|Posted: Wed Nov 27, 2019 2:48 pm Post subject: 40mm lens made from Thorlabs parts
|After working on UV and IR imaging for a while, I found myself getting more interested in quirky lens, and especially lenses for UV. A while ago while searching for information I came across something called a Hanovia Kalosat Spectral Diffusion lens, which was a large format lens with a single quartz element. Having failed to locate one for sale (I guess they are very rare) I thought, why not make something similar. I also came across one of Dr Klaus Schmitts posts where he had done something similar.
My version is based around a Thorlabs 40mm focal length, 1" diameter, UV silica biconvex lens with no coatings on it. I got a bunch of other fittings from Thorlabs at the same time so I could add an adjustable aperture, make it M42 mount, and add a 52mm filter ring on the front. Very impressed with Thorlabs, great to deal with. This is what it looks like;
The aperture is quite a nice one - opens up to 25mm, and closes down to a couple of mm. So in theory, this is a 40mm f1.6 lens wide open. After fitting this together, I fitted it on the Eos 5DSR camera, just fitting it direct on to the M42 adapter. Of course, this was macro only, given the flange to focal plane distance on the EOS. Wide open it was very very soft and glary. Below are a couple of shots in my garden with it stopped down. Nice, typical rainy day here.
As another quick experiment, I wondered what it was like in IR. This is a shot of my garden with a 40mm EOS pancake lens on the EOS 5DSR camera, taken through the window (rain had returned again at this point). Normal colour image.
The a couple of shots from my multispectral converted Sony A7III. These are IR shots with a 720nm longpass IR filter. Firstly with the Eos 40mm lens and IR filter.
Secondly with the Thorlabs 40mm silica lens and IR filter. Same exposure as image #8.
Contrast was boosted for bothe IR images, curves tweaked, but both in similar ways. Yes, my sensor is dirty. The small aperture on the silica lens really shows that up. On the A7III the is much further away as the FFD is much smaller than the EOS. Will be able to use helicoids on this setup. The blob on image #8 towards the bottom is a rain drop from the window I was shooting through.
I'm looking forward to trying it in the UV (when we get some here). I also plan on getting a 40mm focal length plano convex lens, and perhaps even a 40mm meniscus lens at some point to compare them with this biconvex one.