Separated Lens Elements

Lens elements are held together by optical cement. In rare cases, the adhesive bond between the lens elements may fail. This results in what is known as lens element separation.

This kind of defect usually manifests as discoloured or reflective areas at the edges of the elements, but can also appear in the form of bubbles.

In mild cases, there is little effect on the image quality especially if it only involves the edge. However, a severely separated lens usually results in poor image quality.

To check for lens element separation, look through the lens at a bright light source. The bright sky or a sun-lit wall works well. By varying the distance of the lens from your eyes, you can focus on the different elements in the lens and look for element separation.

If you notice bubbles or patches of discolouration, you should not buy the lens. Repairing it will cost you quite a lot of money because the lens has to be dismantled, the old cement removed and the elements recemented. It is generally not worth repairing such a lens unless it is very valuable or rare.

Loose Lens Elements

Some used lenses may have loose elements. These lenses will rattle when you shake them. Just don't shake them too vigorously!

A single loose element will generally not cause any issues since alignment is still maintained within the barrel. However, if a lens group or several elements come loose, you may find that it is impossible to auto-focus or focus to infinity.

You can check by trying out the lens with your camera. Try focusing on various objects at different zoom lengths (if it's a zoom lens). You should also try focusing on something far away, like the sky or a distant building. If the lens hunts around and cannot lock focus, then it's a significant problem.

Even with this problem, you can consider buying the lens if the seller is willing to offer it to you at a low price. You can get it serviced and repaired by a technician for relatively little cost.

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