Before the rush

Before the rush
by evan-pak

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Suppose I have a lens with few elements cemented together. If one of those groups begins to separate over time, what effect could it have on images taken with the lens?

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up vote 4 down vote accepted

Do you actually have this lens or is it just a hypothetical?

The effect on the image would probably depend on how defocused the image is at the point that the sandwiched lenses meet or exactly how far the lens elements have shifted out of place. But if you permit me to make wild speculations, it could be one or more of:

  • Lower contrast, more "haze"

  • Increased flare or ghosting.

  • Change in corner softness or sharpness across the frame.

  • Change in ability to focus at the extremes, such as not focusing as close or not reaching infinity, or vice versa.

  • Imprint of what looks like blurry, dirty marks on the image caused by remnants of whatever of the "cement" remained between the lenses.

  • Or, it could have very little noticeable effect at all.

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I do have a lens with this problem, but I wanted to make the question as hypothetical as possible to fit with the site's format. – keyofnight Aug 12 '13 at 8:32
In that case I'd be curious to see pictures if you have any! – thomasrutter Aug 12 '13 at 10:41
One image, coming right up! (Note: I'm not absolutely sure that's separation, and it's only on the rear element/group.) – keyofnight Aug 12 '13 at 12:34

Mild separation near the edges of the lens will have a very slight impact on image quality. Full separation is usually pretty disastrous as neither of the newly separated surfaces will be coated and more importantly one or other element will be out of alignment.

If you have no baseline of the performance of the lens then it is easier to detect the beginnings of lens separation by inspecting the lens in front of a bright light (holding it up to a window will do). Look for a pattern of bubbles or a discoloured area around the edge of the lens element.

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