Thorlabs 79mm UV aspheric lens for imaging

[JMC]

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.com/thorproduct.cfm?partnumber=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.ultravioletphotography.com/content/index.php/topic/3754-back-to-basics-40mm-singlet-lens-from-thorlabs/page__view__findpost__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.

 

And mounted on the camera using a range of parts.

 

 

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.

 

And a crop from the original, kept at the original image resolution.

 

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.

[Stefano]

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

[colinbm]

This is placed at 79mm from the sensor ?

[JMC]

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.

[JMC]

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.

[colinbm]

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...

[Stefano]

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.

[JMC]

No problem guys.

[ulf]

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.com/thorproduct.cfm?partnumber=LA4725

[JMC]

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.

[Stefano]

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.

[Andy Perrin]

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?

[Stefano]

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

[Andy Perrin]

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

[JMC]

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.

[ulf]

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.

[JMC]

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.

[ulf]

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.

[Andrea B.]

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.

[JMC]

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.

[Andrea B.]

Looking forward to that, Jonathan.

 

A quick question: Do you think this is a good way for members to have a UV-capable lens without the expense of trying to find something on Ebay? We should look at this further methinks. I always try to support low cost alternatives for those who want to try reflected UV photogrpahy without a huge expense.

[JMC]

It's still not cheap Andrea, and fir the majority of users (who will be dealing mainly with UVA) the deep reach of this is not necessary. A better bet is still a 80mm El nikkor.

[Andrea B.]

side note: But there is that flare-y thing with the El 80 that bothers me. Altho I certainly agree it is a good less expensive alternative.

[Stefano]

For UVC imaging it might be useful. I tried to take UVC images with one of my 265 nm LEDs (8-10 mW of output power), using a pinhole camera with common copy paper behind the hole (it fluoresces under UVC) and photographing the paper with my camera. I tried very long exposures, being careful to eliminate all light leaks, but I just couldn't do it. Not enough light to see. Putting a simple fused silica/quartz lens in place of the pinhole would have made a huge difference.

 

The thing is, the full UVA range (320-400 nm) is easily accessibile with common, old UV-capable lenses (not all of them, anyway), and the upper UVB range (~305-320 nm) is accessibile too, even though with limited transmission (according to Ulf's data, a Focotar-2, probably one of the best UV-capable lenses in terms of reach we know, transmits only ~20% at 320 nm). Lenses like these can be used for UV TriColour, for example, as Bernard nicely showed.

 

But for the lower UVB range (280-~305 nm) and UVC range (below 280 nm), one has three solutions (more perhaps):

-Buying an expensive UV-dedicated lens;

-Using a pinhole;

-Building his/her own lens.

 

The latter to me seems the best one for people who want an actual UVC lens but can't afford those super expensive lenses. The quality can be quite good, they are faster than a pinhole and also I think they give you satisfaction when you build them.

 

But for "general purpose" UV photography, a Soligor/Prinz Galaxy/others is a cheaper option (usually) and the image quality is better (those are still actual camera lenses, even if not designed with UV in mind).

[rfcurry (1950-2024)]

Has anyone used Topas COC for lenses? Do we really require the depth of UV that fused silica offers? Since COC is good enough for optical lenses, e.g., eyeglasses, and can be produced in endless prescriptions at a very low price, why not for other optical needs?

 

 

I'm curious what others have tried. I have a kilo of Topas 5013L-10 that I am hoping to dye and then have extruded as flat filter components. Does anyone have a hot-melt extruding company that does small batches that they can recommend?

 

One source for plastic lenses - https://www.alignoptics.com/plastic.html

 

Thanks.