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UltravioletPhotography

Sigma camera for UV reflectance imaging


Adrian

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I have been lent a Sigma Quattro sd camera, to try out for UV reflectance imaging. For those not aware of it, Sigma cameras have a user-removeable “dust protector” filter, which seems to incorporate a hot mirror filter. I used the camera some time ago for IR, and found it to be very good for that. At the time I was not able to use it for UV as I couldn’t find a lens adapter (the Sigma has a proprietary lens mount), but have now got one (see picture)

The camera proved to be highly sensitive to UV. The UV image of the Cleopatra butterfly (Gonepteryx cleoptra) was shot at f/22, at 200 ISO.

For the visible light control I put a Kolari hot mirror filter back on the front of the camera, and found the colour to be remarkably good.

 

Technical details: Sigma Quattro sd camera with dust protector removed. Lens: El Nikkor 105mm. Baader U filter. Lighting: 2 x Metz 45CL1 full spectrum modified flash guns.

I have not been able to “white balance” the UV image as the Adobe raw converter does not recognise Sigma raw files I will see if Photo Ninja does.

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Oooh, nice photo and an interesting result. What was the exposure time for the UV image? And what is a normal exposure time with those settings and that lighting and lens and filter for your usual camera?

 

Photoshop usually can’t white balance UV regardless of camera.

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Very interesting. The SD14 I tried was, frankly, pretty rubbish in UV. Be interesting to see how this compares, in a 'like for like' situation.
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Thanks for the comments.

Andy, the exposure for the UVR image was f/22 @ 200 ISO. My "usual" UVR exposure with my full spectrum Nikon D300 is around f/8 at 400 ISO, so this is around 4 stops more sensitive (very unscientific!)

 

Birna, Photo Ninja also did not recognise the Sigma raw files . I use a home made "grey" scale made from grey PTFE tape. This is how it balanced the JPEG file: I will see if I can get hold of the Sigma software, which should convert the files to .DNG. This camera costs around 700 GBP, so could be a good relatively inexpensive camera for UVR.

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4 stops more sensitive!!! Blimey, sorry, nearly dropped my coffee there. Definitely very different to the SD14. My current thoughts on the whole monochrome vs standard sensor comparison on my EOS 5DSR cameras, is that the Monochrome one is between 6x and 10x more sensitive depending on wavelength in the UV. I can imagine that the Foveon without its Bayer filter could have increased sensitivity in the blue especially, but it'd be interesting to see about the other layers.
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Adrian, I was wondering if you would be able to make that Sigma Quattro raw file available so that I could run it through Raw Digger to determine where the UV light is being recorded? (Mostly blue channel, most likely.) Typically we pass raw files to one another using Dropbox or some similar app.

 

Have you tried the open source Dark Table app for the raw file?

Added: I just looked and Dark Table does not support the Foveon sensors.

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This really makes me want to rent and directly compare.

 

Adrian, you can in camera save files as DNG if the firmware is updated. This may make software choices easier.

 

Andrea, last I looked Raw Digger did not open the raw sigma files but pulled out the imbedded jpeg file. This may have changed or it maybe able to open the in camera saved DNGs. I haven't looked into this recently.

But since Lens Rentals has 15% off right now and stocks the SDq I might just do a range and sensitivity test.

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Andy,

 

I used flash, so the exposure was 1/160th second at f/22. (2 x Metz flash guns with plastic window removed to convert to "full spectrum"). The flashes were approx. 30cm from the butterfly

 

I will check the firmware. As it is a loan camera from Sigma I probably need to ask their permission before updating the firmware!

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Adrian,

I just noticed you used a flash hotshoe adapter to pc sync. Is there a reason why you just didn't use the sync port on the front of the camera?

 

I have finally decided to do a direct comparison of my Sd14, Em1 modified and the SDq. My rental unit hopefully will arrive on the 26th. I will then have to test range and sensitivity using my lights and filters. The 2 minutes maximum bulb time I think will still be a disadvantage. As I do like the max 30 minute live bulb time on the Em1. Seeing the image develop over time is actually fun.

 

Andrea,

It looks like Raw Digger can fully open the Sigma files. I mixed it up briefly with their fast image viewer. Which just looks at the jpegs. Sorry for any confusion.

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Andy

In a nutshell: electronic flash is a powerful source of light, which lasts for a short period of time, from 1/400th second to 1/25000second (or less). If you use a shutter speed of say 1/160th (called the sync speed), and fire the flash in the middle of it, then the flash will (usually) swamp the ambient light and expose the image. There are some scenarios when you can combine flash and daylight. So, the shutter speed usually has very little bearing on the final exposure.

DA Bateman: There have been many stories over the years of old flash guns with high trigger voltages damaging the electronics in modern cameras. I have not experienced this, nor know anyone who does. As this was a loan camera, I thought I ought to use the PC hot shoe adapter to try tp prevent that.

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Adrian,

Yes if mounted on top of a camera, on the hotshoe an older 283 or 285 will have a volt spike greater than the cameras maximum allowable voltage of 6V and can fry the camera.

This is why you should allways use the PC sync port, if provided on the camera. The Sigma pc sync port and most other cameras, including my Olympus Em1 can handle a maximum of 250V. I know the Panasonic G9 is special and its sync port can handle 400V.

 

Yes Andy buy the Canon 199A. Its conversion to UV is easiest. Just unscrew two screws from head and you are miles away from the deadly transformer. Also you can sometimes get fully working units from KEH for only $2 plus shipping. I have 2 of them.

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Adrian, do you still have buttercups or dandelions in the garden? It'd be interesting to see whether this new Sigma gives a range of colours in the UV like the Bayer filter cameras.
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Unfortunately most of the flowers have gone. I might see if there are any sunflowers around in my local florist. Hopefully Sigma will let me have the camera again next spring. I have just shot a carnivorous plant with it, and will post that soon.
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It used to be that the Foveon blue photodiode layer junction depth was 0.2micron and designed for 420nm blue light but the overlying structures severely attenuated any UV and some blue light. Maybe the Quattro sensor has reduced overlying structures in a bid to enhance capture of blue light and in doing so has improved its UV capability.
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Jonathan, I don't think you will ever see the color that we get with bayer cameras. The Sigma is really just like monochrome camera.

 

However, your comment does offer an interesting idea that I will try to test. You may be able to use the Sigma for large channel separation. Using a UG1 or U330 filter which have nice UV band and a large IR band, it may be possible to illuminate a subject and get both UV reflectance in the the blue channel and UV induced IR fluorescence in the Red channel. I tried to quickly test this using ug1 1mm filter and two 365nm LED lights, but my subject was not very fluorescent in the IR. Using just a 720nm IR filter I needed 15 second exposure, where the uv reflectance was 1 second. I will have to find a better subject, but testing this out might be interesting. Getting both types of images for the same exposure. The U330 filter may give some green information, in the mid wavelengths. I also have a 2.5mm UG5 which might work, as the green band is very low at that thickness.

Something I will also have to test with the Quanttro when it arrives.

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When the Foveon sensor first came out it theoretically offered great potential for UV and IR work due to its stacked photodiode channels. Although some pushed it into service for these purposes, it never really reached its theoretical potential. I tried it for both UV and IR, and quickly determined that even the unconverted Nikon D40 outperformed it for UV work, although perhaps due to better in-camera firmware processing. Foveon UV was very noisy and IR, although not bad, had some strange in-camera processing going on. Theoretically, UV should be in the blue channel which it was primarily, and IR in the red channel which certainly wasn't the case. Sigma processing showed IR data in the blue and red channel but none in the green......not possible! Further investigation bypassing Sigma processing revealed that it occurred in all 3 channels increasing in intensity from blue through to the red channel but certainly not confined to the red channel only.

 

The presentation of the Quattro UV is certainly promising and looks to far exceed what I have seen in the past with regards to Foveon UV capture. I'm certainly interested to see more but I would also like to understand what has changed to produce such an outcome. Certainly in 10 years there must have been some improvements in firmware noise reduction but what else?

 

Ideally we need 5 stacked photodiodes (and decent firmware), for UV, B, G, R and IR, although the physical limitations for producing a true UV channel are extremely challenging.

 

The Foveon is similar to a monochrome camera in that it does not have a Bayer array and so demosaicing is not really part of its processing, but it still requires colour interpolation of the three stacked photodiodes.

 

On a side note - I tried to find links to postings I had made about this in 2009 on various UV and IR forums but those forums and important postings made by many have long gone. This often leads to trying to reinvent the wheel. This site houses some extremely useful postings and I hope that these also don't eventually disappear when the site is longer active.

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