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I was reading this answer where it mentions, "..it's a lens that can lose sharpness as it ages.." and was wondering if this is true and if yes, true for all lenses. Reasons as to why this happens and ways to prevent it if possible from a consumer's perspective would be great to know.
Please note, I am not talking about general wear and tear, or dust inside the lenses, nor am I talking about fungus in the lens.
Given that you have explicitly disqualified fungus and dust inside the lens, then the answer is no. A lens will not "naturally" lose sharpness with age. Glass is glass. It is a fixed medium, and assuming a 100 year old lens is in good condition without any extraneous wear and tear like fungus, dust, or a strong enough jolt to misalign one of the internal elements or element groups...a lens will not lose sharpness with age. A 100 year old lens should perform just as well at 100 years old as it did on day one.
One thing that probably should be pointed out is that lens design has improved over the years. While certain classic designs are often held in lofty positions as they were revolutionary in their time, they may not be the best designs still today. New materials sciences, (frequently pioneered by Canon...such as fluorite elements or diffractive optics), even other general innovations like image stabilization, multicoating and nanocoating, aspheric and apochromatic lens elements, etc. have all lead to progressively improving sharpness over the decades.
With improved lens technology comes improved image quality. Even if a 100 year old lens is in pristine condition, it likely lacks the purity of glass that a modern lens has, probably lacks multicoating and probably even a single coating, and each lens element may not be as precisely aligned as we can do today. Two similar lens designs, one from a century ago and one from today, would likely not be directly comparable. The modern lens design should be producing much higher image quality than the century-old lens.
That does not mean the old lens as "lost sharpness", however...it just means it doesn't compare to a modern lens design that's benefited from all the modern advancements in materials and optical sciences.
You ask whether a lens can lose sharpness over time, but then go on to say:
Please note, I am not talking about general wear and tear, or dust inside the lenses, nor am I talking about fungus in the lens.
Which are exactly the reasons lenses lose sharpness over time. So the answer is no - once you exclude all the factors which causes lenses to lose sharpness over time, lenses don't lose sharpness over time!
Clearly fungus on the surfaces of elements will degrade performance. Dust will too, but to a lesser extent. The other major factor is wear, lens elements are ground to very precise specifications likewise the position and alignment of elements is very important. Any shift in the position of lens elements over time, as a result of wear in the mechanical portions of the lens body (zoom or focus mechanisms) or knocks which cause parts to become dislodged, will degrade image quality.
Aside from surface effects (scratches and the degradation of coatings) the glass elements themselves will not degrade except in some rare cases (such as lenses with thorium added to the glass which brown over time).
For validation of the idea that lenses don't inherently lose sharpness, look a Panavision. Panavision is a company that rent their own very high end lenses and cameras to the movie industry. Renting out their own gear means they take a slightly different approach to lens design.
Repair and replacement are both profit generating enterprises for most camera manufacturers, i.e. when you send you lens in for repair once it's out of warranty they will make a profit on the work done. Likewise when you replace your old lenses.
Any time Panavision spend repairing their lenses is pure loss as those lenses are not available for rental. Likewise replacing lenses only costs them money. As a result lenses are designed to be easily maintained, easy to disassemble and clean. Elements are not fixed in place but sit in adjustable mounts with grub screws that can be turned to correct tiny errors in positioning as the lens barrel wears.
As a result they have been renting out the same lenses for location shooting in extreme environments for decades, and they're still as sharp as the day they were first made.
There is one factor not yet mentioned by any of the other answers: separation of elements glued together. As a lens ages the glue that holds some elements to each other can degrade, allowing air to infiltrate between the two lens elements. The areas with air will refract light differently and affect the overall sharpness of the lens.
In the context of the quote, I think it is more likely to infer that certain lens designs are more susceptible to misalignment and de-centering due to the wear and tear of normal usage. The EF 100-400mm f/4.5-5.6L IS with its push-pull zoom is one such lens. The EF 24-70mm f/2.8L, with the centering and tilt adjustments just behind the front element in the barrel that extends when zoomed, is another. You can read Roger Cicala's blog on the differences between it and the newer EF 24-70mm f/2.8L II here. Roger is CEO and technical guru at LensRentals.com which sees a lot of lenses with wear and tear, both from being heavily used and being bumped around by UPS and/or FedEx twice between each time they are used.
The reason a lens would loose sharpness would most likely be because of wear and tear, dust, or fungus. I would think the glass itself is unlikely to warp unless faced with extremes in temperature.
I imaging if you kept a brand-new lens in a dust sealed, climate controlled environment, it would stay sharp for quite some time. But most lenses will need service or repairs to stay at their best.
I confirmed this from Nikon Support to get an official answer to the above question:
"Thank you for contacting Nikon Support. No, a lens will not lose sharpness with age. The optics, glass will not age, however the seals, coatings and glue may.
ref# 130409-001818"
Since 100year old lenses can still be sharper than your average kit lens, I don't see this as an aging issue. "objectives" consist of many lenses, typically 4-15, in groups, and these are carefully places in a constellation that can be misaligned through heavy wear - like hard bumps. This - except fungus, excessive dust, misting, etc., is the only way I can see it losing sharpness.
