Some lenses have a very colorful defect - and this is one of the worst lens defect for most

Delamination / seperation is a defect of the glue between paired (ore more) lenses.
Some lenses in lens systems like photo lenses are glued together to one group. The reason for this is to minimize reflection on lens air surfaces. A larger refraction index differenc results in more reflection. When anti reflex coating was not avialable lens designer started to have lenses glued together to minimize the reflections - and this minimize the stray light.

But even with todays anti reflection coatings lenses are glued together - at non normal light incident anti reflection coatings are still far away from 99.9% transmission. Furthermore total reflection could be avoided, and lens groups could minimize the effort to build and assemble the lens system.

For this glueing was Canada Balsam used, this is a speical tree resin. It has about the same refractive index of optical glass.
Today are UV curing adhesives common, they are faster to work with.
Booth types of glue could start separation, the glued surfaces are no longer bonded together - the optical properties are changed.
In some cases these small distances result in rainbow like colors - a typical sign of delamination.

Agfa Magnolar 75mm/4.5 enlarger lens with delamination - all three images show the same lens - the rainbow color splitting efffect depends on illumination!

Here a nice looking delamination like a leaf or a flower - some may think such a regular shaped form is lens fungus, but it is not.
Such a delamination near the entracne pupil can be seen in the bokeh - the blur of unsharp regions. Delamination can have visible effects in the image!
This was inside a Wirgin Edixa 85mm 1:1.9 lens:

Here in this case one could see reflection and the rainbow colors. Rodagon 80mm f/5.6

Broken lens glue in a Isco Kiptar 110mm f/2.0 projection lens:

Lens glue broken - Canada Balsam

Delamination could be repaired - but this is a very special work. Not many camera service centers will do this, and for amateur lens tinkeres it is most times a bit risky to disassemble the lens group, and bond it together correctly aligned.

Looks like delamination - but is not!

Sometimes a thin layer of oil could cause a similar looking rainbow color defect - and this is much easier to repair - there is only the need to clean it away!
Here an example of this:
Looks like delamination on Mamiya 50mm f/1.4 lens - but is only oil residue Sieht wie ein Verkittungsschaden aus, war aber nur eine ├Âlige Verschmutzung die sich wegwischen lies

Looks like a seperated lens, only after opening the lens I learned this is not lens seperation - it was probably oil on the surface, nothing between glued lenses!

Lens separation/delamination repair - a relative easy way for older lenses original glued with Canada Balm.

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