Revisiting the 340BP15 on a GH-1.

[Andrea B.]

I was looking at some photos made with my old Panasonic Lumix GH-1 full spectum conversion using a UV-capable lens equipped with the 340BP15 UV-pass filter, which was purchased on Ebay from Omega Bob about 12 years ago. 

 

The photo displays a round of PTFE, 3 Labsphere standards (99%, 75%, 50%), a Rudbeckia flower, a Bidens flower, and a B+W 093 IR-pass filter. Here is the Raw Digger raw composite of that image. Raw composites have no white balance applied. For display here as a JPG, an sRGB profile was embedded. 

 

 

In Photoshop Elements I selected the large PTFE area and averaged the color to get a saturated, moderately bright color. The yellow dot next to the text is that average color brought to full saturation and brightness. Yellow is at 60° on the color wheel, green is at 120° and halfway between at 90° is lime-green. So this averaged near-yellow color is nowhere near lime-green and even further from green. But because the color is just 67% bright, it appears to have more green than it actually has. We have discussed this color illusion before. As pure yellow becomes darker we begin to perceive it as a type of green usually termed olive green.

 

 

The 340BP15 filter is too narrowband to produce much false color when white balanced by the usual methods. (We've discussed this before also.)

Here is the image after white balancing in Photo Ninja. It is almost black & white. There are some hints of dark yellow. And there is a lot of blue noise. But the false color is not saturated and not bright.

 

 

I'm not sure where all those blue specks came from. After applying Noise Ninja, most of them went away. I think the two little blue dots at the top middle are actually blown pixels. This is the de-noised image.

 

 

Here is the raw histogram. You can clearly see why the basic raw colors are yellows.

 

 

We are all intrigued by the yellowish-green and green colors seen around 330/340 nm. I found that the 340BP15 I was using produced a nice closer-to-lime-green color when a 2000°K white balance was applied in Silky Pix (a native converter for Panasonic Lumix images). So keep that little trick in mind to try the Kelvin scale when shooting or converting narrowband 330/340 nm images. The false color still did not get to 90° lime-green, but I like the outcome better than the raw composite version. 

 

 

If I were finishing this image, I would work on some of the dark areas which are somewhat blocked. The GH-1 doesn't have much dynamic range compared to newer digital cameras. But this topic was just about the color and the white balance, so I skipped any extended processing.

 

 

Here is a sampler of yellow becoming dark, left to right.

Brightness decreases 10% per block. The rightmost block is black, 0% bright.

 

[Stefano]

On my camera (Canon EOS M), the raw color at 340 nm is orange. An example can be seen here: https://www.ultravioletphotography.com/content/index.php?/topic/4650-how-to-make-a-tricolour-image-my-method/&do=findComment&comment=55625

 

I remember you also got orange (link) Now it's greenish yellow. I guess it's the different sensor.

 

[Andy Perrin]

 

15 hours ago, Andrea B. said:

I'm not sure where all those blue specks came from. After applying Noise Ninja, most of them went away. I think the two little blue dots at the top middle are actually blown pixels. This is the de-noised image.

 

Andrea, you might want to explore "developing" those image from the RAW using some other converters, like Raw Photo Processor 64, for example. PhotoNinja's RAW conversion has been known to make artifacts, especially with narrow bandpass filters...

[Andrea B.]

Stefano, yes, it's strange. The 340 here is BP15. The Edmund filter you referenced is BP10. It doesn't seem like that could make much difference. But we are seeing it.

 

ADDED: Other variables are the lens used and the lighting. The photo here was in sunlight with a lens I didn't record. My photo referenced by Stefano was made with Nikon 140 UV-Flash and the UV-Nikkor

Andy, I have RPP. I'll try it.

[Andrea B.]

OK, here we go.

 

This is a raw composite from RPP64. (It is a bit more saturated than what we get from Raw Digger.)

 

 

 

 

Here is a white balanced version from RPP64. I white balanced on the middle labsphere (light grey). No noise reduction has been applied. Immediately obvious is the large amount of noise (blue-purple dots). The GH-1 is very old and its sensor quality is not good. Additionally, white balancing a very narrow bandpass file, is quite hard on the pixels.

 

 

 

Here is an unresized crop of the noise. It is slightly different in quality, perhaps, than the noise produced by Photo Ninja, but it is equal in quantity. The old GH-1 is simply a noisy camera.

 

[Andrea B.]

JMC just reminded me to test this 340BP15 with an IR blocker. Good idea, just in case the weird tint is due to some minor IR leakage. When we get some clear skies back, I will give that a try.