Balsam separation is used to describe lens elements that have started to delaminate. The description was originally specific to Canada balsam. However, also applies to modern lenses that use various epoxies.
What balsam separation looks like will depend on the lens coatings. It most commonly looks like a white or reflected rainbow halo around a group of elements in the lens.
White colors will appear on lenses that have no or few coatings. You'll see a rainbow of colors with lenses that have more coatings.
The delamination does not always occur around the outside of the elements. It can also look like a white/rainbow semi-circle on one edge, or be in the center of the elements.
You will be able to see the separation more easily than a small amount of haze or fungus.
Canada balsam is turpentine made from balsam fir tree resin. The resin is dissolved in oil, which allows it to be applied to lens optics.
It is important because it has a refractive index that is similar to crown glass and is invisible when dry. This is why Canada balsam was used for cementing lens elements together, as well as making permanent microscope slides.
Once dried, the balsam is amorphous, meaning it does not have a crystalline structure. This is desirable because the optical properties will not degrade over time.
Unfortunately, Canada balsam is not resistant to temperature extremes or solvents.
During World War II, the use of Balsam was replaced by epoxy, polyester, and urethane-based adhesives. Modern lenses use UV-cured epoxies to cement lens elements together.
Balsam Separation Effect on Images
A small amount of separation along the edges of the lens group will have very little effect on image quality. As the separation gets worse you can encounter:
- A decrease in contrast. Similar to what you would see in a lens with haze.
- More flare and ghosting.
- Softer corners and a loss of sharpness over the frame.
- Focusing may become difficult.
- "Dust spots" on the images from remnants of the adhesive used to bond the elements.
If the elements come completely apart, the lens will be unusable.
Lens Balsam Separation Repair
There are several options for repair. You're likely going to have a hard time finding someone that can re-cement the lens elements back together.
If you are able to find someone that can do the repair, it is unlikely to be worth the cost. The exception would be for Leica lenses.
That leaves you with DIY options.
Harvest a Replacement from a Donor Lens
By far the easiest option is to buy a damaged donor lens. Lenses that need repairs sell for much less than fully functional lenses.
All you need is a damaged lens where the lens group you need is undamaged.
This has the added advantage of giving you some practice on disassembling the lens. That should make replacing the lens group easier.
Re-Cement the Lens Group
If you're feeling bold you can try to re-cement the elements that are separating.
I have not done this and I don't recommend trying to. It is going to be expensive, time-consuming, and you're likely to end up with poor results.
SK Grimes also has a short tutorial on the process.
Remove the Lens Group
First, you're going to have to remove the lens group from the lens.
Depending on the construction of the lens, getting the bare elements can be difficult. On newer designs, the elements can be epoxied into metal or plastic.
Any additional parts epoxies to the elements will get in the way of scribing the edges of the elements so they can be properly aligned when cemented back together.
If the elements are not properly aligned, you'll end up with a lens that is decentered. This will cause the focal plane to no longer be flat.
Separate the Elements
Before you separate the elements you need to scribe an alignment mark on the edges of the elements. You'll need some sort of diamond-tipped tool to scratch the glass.
If this is not done, you'll need access to expensive optical alignment equipment. At that point, buying a new copy of the lens would be cheaper.
In order to separate the elements, you can use heat or a solvent.
Canada balsam will be the easiest to work with. Newer epoxies will be more difficult.
Acetone or isopropyl alcohol are solvents you can try. Place the elements into a container, and cover with acetone.
Use a container with a lid in a well-vented area, as acetone and isopropyl alcohol will produce harmful fumes. Both are also highly flammable.
For heat, you can try using an oven. Place the lens group on a cookie sheet and one side up so the elements can slide apart.
Start at a temperature of 100° F or 38° C. Increase the temperature 25°F or 14° C every 30-60 minutes until the elements come apart.
If you use heat, you're still going to need to use a solvent to clean the epoxy off.
Epoxy the Element Back Together
You need to thoroughly clean both elements and make sure there is no dust on them. Ideally, you would do this process in a laminar flow cabinet to ensure dust-free working conditions.
Apply a small drop of UV epoxy and use the scribe mark to align the elements.
Once everything is correctly aligned, use a UV light source to cure the epoxy.