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UltravioletPhotography

GBU in the Fog


OlDoinyo

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The GBU-->RGB cross-sampled image is the rarer counterpart of the better-known IRG image, and unlike the latter, has no practical history or technical application; it is largely a curiosity. No film ever produced this sort of image. The only way to obtain this type of image is by a two-exposure technique. My only previous forays in this direction have been on sunny days; but the other day was quite foggy and I was curious to see how this genre interacts with such conditions. I like to shoot in the fog, anyway; give me trees and give me fog and I can usually come up with something, and the best of what I came up with on this occasion follows.

 

Here are some things you might find interesting to look for:

 

-Vegetation is rendered in hues ranging from maroon to rust-brown.

 

-Objects take on a greenish-yellow cast as they become more obscured by the fog. I am not completely sure if this is an artifact of workup or if this reflects some underlying physical reality such as differential Rayleigh scattering or absorption by water (which has a transmission maximum around 466 nm). I have suppressed the effect in several frames where it seemed excessive, but otherwise left it largely alone. I have rendered the fog colorless; it could be argued that this is an artifice, as the ambient UV flux is in reality much less than either the green or blue flux. However, I did not want strongly tinted images.

 

-Yellow: This denotes surfaces which are UV-dark while reflecting visible. Lichens, apple blossoms, and titanium dioxide paint show yellow.

 

-Magenta: this appears on strongly blue-absorbing surfaces that are UV-bright. To my surprise, it shows up in the foreground of one of the frames.

 

All images obtained with Sony A900 and Steinheil 50mm lens at various apertures (aperture matched between frames of same shot.) UV component obtained through Baader U2 filter with manual exposure and custom white balance. Visible component obtained through KolariVision deconverting filter with aperture-priority automatic exposure and +.3 to +.7 exposure compensation. and "cloudy" white balance.

 

"The Bashful Bush 1"

post-66-0-86258100-1429106278.jpg

 

"The Bashful Bush 2"

post-66-0-25796700-1429106376.jpg

 

"Outcrop"

post-66-0-32959200-1429106418.jpg

 

"False Autumn"

post-66-0-91099200-1429106478.jpg

 

"Among Strange Flowers"

post-66-0-46961700-1429106522.jpg

 

"Triple Echo"

post-66-0-47424700-1429106561.jpg

 

"Apple Blossom Time"

post-66-0-59838300-1429106603.jpg

 

"Rake and Sand Trap"

post-66-0-11113100-1429106673.jpg

 

"Right, Left, Right"

post-66-0-10419600-1429106753.jpg

 

"A Choice of Uncertainties"

post-66-0-69571200-1429106790.jpg

 

GBU images often do not have the in-your-face surreality of some other cross-sampled image genres; some of these images might be mistaken for ordinary color images. In every case, however, the scene as depicted does depart substantially from what the naked eye saw (or the visible source frames, for that matter.) Green grass is transformed into a quasi-autumnal brown, for example.

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This is really beautiful photography! The fog plays such an interesting role here. I like your decision to render the fog colourless.

 

I want to say that these photos are very "atmospheric" without intending any play on words. We can feel that fog when we look at these photos.

 

The photographs would stand 'on their own' even if we did not know about the GBU -> RGB mapping.

 

A lovely contribution to our pages - thank you!!

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Looks very interesting Clark.

Unfortunately fog is not very frequent where I live :lol:

I am not familiar with the term 'KolariVision deconverting filter' ?

Col

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Colin---

 

The filter in question may be found here.

 

http://www.kolarivision.com/hotmirror.html

 

It is marketed as a "hot mirror" but in reality it is nothing of the sort. It is a purely absorptive filter with no dichroic components. It is also an excellent match to the A900 in terms of color balance (but not the SD14: it gives rather blue images with that one.)

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Thanks Clark

I have that KolariVision filter, yes, it appears to be a good match for a IR cut filter for many Bayer CFA cameras, but certainly not the Sigma Foveon cameras.

I am setting up a, Panasonic Lumix DMC G3, for UV photography & I will test the KolarVision IR cut filter for Visible imaging.

http://www.ultravioletphotography.com/content/index.php/topic/1316-ebay-uviroptics-is-being-slandered-on-youtube-ebay/page__fromsearch__1

Col

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Very ethereal for sure. My favourite is "Outcrop".

 

Did something vaguely similar with D70 and the UG11x back in the late 2007-2008, but my processing was less subtle. Did manage with a single exposure, though.

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  • 2 weeks later...

Postscript: what the magenta flowers were...

 

post-66-0-65808400-1430061558.jpg

 

I don't think I need tell anyone here what this is.

 

My flower macro skills are not up to scratch by group standards, and I was dealing with a large focus shift at close range, so try to be understanding. I put the Steinheil on an extension tube to get this.

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It's cool, Clark !!

 

I love this close-up. It is fun to see the typical bullseye in this false colour range, too.

 

Do you have a UV flashlight in order to focus via Live View when the UV filter is in place?

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No live view on the A900--just a standard ground-glass vewfinder screen. I tried to focus the visible by raising or lowering the tripod center-post. As for focus-matching with UV--crank down to f/16 and hope for the best. If I had been able to focus both frames independently, the image sizes would not have matched (different focal length.)
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