Example of Monochrome setting for better exposure in UV images. Or something like that.

[Andrea B.]

It has been suggested more than once by several members that it is helpful to use a Monochrome setting when shooting in UV light in order to make better use of the non-raw, JPG histogram for attainment of an optimal exposure.

 

I'll use as an example (below) a monochrome UV photo of a very small flower which I photographed on the 27th October - the last flower of the season before the big freeze and some snow arrived yesterday.

 

Gear:  Nikon D610 conversion + UV-Nikkor + BaaderU UV-Pass Filter + Nikon SB-14 UV Flash

 

Exposure: f/11 for 15 sec at ISO-200 during which 3 handheld flashes were made (left side, middle, right side). The brightness was controlled in this exposure by increasing/decreasing the distance of the flash from the flower.

 

UV photograph as shot, straight-out-of-camera.

 

 

Next is a view of the photo in Photo Mechanic, zoomed in and with the histogram to the right. Note that the JPG brightness histogram has two peaks. The right-hand peak represents the Spectralon background which is about 98% reflective in UV/Vis/IR. The left-hand peak represents the dark unlit background stuff.

 

And now note that JPG brightness histogram has gotten near to the right-hand wall, but does not touch it. If the brightness histogram hits the right-hand wall, you will be in danger of blowing all three channels. This can be quite difficult to recover from when converting a photo.

 

 

However.....(there's always a "however").....even though the brightness histogram is not hitting the right-hand wall, that doesn't mean I didn't blow a channel as we will see later. But a 1-channel blowout is usually easy to handle during conversion. Here is the Raw Digger non-linear view of the photo prior to the application of white balance. The 4 channels of linear raw data has been demosaiced into 3 channels together with application of a gamma curve and some auto-scaling.

 

 

 

Let's see what Raw Digger had to say about this exposure. Here are the Stats for over/under-exposure. I'm going to disregard the underexposure information because the underexposed areas are all outside the Spectralon rectangle and have nothing to do with proper UV illumination of the little flower. But the overexposure stats are showing a blown red channel which is likely in the Spectralon, so that will need to be dealt with as it is the background for the little flower.

 

 

So now I will select the Red channel and check the OvExp box to highlight the Red blowout. Note that the flower itself has no highlighted areas.

 

 

 

 

At this point I know that I have a useful exposure of the flower due to my use of a Monochrome picture control and my use of exposure settings (including distance of flash from flower) that ensured that the JPG brightness histogram got close to the right wall but did not hit it.

 

Side Note: To reduce noise, we need to capture all the light we can - especially when shooting in UV light. So it is good to expose-to-the-right while taking care not to blow more than one channel. Some cameras are better at this than others. I can only speak for the newer Nikons. They have a large amount of "headroom" which means that an increase over the metered stop by 1, 2 or even sometimes 3 stops can successfully be "pulled back". How converters handle blown channels also plays a role in this process. Again, some are better than others at recovering a blown channel while ensuring there is no off-color cast produced in the blown area.

 

 

It's time to open the file in Photo Ninja.

First, the Color Enhancement tool is initially set to Plain with a default Saturation of 50.

Second, the Color Correction tool is used to set white balance. The WB dropper can be used a non-blown part of the photo. Or a white balance preset can be applied. Here I applied a WB preset created for the D610 with UV-Nikkor.

 

 

The third step is to deal with the blowout in the Exposure and Detail tool. Again the blown areas are highlighted in red. 

 

 

Because I had pushed the original exposure to the right, I'm not going to use the Highlights slider to deal with the blowout. That slider is best used only for small too-bright areas. Instead, I'm going to deal with the blown area by first pulling back the Exposure slider until all the red is gone. Then I'll push the Illumination slider to the right to add back just enough brightness so that the image (subjectively) pleases me. The Illumination slider in Photo Ninja only pushes those areas which can become brighter without hitting the wall. Nice tool to have

Here's the final conversion. (Although this is not the final image as detail work and cropping needs to be done.)

 

 

 

The final image might look something like this.

 

Stephanomeria pauciflora: abaxial view, 27 October 2023, Eldorado at Santa Fe, New Mexico, USA

[colinbm]

Thanks Andrea

[Andrea B.]

Does anybody else shoot Monochrome to control blowouts? 

Has anybody heard of this method before?

Are more examples needed?

[photoni]

24 minutes ago, Andrea B. said:

Does anybody else shoot Monochrome to control blowouts? 

Has anybody heard of this method before?

Are more examples needed?

 

I did it for a while.
this display with all three histogram channels the same... is misleading if you don't put the right white balance in the setting.

.
I prefer to set a standard white balance (UV-VIS-IR) and see the three color channels, so I can see if one of the three is out of range;

if the other two are very underexposed with low ISO you can recover from 3 to 5 stops in the shadows.

[Andrea B.]

Yes, we have to be careful when using any JPG histogram for brightness assessment. 

 

I've always had more problems recovering shadows in UV work. So much noise!

[Lou Jost]

That is an elegant image, Andrea.

 

I purposely try to blow out my background! I want the final background to be white anyway, so blowing it out (being careful not to overdo it so that light bleeds into the flower) lets me devote more dynamic range to the flower.  The flower pixels would be represented throughout the histogram except at the extreme right, and the background would be a single sharp spike on the far right edge.  Am I missing something?

[Doug A]

Haven't tried monitoring B&W jpeg for better Raw exposures. I will now. Andrea, thanks for the tutorial on UV exposure and processing. Looking forward to trying this with UV flash.

Thanks for the tips,

Doug A

[Andrea B.]

Lou, the primary problem one might encounter when blowing-out-the-background is that some converters will not render blown white as white. It often gets rendered with a cyanish cast especially when more than one channel is blown. And trying to remedy a cyan cast can lead to other problems. So there is that.

 

The next thing I can think of is that blown backgrounds are not easily accomplished with UV-reflective subjects. Even with the UV-absorptive flower above, there was some too bright areas on the upper petals due to uneven lighting (which was my fault).

 

Finally, when printing, I've found that stark white 255,255,255 can be somewhat harsh. But tastes may vary there. (And most folks don't print anyway!) It's easy enough to tone down white backgrounds to 250,250,250.

 

Like any technique, when it works, it works. But sometimes it doesn't work. 

 

Doug, experimentation is key! Try using the JPG brightness histogram to determine whether it works for you, your camera and the subject.