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UltravioletPhotography

Learning In-Camera UV White Balance


JCDowdy

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After studying the thread How to make UV colours reproducible http://www.ultraviol...s-reproducible/ I am please to post what to me is welcome results.

Like any gadget a digital camera can quickly turn from frustrating to delightful once things "click"! Both of these images are camera jpg and my live view focus target was the window panes in the cupola of the building across the parkway ~125 meters away.

 

First photo, LifePixel modified full spectrum DMC-G3, Badder UV/IR-Cut/L, "KA" Soligor 35mm f/3.5 set to f8 camera auto select ISO 160 and 1/2500 shutter.

The colors look over saturated using default camera sunlight WB.

 

post-24-0-89375200-1377454694.jpg

 

Second photo was shot with Baader-U and was white balanced in-camera to a 1.25" Labsphere Spectralon 99% diffuse reflectance standard plus a black anodized Al disk (filter stack cover) both placed on a grey foam core sheet lying flat in full sun. The camera auto selected ISO 800 and shutter speed of 1/60 but I extended that to 1/10 to lighten the image. The UV image seems to have a slight green hue compared to the pale blue cast I observe in other postings.

 

post-24-0-74954000-1377454840.jpg

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Find the 'Edit' link below your post.

 

Edit -> Use Full Editor -> Attach Files which will present you with Browse functionality. Locate file, then click Attach, then when the thumbnail is shown, click Add to post. The image will be added at the current position of the cursor within your post, so pay attention to that and you'll be just fine.

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Thanks, but that is what I did the first time. I also tried both versions of the file uploader on two different browsers.

I tried to upload again today with the same result.

Since I have not uploaded a photo on this site before, perhaps some type of permission setting is needed?

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You are a member with all necessary permissions to upload material.

 

However, there might be a constraint as to the file sizes. Make each file

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You are a member with all necessary permissions to upload material.

 

However, there might be a constraint as to the file sizes. Make each file << 1 MB to see if such a limitation is in force.

 

That worked, original file was ~5.6 MB, reduced to <1 MB uploaded.

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You should have gotten a message that the file was too large for uploading. This did not happen?

 

*********

 

But about your photo. Yes, it has a colour cast still. Actually a bit too yellow I think. But you're in the ballpark.

 

What looks green is really an unsaturated yellow, a kind of greyish-yellow. The appearance of this grey-yellow is not wrong, but there seems to be a bit too much of it. There is also still some purple in the car wheel.

 

For pure saturated yellow R=G and B=0. For grey-yellow, R=G and B>0.

 

In your photo the car window in the foreground is (97,92,49) which is indeed grey-yellow, not green. The grass is (66,63,49) which is also grey-yellow, not green. Those measurements were made in only two spots, so there will be variation in the exact numbers.

 

Your in-camera white balance step was made in conjunction with a Visible colour profile that does not take into account the altered RGB curves caused by removing the internal UVIR blocking filter. Then when you opened the photo in the converter or editor, a Visible colour profile is again applied that doesn't match the RGB curves in the camera. So a bit more work is needed to get closer to the usual standardized look.

 

I don't know what editor you are using, so I can't offer any specific suggestions.

Asphalt is usually UV neutral, so try a white balance or colour cast removal in the editor on that area of the parking lot.

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Thank you for the thoughtful observations.

 

I do not recall a file size error notification. I’ll know better next time.

 

From some of your remarks I realized that I may not have described what I did clearly. I attempted to do what is shown in figure 5 of How to make UV colors reproducible. Both images shown were shot as jpg and are unprocessed other than my rudimentary in-camera white balance for the second image and rescaling to an uploadable file size. I failed to record an image of my w/b targets at the time but here is a reasonable facsimile shot today with the UV/IR-Cut/L. The lens as mounted is just off the edge of the focus range, or else I am not tall enough.

 

post-24-0-98939100-1377557371.jpg

 

If I understand Bjørn’s comments in figure 5 this w/b is conducted with the filter in place. I suspect I do not yet have “adequate targets” arrayed to achieve comparable results. I assumed, perhaps incorrectly, that the anodized aluminum filter stack cover and the gray foamboard are UV neutral. Is the color passport needed for this in-camera method, or is it there simply for reference?

 

- John
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It serves several purposes of which I'm certain Andrea is about to tell us a lot in the near future. Some of the patches are UV neutral and can be used for UV w/b (black patch in particular). Also allows though various software to generate a camera/lens specific profile which can be used for fine-tuning the camera profile. Andrea tells me this is crucial but I'm not convinced I'm following her all the way this time. Suffice it to say, however, that the Color Checker is a valuable tool whether you conduct visible- or UV-band photography and well worth the cost.

 

Foam inserts for packaging (the grey type) can be surprisingly good for UV balancing and they further have a diffuse light field character which is a worthwhile bonus. Whenever I find a good sample (tested with a profiled camera) I put them in the dashboard of my car so a passable UV balancing target always is available.

 

White or grey PTFE discs tend to be fairly UV neutral, but I recently came across a sample that definitively added a clear cast (in blue so the end balance would be biased towards yellow).

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John, to answer your question about the CC Passport -

 

We use the Color Checker Passport primarily to correct Visible colour for photos made with converted cameras and the Baader UVIR-cut filter. Even with the Baader UVIR-cut filter, a visible shot with a converted camera will not have correct colours because of the removal of its internal UVIR-block filter. The white balance step will therefore no longer completely correct colours in such a photo. The way in which the CC Passport is used is to create a new camera colour profile for the converted camera.

 

If you want to do an in-camera white balance with your Spectralon standard, then you must try to fill the frame as much as possible with it. It need not be in focus. It should be fully illuminated by whatever light is in use for a given photo session. The white balance preset you create with such a standard is dependent on camera, lens, filter and source of illumination. If lens, filter or light changes, then you must create a new preset.

 

If you plan to do any Visible photography with your converted Lumix, then the CC Passport is mandatory for creation of a new converted camera colour profile.

 

The CC Passport has a secondary use as a simple indicator that our UV white balance step is producing the standardized blues under the Baader-U filter. We do not use it any formal way for UV photographs.

 

Q & D SHORTCUT: Somewhere along the way both I and Bjørn discovered that the black patches on the CC were very close to UV-neutral and could be used for a quick-n-dirty UV white balance. White balancing on black does tend to be less accurate than white balancing on the typical neutral greys or white. We have no information about how far into the UV the CC black patches remain neutral so this characteristic may be filter dependent.

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