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Q: Sigma 60 mm f:2.8 MFT in UVA?

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#1 Pedro J. Aphalo

    Pedro J. Aphalo

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Posted 29 December 2017 - 16:35

Hello all,
As both the current 19 mm f:2.8 and 30 mm f:2.8 Micro Four Thirds objectives from Sigma are usable in UVA, I wonder if anyone in the forum has information on the UVA performance of the 60 mm f:2.8 from the same Sigma DN Art series. If it is known to work to the same extent as the other two, I would buy one.
Thanks in advance,
Pedro.

#2 Bill De Jager

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Posted 30 December 2017 - 01:53

Pedro, I have the lens but I sold off my broadband MFT camera recently. However, I also have the E-mount version. I'll see if I can check it out this weekend on my converted A6000.
Studying the botany and plant geography of California and western North America for almost 50 years.

#3 Pedro J. Aphalo

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Posted 30 December 2017 - 16:15

Thanks!
Pedro.

#4 Bill De Jager

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Posted 30 December 2017 - 20:56

Pedro, the quick answer is about 2-1/3 stops less transmittance of UV compared to the Coastal Optics 60mm f/4, a standard UV reference lens. That's using a broadband-converted Sony A6000 and a Baader Venus filter. I suspect the transmission doesn't go anywhere near as deep into UV as with the Coastal Optics lens.

I'll be going out a bit later to get photos on a tripod and see how well they hold up under close examination. No flowers this time of the year, but we'll see what else I can get. Unfortunately I'm having a problem with cheap adapters so my F-mount Coastal Optics lens won't focus beyond a meter or two at most. That will make comparison shots more limited in scope.
Studying the botany and plant geography of California and western North America for almost 50 years.

#5 Pedro J. Aphalo

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Posted 01 January 2018 - 10:03

Bill,
Thanks! I do not expect the Sigma objectives to get close to the Coastal Optics or the Kyoei. The question rather is, are they usable at all for UV. I have compared the 19 mm f:2.8 to a Soligor 35 mm f:3.5 (Kyoei Acall rebranded). The 19 mm is behind by approximately 1/2 stop, but only when using the StraighEdgeU filter. With a Baader U-Venus, the difference seems to be about 1 or 2 stops, so not very far from what you observed for the Sigma 60 mm f:2.8. The Sigma 19 mm "sees" LEDs emitting at 385 nm and at 365 nm but it is "blind" to 340 nm LEDs. However, the emission peaks of LEDs are rather broad... I am in Finland, so we do not have much sunlight at the moment.
Pedro.

Edited by Pedro J. Aphalo, 01 January 2018 - 10:17.


#6 Bill De Jager

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Posted 02 January 2018 - 01:33

Pedro, I did more tests and it looks like this lens is usable in UV. I hope the information below is helpful.

Camera: broadband-converted Sony A6000
Lenses: Sigma 60mm f/2.8 DN (E-mount) vs. Coastal Optics (Jenoptik) 60mm f/4 APO macro (reference lens)
Exposure: aperture priority, varied by lens and exposure
Filter: Baader Venus
Date: December 30, 2017 (EXIF date is wrong)
Latitude: ~38 degrees N.

First, unprocessed JPGs to show the difference in frequency response. These have been downsized and cropped to approximate the processed photos. The plant is coast live oak, Quercus agrifolia.

Coastal Optics

Posted ImageCoastal UV unprocessed by Bill de Jager, on Flickr

Sigma

Posted ImageSigma 60 UV unprocessed by Bill de Jager, on Flickr

Then I converted the photos to monochrome as a quick way of assessing performance. Due to lack of practice I took with me my ColorChecker Passport instead of my reflectance standards which would have allowed proper color UV balance, so monochrome was my best choice. These have been downsized and cropped; sorry the sizes don't quite match. While the camera was on a tripod, the combination of slow shutter speeds and wind was an issue so I didn't feel that pixel-level crops would allow a fair comparison.

Coastal Optics

Posted ImageCoastal UV mono by Bill de Jager, on Flickr

Sigma

Posted ImageSigma 60 mono by Bill de Jager, on Flickr

These have been processed to try to make them look their best, but since their starting points were so different the comparison merely gives some idea as to what is possible. I note more noise in the Sigma image, which is to be expected given the higher ISO (3200 vs. 1600).

I also noted that the A6000 had some difficulty focusing the lens in UV, as most shots were blurry. I tried to take photos with the Sigma 19mm DN for purposes of comparison and all of them were blurry. UV transmission of both lenses appeared to be similar, but can't be compared exactly since the light was changing at the time.

I conclude that the Sigma 60mm f/2.8 (available in Sony E-mount and Micro Four Thirds mount) is a usable lens for UV photography. However, as of now I don't have sharply focused UV photos taken with this lens that would allow close examination for optical faults.

I also want to put in a plug for the Sony A6xxx cameras for UV purposes. These cameras are no larger than Micro Four Thirds cameras but have a much larger (APS-C) sensor. The A6000 is quite affordable; wait for sales which will bring the price down to US $450, or find a refurbished or used unit. They also have a very short register distance and can mount a large range of lenses. The downside is that it's very lightweight and not very sturdy, so I wouldn't trust a very heavy lens that lacks a mount to not damage either the lens mount or the mounting hole.

The A6000 comes across as a high-tech plastic toy, so be prepared to deal with that. It has its uses, especially as a lightweight coat-pocket camera, but probably won't satisfy in use in the same manner that a more substantial camera would.

Edited by Bill De Jager, 02 January 2018 - 01:38.

Studying the botany and plant geography of California and western North America for almost 50 years.

#7 Pedro J. Aphalo

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Posted 02 January 2018 - 08:00

Thanks a lot! This is very useful information. For now I will stick to my Olympus E-M1, but I will keep in mind the Sony cameras.
Pedro.