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UltravioletPhotography

White balance correction only is not enough for good Visible colour in a converted camera


Andrea B.

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I finally got around to adding a nice illustration to the end of the tutorial Photo Ninja: How to Make a Custom Light Profile. I had been meaning to do this for quite some time! I'll ponder at another time why it takes so long to get these things done. :D

 

The point I had wanted to make at the end of that Tutorial seems important for anyone using a converted camera for Visible colour photographs: Any white balance adjustment alone does not fully correct the colours after removal of the internal blocking filters.

 

Now the results of my particular experiment make use of a Visible shot made under a Baader UVIR-Cut Filter. I do not have any external filter purporting to restore the original colours of the converted D600, so it is possible that the results under this missing filter would not be so dramatic as shown here. But whatever. It is easy enough for anyone to repeat this little illustration with whatever filter they use for Visible shooting on their converted cam to see what happens.

 

Here is the excerpt from the end of the Tutorial.

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

 

Example of CC Passport Before and After

The original shot of the CC Passport was made in sunlight using an in-camera preset white balance. It can be easily see that white balance alone did not fully correct the colour in my converted D600 even though it gets fairly close.

 

Note: There may be some inaccuracies in both photos due to application of the sRGB setting for browser

viewing and due to resizing for posting here.

 

Equipment: D600-broadband + 60/4.5 UV-Planar + Baader UVIR-Cut Filter + Sunlight

 

BEFORE: The original photo with only in-camera white balance does not look too bad,

but the yellow & purple are obviously off. The other differences are more subtle.

Photo Ninja does a good job (at least in this example) of preserving the in-camera white balance preset.

before.jpg

 

 

AFTER: The Custom Light profile was applied. Corrected colours look better.

Saturation can be further adjusted if desired.

after.jpg

 

 

The differences are not so easy to see when the two versions are separated. In Photoshop Elements I made a difference

layer to show where the differences occur. The difference layer was brightened slightly to make the difference locations more

apparent because some of them are subtle.

 

DIFFERENCE LAYER: Between preceding Before & After version of the CC Passport.

This is just to show the location of colour differences between the two versions.

Diff.jpg

 

 

To make the colour changes even more apparent, I overlaid a grid cut from the After version onto the Before.

The grid was made in Photoshop Elements with a flower cookie cutter. Looking at the differences this way makes

it quite apparent that white balance adjustment alone is not enough to correct colours in a converted camera.

 

AFTER: Flower grid.

flowerGrid.jpg

 

 

LAYERED GRID: The After flowers show the adjusted colour against the Before background.

Again, note that this shows that a white balance adjustment alone is not enough to correct colours in a converted camera.

There are only the minorest of differences in the top monotone row. This means that the in-camera white balance setting made it through the converter (Photo Ninja) without major changes. Do be aware that this may not always happen depending on what converter is used.

AfterOverBefore.jpg

 

 

As a final observation I must note that the colours from the converted camera are being corrected to the CC Passport colours. It is possible that there is some variation between such corrected colours and the actual colours which would have been produced by the camera in its original, unconverted state.

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How would your 'Before' image have looked if you had done a one-click white balance in PhotoNinja on one of the top line neutral squares?
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Dave, there would be no change to what you see in the Before photo. But let me explain further.

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FIrst, let me review the conversion in Photo Ninja during which the raw Before Photo had the following settings applied.

  • Color correction: Daylight/Flash (default) + From Camera (set my me). No further white balance sampling was done.

  • Exposure and detail: All default (0 settings) except for Photo Ninja's automatic highlight correction, if needed. Note that activiting the Exposure & Detail tool also applies the default gamma curve to the linear raw.

  • Color enhancement: Plain (set by me) and Intensity 50 (default).

For this particular Before photo, a click on the white standard in the Photo Ninja Color Correction page does NOT change anything from these settings. This is because I created an in-camera white balance measurement immediately prior to shooting the CC Passport. And subsequently, Photo Ninja did a good job of preserving that in-camera setting when I made the "From Camera" choice in the Color Correction tool.

 

But do note that you sometimes can get a small shift in the Temperature setting if you sample a white standard in the Color Correction tool after setting "From Camera". This is because white balance is constantly changing while you are out in the field shooting. So if some time has passed since making your in-camera white balance, the white balance may have shifted to become a few degrees warmer or cooler due to the changing ambient light where you were shooting.

 

Also note that not every camera is entirely precise every time in setting an in-camera white balance. This could cause some shifts in Temperature when a white standard is sampled. Older cameras may not nail WB perfectly. Newer cameras seem to get pretty close most of the time.

 

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

 

I recommend shooting white and colour standards at the beginning of every shooting session and to also making a new in-camera white balance at the beginning of every shooting session. Then you will always have the data needed to recover the best colours from your work.

 

[[Note: It is outside the scope of the article and this post to discuss the steps needed to colour correct photographs while preserving a certain tint to the ambient light.]]

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Shouldn't this be part of the service of a "professional" full spectrum camera conversion? At the least a conversion service should be able to provide a spectrum of the removed ICF to construct a WB correction and/or identify an external lens filter to compensate as best as possible for the missing ICF.

 

Is it possible to use the removed ICF and make an external filter out of it? Assuming of course it is not broken during the removal.

 

 

 

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The removed ICF is rectangular and sensor-sized. So even if full frame, that would be about 36 x 24 mm and could likely only be used as a rear filter on certain lenses if you could figure out how to mount it.

 

The manufacturer's ICFs are proprietary. A couple I've seen appear to have a blue-green tint so its obvious some kind of BG glass might have been used. But they never quite look like the BG filters I own? (I have about 6 of those of differing kinds.) And coatings may have been added to the ICF by the manufacturer to further the UV/IR blocking.

 

With the Nikon conversions I have owned, I have tried various BG glass to restore the original colours when shooting Visible light. They never worked as well as the Baader UV/IR Cut filter used in the preceding work and, as you see, it doesn't hurt to profile even that result to get further correction.

 

Other cameras may respond differently, of course.

 

Kolari Vision does offer some kind of BG filter for visible shooting meant to restore the original colour. I don't have theirs however because I got an IR conversion from Kolari, not full spec.

 

**********

 

I should probably add to the post above that we (that is to say, I, Andrea) should not get overly obsessed with this colour correction thing. The differences appear very obvious when the After flower grid is placed over the Before colours. But, in practice, a good white balance setting under whatever UV/IR block filter you are using goes most of the way towards correct colours, and in many general scenes you will never notice that some of the colours might be a few degrees off true.

 

OTOH, did you know that each example of a particular model of camera may actually have a very slightly different white balance under identical light? And the same holds true for everything else involved in the production of proper colour in a photographic print - monitors and printers, for example. This is why we have gear for colour calibration in the first place.

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Then after spending mega-bucks to get every step in the chain to a print colour managed (monitor profile, printer profile for the specific printer and paper used) you need to specify the colour temperature of the light under which the print will be viewed. Which is why specialised colour booths are used when professional photo organisations do their annual print awards. But Joe Blow looking at your perfect images on his mobile device will see something completely different :wacko:
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You know you are in DEEP trouble when you go to print one of your favourite UVIVFL images which has a lot of bright fluorescent blue which looks fine on screen and the thumbnail in the printer dialogue shows grey meaning you are way out of gamut for the printer/paper combination. So you print it anyway and it just looks dull and boring. Then you remember being told that you can never print some things you see on screen.

 

That must be why we call them false colours B)

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There's another unseen factor at play, call it the "room in the elephant" since it's so big.

 

Do you know what the white point of the monitor that you are looking at this with is set to?

 

Straight out of the box most monitors would be set to D65 (if anything) which means a colour temperature of 6500K, nominally daylight but somewhat on the blue side. Calibration software lets you set your monitor white point - if the monitor lets you!

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I might add that the same UV/IR-cut filter doesn't work the same with all cameras.

I only have two UV/IR converted cameras to sample from, Nikon D7000 and D90.

I have a Baader UV/IR-Cut filter (dichroic), and absorptive types BG38, BG40, BG39, S8612, and various other 'hot mirror' type (dichroic / reflective type) filters of other brands, such as Tiffen, Hoya, MaxMax...

With the absorptive filters the thickness will change the results, but I have usually used 2mm thick glass, such as the CC1 (2mm or 2.2mm if I remember correctly = BG38, 99% sure).

My original visual filter was the CC1 and BG38, which work the same on both of these camera models.

I have also used the BG40 with about the same results as the BG38, but I like the look of the BG38 slightly better.

I have used S8612 at times 'in a pinch', but find that it makes things look a little more blue, with not enough red (with my cameras).

Many people use the Baader UV/IR-Cut filter, but with my two cameras it looks too red, and the BG38 looks more natural both when set to Auto or when White Balanced out of the camera.

I tend to shy away from dichroic type clear UV/IR cut filters because of uneven color problems from center to sides, which I have encountered with some dichroic versions (aka 'hot mirror' types) but I have not tried all of them, and I have not seen this problem with the Baader UV/IR-cut filter, which I have used with the Nikon 18-55mm VR and the Kuri 35mm.

For many cameras it may have better visual color than the BG38, as many of you prefer it to all the other filters for visual shots, but after using it in many landscape situations I decided the BG38 looks best for my two cameras.

I have also tested the method of stacking the Baader UV/IR-Cut filter with a BG40 or BG38 to 'round out the curve', but I simply didn't see a noticeable difference in doing that (with my cameras),

but it may improve results with other cameras, I don't know.

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Yes, it is true that none of the UV/IR-cut filters - Baader, BG38/39/40 or S8612 - will work the same on all cameras.

 

So to obtain accurate colour profiling, we must create a colour profile for each combination of camera + lens + filter + light.

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Plus EYES, Andrea. Haha, the real joke here is on me, because I'm red/green colorblind. (Not totally, but my ability to distinguish them is definitely much less than most people's.) I really thought this was the one branch of photography, aside from monochrome, where it truly would make no difference.
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The white balance process presented above is for processing Visible output from a converted camera with a Baader UV/IR-Cut filter (or other IR blocking filter). If you are not planning to shoot Visible counterparts of your UV shots, then no worries.

 

For UV photography, white balance is optional (unless you are preparing a formal botanical presentation). So no worries.

 

If for any reason you do want to enable white balance and/or colour profiling for your converted camera, you would be following a purely mechanical process which does not require that you be able to discern the colours accurately either before or after. So, again, no worries !!

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Well of course I do plan to shoot visible counterparts, but that bit certainly didn't occur to me when I started all this. I just want an out in case all my greens turn out to be oranges someday. I can point back to this post. "See?? You were warned!"
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To ease your mind, one of the best photographers we know is colour blind and makes wonderful photos in both Visible colour and in Infrared. I'll say again, with the right tools, white balance is purely mechanical so you can achieve it when you want to if colourblind. But the relationship of tones within a photograph is extremeply important too and does not rely so much on WB. Try changing one of your photos to B&W when converting/editing. Adjust black & white points and figure out proper contrasts using a curve tool. You will get a photo which should look pretty good when you "add back" the colour in the last step (subject, perhaps, to some saturation adjustments.) Colour photography is not all about just colour. :D
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