Testing an (achromatic?) doublet lens for UV

[Stefano]

I have some lenses and prisms my dad found somewhere and gave me years ago. I didn't use them much, but I recently tried to use one of the lenses for UV photography. I have two of those lenses, but one of them is chipped and the other one is in good conditions, so I used the latter. It is clearly a cemented doublet, and my guess is that it is a crown/flint achromatic doublet. One surface is almost flat (it looks convex), and the other one is slightly convex. I don't remember how I ended up mounting it, but if you want to do a similar experiment you have to orient the least curved side towards the sensor and the more curved side towards the subject. The diameter is 22 mm and the focal length is about 80 mm.

 

I assembled together a bunch of step-up and step-down rings, and at the base there was a 42-39 mm step-down ring to screw everything on a M42 helicoid. The lens was fixed in place with a generous amount of putty.

 

This is how the lens looks like:

 

 

(Sorry for the bad focus)

 

It does have a very slight yellow tint (visible in the first photo) that I also see with my eyes. This lens transmits UV pretty well, but it completely blocks UVB at 310 nm.

 

Photos with the lens fully open (about f/3.63). Full-spectrum Canon EOS M, in-camera white balance (I think it was a preset made with the Soligor 35 mm f/3.5). Filter: ZWB2 (2 mm) + Chinese BG39 (2 mm).

 

ISO 12800, 1/30 s exposure. Brightened by 50 in Windows Photo editor:

 

ISO 100, 1/2 s exposure:

 

Full-size crop:

 

Then I stopped down the lens by addind a ~3 mm diameter hole made with black tape (about f/26.7):

 

ISO 100, 8 s exposure:

 

Full-size crop:

 

ISO 200, 8 s exposure:

 

ISO 100, 30 s exposure:

 

The edges were still a bit soft, but in the middle the image was very sharp. There is what looks like a hotspot. Chromatic aberration was present (see the net in the second photo) but quite contained. I would still suggest to buy a known UV-capable lens, but an achromatic doublet (assuming this is that) can be used as a lens for UV. If you can, try to find one with some correction for spherical aberrations.

 

Note: crown glasses can easily transmit well in the 300-400 nm UV range (BK-7, a common crown glass, can even transmit UVC if thin enough), but flint glasses are not as good, and they will be the limiting factor. Some (like Schott F2) have a good reach in the UVA but block UVB. Very dense flint glasses will start to absorb in the blue-violet and will have a yellow tint (and practically no UV reach). Also, in cemented doublets, the cement itself might absorb UV. Keep that in mind if you want to search for achromatic doublets for UV photography.

[Andy Perrin]

Interesting. Stefano, add some flocking (see the black materials thread) to help that hotspot. Also make sure the edges of the lens are dark/covered. Putty transmits IR in my experience.

[colinbm]

Good to see Stefano thanks for sharing

[photoni]

it is probably the front doublet of a Petzval, (a lot of incision in the center but a lot of swirl at the edges) in 100 years millions of them have been produced, they were easily disassembled to clean them inside as they had an opening to put the diaphragms (waterhouse) in the center

[diant]

The edges were still a bit soft, but in the middle the image was very sharp.

Stefano, the reason is that an achromatic doublet has a strong off-axis astigmatism and field curvature, so it is suitable only for a work with very tight field.

[Stefano]

Thanks all. That's interesting, this could be part of a Petzval lens.

 

If I take this achromatic doublet (or another one) and try to correct the spherical aberrations with other elements, is there a chance that chromatic aberration comes back again? Intuitively, I think so.