Talk:Achromatic lens

Quality Rating
I think this is a highly important article because the achromat is basically the first fundamental invention in applied optics beyond the single lenses. Its content is still rather rudimentary at this point, though. Should it be transfered to "Start" status? Best regards, --Trinitrix (talk) 15:42, 2 March 2010 (UTC)

Old Discussion
Very handy to have these linked pieces on many aspects of lens and optical device design, but I came here wanting to know how the achromatic doublets in an old pair of binoculars had been stuck together, and I am still in the dark about this.

If anyone knows about this, and would like to add the information to the piece, I would be grateful, and the piece would be even more informative than it already is.

The achromats in my old binoculars are very nice and clear, but the material holding the two elements together is deteriorating in gold coloured concentric rings from the edges of the lens. Might it be 'balsam' as we used to use on school microscope slides?. Gelatin?

Ideally I would like to separate the elements, clean, and reset them with new 'junctive'. Is this possible (Immerse in warm water perhaps?), and, if so what should I remount them with nowadays?

Also: Is the Dollond mentioned in the various pieces, related to the modern 'Dolland' of Dolland and Aitchison opticians? If so, it would be nice to see this family continuity of trade mentioned.

Any answers?

Spamlet 19:09, 26 September 2007 (UTC)


 * Canada balsam is a traditional adhesive for optics. According to the article on it, it does turn yellow when the essential oils evaporate. You should be able to dissolve it in xylene, which you can get at your local hardware store (with the paint thinners). In modern optical engineering, UV-cured epoxy is the material of choice. It's convenient because you can put your parts together, move them around if necessary to get them aligned or to get air out, and then "zap" them with a UV lamp to tack them in place. The bond is then finished by a longer UV exposure or by baking (depending on the epoxy). Norland is the brand I'm familiar with, and it is readily available in small quantities. You would need a powerful UV source and safety glasses, though. I'm not sure what other modern adhesives you could use. You need something with an index of refraction close to that of glass, and good optical clarity.--Srleffler 03:45, 27 September 2007 (UTC)


 * Adding two cents to an old discussion on a non-how-to talk page but - xylene will work on old balsam. If it is modern UV cured cement then you will have to soak the lenses in Benzene (nasty stuff if you can get it) for about a week. Cementing back together is simple. UV setting auto glass cement (the stuff in the little yellow syringes for injecting into cracks and dings in windshields) works great for cementing lenses, we used all the time when I worked in an optical shop. It sets fast in bright sunlight, no UV source needed. 75.196.125.136 (talk) 20:23, 7 November 2008 (UTC)

Thanks very much for the tip. I am impressed as I had expected this to be a topic that would not get many visits! I think that the traditional method is probably better, as this does allow for redoing things if necessary, and though these binocs are very old, most of the lenses are still good, so balsam seems fine - if this is indeed what was used. I would imagine that, if I can get some balsam and xylene, assembling the lense elements under immersion would minimise the trapping of air and dust.

Very helpful, Cheers, Spamlet 14:49, 29 September 2007 (UTC)

Just checked the Canada Balsam link, and added a note there too. Assuming I have balsam sealed lenses. Will the adhesive swell back up and become clear again simply by immersion in xylene, or would loss of 'essential oils' mean the resin base must be changed as well?

Spamlet 15:21, 29 September 2007 (UTC)


 * Zylene isn't an essential oil. It won't fix the discolouration of the balsam. Be careful with the zylene, by the way. It is a powerful solvent. Wear gloves and don't get it on anything you want to keep. It shouldn't hurt the glass of the lenses.--Srleffler 15:58, 29 September 2007 (UTC)

Not to worry. I used to be a lab technician, (Gk xulon = wood. Xylene was originally obtained from distilling wood as a source of hydrocarbons.), and the main problem with handling xylene is, to me, its particularly nasty fumes, which used to give me a headache almost as soon as I opened the tin!

Thanks for the continued feedback.

Spamlet 20:01, 30 September 2007 (UTC)


 * John Dollond was the father of Peter Dollond, founder of the opticians maker, much later merged with Aitchison. Michael Daly 21:20, 1 October 2007 (UTC)

Oil-spaced?
The article says:
 * The use of oil between the crown and flint eliminates the effect of ghosting, particularly where R2=R3. It can also increase light transmission slightly and reduce the impact of errors in R2F and R3.

I don't understand. immersion oil has n=1.515ish, which is very close to that of crown glass, so I guess it's saying that the oil and element 1 are essentially one element. But optical adhesives come pretty close so wouldn't a cemented doublet perform the same? —Ben FrantzDale (talk) 13:31, 2 August 2011 (UTC)
 * Just guessing, but with oil you wouldn't get stresses building up with differences in thermal expansion between the lenses. In the past, oils may have had better optical and aging characteristics than the cements that were available.--Srleffler (talk) 17:31, 2 August 2011 (UTC)

Focus in the plane
I disagree with this edit. I believe the wording was better as it was. The editor's comment illustrates his misunderstanding: yes, you would speak about focusing light onto something, if you were focusing light onto a physical thing. In this case, though, the light is being brought into focus in a plane. The plane is a mathematical abstraction, not a physical surface. An achromat brings two wavelengths into exact focus in the same plane, as opposed to bringing them into focus in two different planes. Using "on" just sounds wrong, and implies a physical surface where there need be none.--Srleffler (talk) 03:34, 26 November 2017 (UTC)