I repaired a Canon 28-70 F2,8 lens for somebody, with complete cleaning of the internal elements. The repair was ok – but the image was very smooth.
Controlled with a strong handlamp, I have seen a haze on one of the inner lenses.
After disassembling, I have found the problem. The double aperture module has a composed lens. Unfortunately, it is composed of two glued assemblies – and the problem is on the contact surface of two glued ones…
Now I have to find out the way of disassembling and reglueing them.