Sometimes you encounter a lens that has some internal blemishes that you cannot seem to clean off … can look like rainbow rings (ring of fire), a cloudy ring round the edge, a crescent beige tint, bubbles, or even a flowery/snowflake pattern … and it always seems like it’s trapped between two lens elements.
This old LTM lens has an almost complete ring haze. Now, it could possibly be just haze on the back of the element … but I suspected it was more than that. After talking it apart it was apparent that it was separation of the front two elements.
Yes, in fact it is between two lens elements. Most lens designs require two lens elements to be bonded/cemented together (doublet) with adhesive to eliminate the glass air gap. By reducing exposed surfaces, cementing reduces refraction and ghost images from reflections, and allows the use of thinner flint elements.
Older lenses used Canada Balsam as it had a high index of refraction, which is optically a good thing to have. It is derived from good ol Canadian trees … yup good ol sap … yes, its not just only for maple syrup.
Development of resins to cement elements started in 1785. Eventually changed to a synthetic resin because of WWII … high altitude aerial photography caused issues with Canada balsalm, so they developed a synthetic cement that could handle low and high temperatures … this is the MIL-A-3920 standard optical adhesive. UV curing adhesives appeared in 1966 to reduce the curing time for high production.
When the “cement” has degraded and no longer seals the two elements together, the separation will create artifacts that will change the refractive properties of the lens. Sometimes it is not very visually apparent … sometimes it is. The only real solution is to separate and re-cement them.
This topic has been discussed much on the web … Fotomozaic, LargeFormatForum, skgrimes
With Canada balsam, it requires heating to soften the resin so the elements can be separated. The newer synthetic resins require special chemicals … and in some cases the elements cannot be separated without damaging it.
Once separated, new resin can be applied to cement the elements. Then new resin can be applied to cement the elements … modern day resins are UV cured, so makes it much quicker to get a lens back to working condition.
Sounds easy … not !!! Heating sometimes does not work, or even worse causes the element to crack … and there are cemented group that are encased in a metal sleeve which has to be removed first (without damaging the glass, and you may have to find something to replace it) … or, as mentioned before, it just doesn’t work.
One day I will make a serious attempt at re-cementing … meaning, my half-ass attempt sucked, and I will try again.
Summers Optical makes many different types of synthetic resins for optical cementing … and you can still buy Canada Balsam, as it is still used for microscopic slide mounting and painting.
Sometimes the lens separation is not so bad in terms of affecting the image, especially if it is around the edges like the lens pictured above … but when it sits right in the centre, it will most likely cause apparent visual effects. Mounting this lens on my Sony NEX-6 and taking some shots, I could not really notice it until I got some light hitting the front and the separation starts adding some flaring/fogging.