Re-Cementing and old Rectilear Picture Taking Lens

S.K. GRIMES
Services for the Large Format Photographer

Old Lenses can be restored by Re-cementing

This is one of the worst "Before" lenses we have ever run across.

See how such an old lens can be separated, cleaned, and re-cemented.
Also learn a simple explanation of "centering" and centering techniques.

The glass achromat is first removed from its metal mount. Old Canada Balsam can be melted by heating to about 300 ° F

After heating on an electric hotplate the Balsam will melt and while its still hot the two sections of the achromat can be slid apart. Cool carefully and slowly. After cooling the elements can be cleaned in acetone to remove the old balsam.

Although not absolutly necessary a good dust free environment helps speed the work. This is a "Class 100 Certified Laminar Flow Work Station" This means highly filtered air is blown across the work towards the operator so that there will be no debris caught between the glass. With such a station handling of the glass is kept to a minimum; once it is clean it will stay clean and free from dust, no matter what the humididy or electrostatic condition of the atmosphere.

Its important to preserve the "centering" of a lens when doing this type of repair. So, just what is "centering" anyway?

This can be demonstrated by handling an ordinary pair of reading spectacles. A light ray traveling through the optical center of the lens will not deflect. The picture shows a way of determining the optical center: When the vertical line above and across the lens is in the same place (undeflected) you have found one axis of the center.

This is a view illustrating the deflection of the rays when viewed away from the center. The vertical line is displaced, indicating that it is away from the center.

Holding the spectacle at another axis to the vertical and doing the same alignment trick will show where the other axis is. Where the two lined up lines cross is the optical center of the spectacle lens.

The spectacle optician's task is to arrange the lens so that the optical center is directly over the pupil of your eye. (In the case of these drugstore glasses, some care has been taken in manufacture to not have the optical center in the mechanical center of the frame instead approximating where a normal eye will be for this frame.) In the case of the camera lens maker the task is to have the optical center in the mechanical/geometric center of the lens. This is not a difficult task in manufacturing, and once manufactured, then its a done job which can't be changed.

Its safe to assume that any photographic lens made for commercial sale has had reasonable care taken in making sure the optical center is close to the mechanical center of the glass. Old lenses have greater discrepancies than new ones and modern German and Japanese lenses are centered like by God Himself.
This means that in re-assembling the lens either or both optical and mechanical aligning methods can be used.

The cleaned lens elements can now be re-cemented using modern UV light curing cement intended for this purpose. (My link list has a supplier) Place a drop of cement (whose size is determined by practice and experience) in the center of the concave surface, be sure there are no bubbles or debris then gently apply the mating lens.

If the size of the drop is right and it is in the center of the concave of the lens then capillary action will distribute the cement throughout the seam in 15 to 20 minutes. Its best to avoid swirling or otherwise moving the lens around to acheive this flow. If you observe bubbles or debris or uneveness in the capillary action its best to simply start over, adjust the size of the drop and try again.

The first mechanical method is to use triangular location using prisms. Binocular prisms are accuratly made and readily available. The lenses must be the same diameter as each other and at least one of them must fit flat on a plane surface.

A similar method is to "Use V-Blocks" The picture shows how accuratly machined "V blocks" (which are an ordinary piece of industrial supply) can be used to keep same diameter lenses in mechanical alignment with each other.

The optical method of alignment depends on the principle explained with the spectacles. By arranging a projecting collimater or laser through the center of an accuratly made turning spindle and observing the focused image with a magnifying video camera, lenses of different diameters can be aligned. By setting the lens pair with uncured cement up so that the image of the collimater can be observed on the video monitor when the chuck is turned misalignment will show up as eccentric movement of the image on the video screen. Since light travelling through aligned optical centers is not deflected when alignment is acheived the image will "run true" as the spindle is rotated.
The picture shows the operater's position. The lens is manipulated to center it while rotating the chuck/spindle (motorized) while observing the eccentricity of the magnified image on the video monitor.

When everything is aligned and you have waited about 1/2 hour or so for the capillary action to taper off and the lenses are settled down and not moving with respect to one another (even slight movement or undue pressure on the glass during curing will cause all sorts of problems: reticulation of the cement, spidery patterns, etc. etc. The big advantage of the UV cement is that you can take your time being sure everything is right before committing to the cure by shining the UV lamp onto it. This curing process takes two or three hours depending on the thickness of the glass and the intensity of the light. Sunlight will also work but may take several days to cure completely.

Click here to see an entertaining "do it yourself" article from 1943 Popular Science.
There follows after that another page about removing lenses from "burnished" lensmounts.

The glass achromat is first removed from its metal mount. Old Canada Balsam can be melted by heating to about 300 ° F
After heating on an electric hotplate the Balsam will melt and while its still hot the two sections of the achromat can be slid apart. Cool carefully and slowly. After cooling the elements can be cleaned in acetone to remove the old balsam.
Although not absolutly necessary a good dust free environment helps speed the work. This is a "Class 100 Certified Laminar Flow Work Station" This means highly filtered air is blown across the work towards the operator so that there will be no debris caught between the glass. With such a station handling of the glass is kept to a minimum; once it is clean it will stay clean and free from dust, no matter what the humididy or electrostatic condition of the atmosphere.

	Its important to preserve the "centering" of a lens when doing this type of repair. So, just what is "centering" anyway?
This can be demonstrated by handling an ordinary pair of reading spectacles. A light ray traveling through the optical center of the lens will not deflect. The picture shows a way of determining the optical center: When the vertical line above and across the lens is in the same place (undeflected) you have found one axis of the center.
This is a view illustrating the deflection of the rays when viewed away from the center. The vertical line is displaced, indicating that it is away from the center.
Holding the spectacle at another axis to the vertical and doing the same alignment trick will show where the other axis is. Where the two lined up lines cross is the optical center of the spectacle lens.
The spectacle optician's task is to arrange the lens so that the optical center is directly over the pupil of your eye. (In the case of these drugstore glasses, some care has been taken in manufacture to not have the optical center in the mechanical center of the frame instead approximating where a normal eye will be for this frame.) In the case of the camera lens maker the task is to have the optical center in the mechanical/geometric center of the lens. This is not a difficult task in manufacturing, and once manufactured, then its a done job which can't be changed.

The cleaned lens elements can now be re-cemented using modern UV light curing cement intended for this purpose. (My link list has a supplier) Place a drop of cement (whose size is determined by practice and experience) in the center of the concave surface, be sure there are no bubbles or debris then gently apply the mating lens.
If the size of the drop is right and it is in the center of the concave of the lens then capillary action will distribute the cement throughout the seam in 15 to 20 minutes. Its best to avoid swirling or otherwise moving the lens around to acheive this flow. If you observe bubbles or debris or uneveness in the capillary action its best to simply start over, adjust the size of the drop and try again.
The first mechanical method is to use triangular location using prisms. Binocular prisms are accuratly made and readily available. The lenses must be the same diameter as each other and at least one of them must fit flat on a plane surface.
A similar method is to "Use V-Blocks" The picture shows how accuratly machined "V blocks" (which are an ordinary piece of industrial supply) can be used to keep same diameter lenses in mechanical alignment with each other.
The optical method of alignment depends on the principle explained with the spectacles. By arranging a projecting collimater or laser through the center of an accuratly made turning spindle and observing the focused image with a magnifying video camera, lenses of different diameters can be aligned. By setting the lens pair with uncured cement up so that the image of the collimater can be observed on the video monitor when the chuck is turned misalignment will show up as eccentric movement of the image on the video screen. Since light travelling through aligned optical centers is not deflected when alignment is acheived the image will "run true" as the spindle is rotated. The picture shows the operater's position. The lens is manipulated to center it while rotating the chuck/spindle (motorized) while observing the eccentricity of the magnified image on the video monitor.
When everything is aligned and you have waited about 1/2 hour or so for the capillary action to taper off and the lenses are settled down and not moving with respect to one another (even slight movement or undue pressure on the glass during curing will cause all sorts of problems: reticulation of the cement, spidery patterns, etc. etc. The big advantage of the UV cement is that you can take your time being sure everything is right before committing to the cure by shining the UV lamp onto it. This curing process takes two or three hours depending on the thickness of the glass and the intensity of the light. Sunlight will also work but may take several days to cure completely.

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