire image in order to get a kind of "average" white balance over large selected area.

 

With View NX-2 or View NX-i or Capture NX-D, the white balance sample size is restricted to a very small area. (Why this is, I do not know.) So when you perform white balance with such a small sampler, you must know to click on a magenta/pink area in order to have the typical yellow/blue/grey stuff appear. And if there is any remaining pink tint, you must know to re-click on it so the pink tint disappears.

 

There are arguments to be made for either the very large sample size or the very small sample size depending on one's final goal.

 

If white balance is made on the entire image with the large Marquee Tool in NX2, it can happen that you do not get a "standard white balance". Now by "standard white balance", I mean a white balance which is made on a 99% reflective white standard and then backed up with a colour profile for the particular lens + camera combination in use. Sometimes a large average WB setting can slightly tint the pure white we would expect on the 99% reflective standard.

 

However, it must be noted that a "standard white balance" is not always the particular look we want for a false colour UV photograph. The examples that comes to mind immediately for me are photos made with a filter like the AndreaU which passes that interesting touch of violet or with a filter stack which passes UV+B+G. White balance against a 99% reflective white standard does not always give the best "look" to photos of these latter two types.y t

 

For documentary botanical UV work posted here on UVP, we have requested adherence to the "standard white balance" so that the documentary photos have a similar appearance even though they come from different platforms. But for any other UV photographs we are free to use any type of white balance we choose.

 

After all, false colours are false. B)

 

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

 

Technical note: Capture NX-2 is rapidly falling into being unsupported by the latest operating systems. And I think Nikon themselves have discontinued support for NX-2. I'm trying to decide what to do when I have to upgrade my Macbooks and eventually lose NX-2. I have loved that app and have always found it fast and easy for converting & editing my NEFs.

[Cadmium]

I simply didn't know that View NX did white balance. I never found a way to do white balance with View NX. Perhaps the newer versions have added a better tool for that.

As far as full frame or small square, I use both in NX-2.

[Cadmium]

I found it in View NX. Yes, it works. I like the way marquee works in NX2 better though.