The Canon 28-70 F2.8 has a special problem: after a while, one of the duplets inside gets foggy. The problem is the cementing, a bad glue or process.
After you disasseble the element, you can glue it with a UV curable adhesive.
Here is my device (built from an old Zeiss focimeter), and the UV curing procedure (with a Lidl nail maker set).
Some technical details:
The middle part is made of PTFE (Teflon), as should not destroy the lens, and should not be glued to the lens – as this happened first time, when I used an another material. Teflon is great!
The 3 small disks are also made of PTFE, and they have a small magnet on the base, so they can be rotated or moved on the polished steel ring.
In the middle of the PTFE ring there is a closed area, in which I can make vacuum (or more precisely, depression). This is made with an ejector, so I just have to switch on the compressed air with the small switch (is a pneumatic switch). I use approx -0,5 bar depression.
When the first lens element (which is a plan-concave one) is cenetered, I switch on vacuum. This holds it on place.
After this I put 1 drop of glue on the lens.
I put the second element (concex-concave) on it. I rotate the lens, to get out the air, and to have uniform glue distribution.
I adjust it to be centered with the lensometer.
When is in center, I set the 3 PTFE disks on the margins.
I recheck the centering.
Switch on the UV light.
By the way: I had to modify the lensometer, as originally was oblique, and MUST be vertical. That was a good job, to cut it. And LED lighting, of course.
I was waiting 2 years to get rid of the lensometer. Was about 45 EUR.
(Now I have 3 of this type, from a flowmarket, and a better one, a japanese Topcon. The Topcon was a bargain: 15 EUR…..)