- FAQ

Repair of Cemented Lenses
Information provided by John Toeppen

Old lenses sometimes develop a cloudy or cracked yellow layer between two glass elements. This is because the lenses were typically cemented using a transparent tree sap called canada balsam. It usually has bits of wood floating in it and it must be warmed to allow it to flow like honey.  Since it warms the optic unevenly it can stress and warp otherwise good optics. If you ever buy and use this material you quickly learn to appreciate UV cured optical cements. Sunlight and blacklights will cure UV materials but tests need to be done on microscope slides first. Canada balsam is easily remelted with a hair dryer and cleaned with many light solvents.

If you do not have a good spanner wrench to remove lenses from cells, it is common to grind a steel edge to key to span the ring and get both slots of the threaded ring at once. Removing a lens from a threaded mount usually requires a suggestion of acetone on the thread of the rings holding the element into the cell. It was common practice to mix Duco Cement with a little acetone and put a drop on the threaded ring to secure the ring. Excessive solvent could separate doublets that are still good and dissolve other plastic parts.

Examine the edge of the doublet and determine if the edges of the lenses were aligned to each other when cemented (typically an autocollimator is not used to align doublets). Before you separate paired lenses it is common practice to use a pencil to mark the clocking (relative rotational  positions of the elements) as it is usually important with custom lenses. Use Norland or Sumner optical adhesives to recement lenses:

If you have lenses that have become separated they will be of little value for imaging. The camera may be collectable or not.  If may be worthless if you try to fix it, or it may be worthless if you leave it alone. It is therefore suggested that the first optics that you practice on are not of high value. 

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Last modified on May 4, 2005
Copyright © 1999 - by and Alexander Klein. All rights reserved.