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Thorlabs 79mm UV aspheric lens for imaging

Lens
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#1 JMC

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Posted 15 May 2021 - 11:01

I recently had to put an order in with Thorlabs and while I was there, I thought I'd try out another one of their lenses for UV photography - a 79mm UV fused silica aspheric (https://www.thorlabs...umber=ASL10142M).

In the past I've played around with some of their 40mm lenses in a setup built from their parts, but I got a lot of distortion at the image edge with that (not a big surprise). See here for the thread on that - https://www.ultravio...dpost__p__33674

Moving to a longer focal length and an aspheric lens design, I hoped would help get some of the distortions under control and make the lens more usable.

Here's the lens.
Attached Image: 578A2890sm.jpg

And mounted on the camera using a range of parts.
Attached Image: 578A2895cropped sm.jpg

Attached Image: 578A2897 cropped sm.jpg

The weather here is dull and rainy today, but I went out and tried to get a photo anyway, taking a Nemo UV torch with me to boost the 365nm UV a bit. Baader U filter used and whitebalanced in Darktable.

This the full frame image from the A7III, just with some denoising and contrast boosting done, with the lens stopped down about half way (I guess about f8). Resized for sharing obviously.
Attached Image: DSC00450mod sm.jpg

And a crop from the original, kept at the original image resolution.
Attached Image: DSC00450mod cropped.jpg

The Buttercup photo was done at ISO6400 and 1/15s with a mix of daylight (no sun) and Nemo 365nm UV torch.

Overall, I'm quite impressed with this little singlet. I'll hopefully do more with it when the sun comes out, but I also want to try it for UVB (or even UVC) imaging.

Edited by JMC, 15 May 2021 - 18:45.

Jonathan M. Crowther

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#2 Stefano

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Posted 15 May 2021 - 12:33

So this is a single lens, right? If so the sharpness is very good.

#3 colinbm

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Posted 15 May 2021 - 13:10

This is placed at 79mm from the sensor ?

#4 JMC

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Posted 15 May 2021 - 13:19

Stefano, yes this is a singlet.

Colin, at 79mm from the sensor it would focus to infinity. Move it further from the sensor and you can focus closer.
Jonathan M. Crowther

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#5 JMC

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Posted 15 May 2021 - 13:22

The sun came out (briefly, this is the UK after all). Quick photo of a Dandelion in sunlight. Again, Baader U (no Nemo 365nm torch this time, just sunlight). ISO1600 and 1/4s. Whitebalanced in Darktable. Again this is the whole of the full frame image, just resized for sharing.
Attached Image: DSC00460mod small.jpg

Edited by JMC, 15 May 2021 - 13:23.

Jonathan M. Crowther

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#6 colinbm

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Posted 15 May 2021 - 13:23

View PostJMC, on 15 May 2021 - 13:19, said:

Stefano, yes this is a singlet.

Colin, at 79mm from the sensor it would focus to infinity. Move it further from the sensor and you can focus closer.

Thank Jonathan
Food for thought...

#7 Stefano

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Posted 15 May 2021 - 13:23

If I will have a UVC-capable camera one day I will consider this lens (or similar). They won't rival a dedicated multi-element lens, but the image quality is very promising and good enough for me.

#8 JMC

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Posted 15 May 2021 - 13:24

No problem guys.
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#9 UlfW

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Posted 15 May 2021 - 13:35

The benefits of aspheric lenses decreases when stopping down.

The change to a longer FL is, I think, a very important part of the improvement, over the 40mm PCX.
I would be interesting to see how well a PCX lens of similar FL would perform.
Those cost less than 1/3 as much as this aspherical lens.
https://www.thorlabs...rtnumber=LA4725
Ulf Wilhelmson
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#10 JMC

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Posted 15 May 2021 - 13:39

Ulf, longer focal length is certainly a benefit here. I got this one so it could potentially be used wide open as well as stopped down. Not going to be buying any more from them anytime soon so no comparisons for now. Be nice eventually to compare them both from wide open.
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#11 Stefano

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Posted 15 May 2021 - 19:14

View PostUlfW, on 15 May 2021 - 13:35, said:

The benefits of aspheric lenses decreases when stopping down.
Can you please explain me why? Doesn't stopping down always increase the sharpness (until diffracton starts softening the image)? Especially with these lenses, since they are very simple and often have soft edges.

#12 Andy Perrin

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Posted 15 May 2021 - 20:05

The more you stop down, the closer you get to becoming just a pinhole, so the benefits of adjusting the curvature of the lens to correct for spherical aberrations go away. Like, why not just use a pinhole if you stop down to F/100?

#13 Stefano

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Posted 15 May 2021 - 20:43

OK, in that sense. You don’t lose quality, you lose the advantage of it being an aspherical lens.

#14 Andy Perrin

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Posted 15 May 2021 - 21:25

And eventually you lose quality too as the diffraction effects become significant and the exposure time gets longer and longer.

#15 JMC

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Posted 16 May 2021 - 13:45

View PostUlfW, on 15 May 2021 - 13:35, said:

The benefits of aspheric lenses decreases when stopping down.

The change to a longer FL is, I think, a very important part of the improvement, over the 40mm PCX.
I would be interesting to see how well a PCX lens of similar FL would perform.
Those cost less than 1/3 as much as this aspherical lens.
https://www.thorlabs...rtnumber=LA4725
Ulf, I've actually just placed an order with a different optical supplier and they had 80mm focal length (a close match to this aspheric one) plano convex fused silica lenses in stock, so I included one in the order.
Jonathan M. Crowther

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#16 UlfW

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Posted 17 May 2021 - 10:21

Great.
I'm looking forward to a comparison, eventually.

I have two Ø40mm PCX FS with focal lengths of 50mm and 75mm.
I will try them out in the future and post the results.
It might take some time until I am ready to do that.

Edited by UlfW, 17 May 2021 - 10:21.

Ulf Wilhelmson
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#17 JMC

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Posted 17 May 2021 - 10:44

Ulf, I suppose really I should do a comparison of the two lenses at infinity as well as macro as that will have a big impact on how far the lens is from the sensor and any distortion at the edges of the image. This comparison is 'growing' and may take me a while to get round to.
Jonathan M. Crowther

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#18 UlfW

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Posted 18 May 2021 - 08:19

That sounds like a good approach.
I think the best usage will be for more close up motifs if utilising the deep UV-reach is the goal.
My guess is that that will also give best image quality.
Ulf Wilhelmson
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#19 Andrea B.

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Posted 20 May 2021 - 00:24

I'm a bit late getting to this topic, kindly excuse.

Jonathan, thanks for posting this interesting aspheric singlet topic. I am always so impressed that our UVP members try such interesting experiments and tests !!
Also, it is so instructive to see how you have used the various pieces to mount this lens.

So who needs a UV-Nikkor anyway? This is really pretty darned good for a single piece of glass. :cool:
Andrea G. Blum
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#20 JMC

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Posted 20 May 2021 - 08:36

No problem Andrea. I was amazed by the quality in my test shot especially given the price. I hope to do some more thorough testing with it in the future and will of course update the site with the results.
Jonathan M. Crowther

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