EF 24-70mm lens coating - known issues / examples...
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zackdezon wrote:
After using Pancro and a microfiber cleaning cloth on this lens that got a little sunscreen on it, this streaky, hazy area has appeared and it doesn't want to go away. My fear is that I've somehow stripped the coating. Anyone familiar with this? I've taken a few test shots with it and everything seems sharp so far. Haven't noticed any haze over the images, either.
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Sunscreen on a coated lens...
Sunscreen typically contains zinc oxide nanoparticles and titanium oxide nanoparticles. These are more abrasive than some toothpastes although they reflect UV light exceptionally well. They are considered so abrasive that skin testing was done on humans and (sadly) animals to determine how irritating the compounds were. The vapor-deposited coating on your front end element is so thin that it is quite easily rubbed off with simply pressure and a "lens cloth"... so applying sunscreen and then trying to wipe it all off carefully would certainly have resulted in a loss of coating. It might not have happened to other lenses but this lens has a long history of delicate coatings...
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This appears to be a well documented issue...
The bad news is that you are not the first person this has happened to. When I first saw your post I assumed that you still had residue on your lens but I revisited the thread and noticed that the coating appears to have genuinely been altered. I read your original post and then spent a few days reviewing other opinions here and elsewhere online before seeking out numerous examples of similar problems from other users. It would appear that this lens is one of several which can lose their external optical coatings with little or no effort. The coating seems to break down and flake off quite easily in some instances. Adding a powerful solvent (sunscreen sometimes contains solvents and penetrative oils) and rubbing at the optical coatings can indeed strip this lens by weakening the bond between the optics surface and the coating. It's resolvable by replacing the outer element at considerable cost. So your solution will be to replace the outer element. Unfortunately, lens elements are no longer inexpensive to replace and the cost has risen considerably over the last decade to the point where people who positively hate using a protective filter will now do so. Another alternative will be to simply continue to use the lens although you may (or may not) notice lower contrast and a "foggy" appearance in some images, especially where the coating is missing or where the transition exists between the coated and uncoated lens.
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The specific problem with this lens is that delamination appears to occur very quickly and somewhat easily. I understand that it's quite easy to lift the coating with oils and especially concentrated citrus oils. Sunscreen (as noted above) contains penetrative oils to aid in the spreading over skin and to retain it when immersed in water. Other methods of damaging the coating include applying window cleaner fluids and solvents of any kind. Using lens cleaning fluid designed for optics SHOULD not be a problem although in this instance, with this lens, it may be. Acetate is used as a paint thinner. It is sometimes referred to as a paint-stripper and with good reason. Using acetate for cleaning a lens in place of isopropol (which is normally quite commonly used for this purpose) would almost guarantee the lifting of a non-baked coating. Canon like to claim that their nano-coatings are baked but this particular lens seems to shed its coating under a variety of non-abusive conditions. I think Canon need to address it.
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Filters for future lens use...
I was once a licensed Canon dealer years ago and I did not come across any examples of coatings wearing off except where sand and abrasives were involved. My own lenses have quality UV filters on all of them for safety reasons since I shoot in a variety of conditions and many of the modern multi-coated filters today (specifically higher-end modern filter options) allow up to 99% of light transmission with anti reflection properties (to reduce internal reflection). I'd encourage you to consider something like this with your future lenses. I'm using the Hoya models on my lenses.
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Possible causes...
It's possible that these new metallic coatings are applied to a porous surface when first annealed to the lens optics. The coatings serve several purposes but if a metallic/ Magnesium is used (and I believe many coatings from Canon are indeed Magnesium fluoride), the contraction and expansion of the coatings against the glass optics may create micro-fissures which then initializes the delamination process. This will occur with temperature changes between hot and cold environments where the materials expand and contract. Obviously the process is not ideal when applied to this particular lens. Using anything other than a soft, dry microfiber cloth at this point is simply going to lift the fragmented surface coating and I suspect that this is what's happening here.
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Some examples of similar problems to your own were easy to find online. Many are fairly recent and imply that the problem is a relatively new one. The first example seemed to be your own images until I compared them more carefully. It certainly looks the same.
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I thought this was the OP's lens.
Almost a spitting image of the original photograph from the OP (above).
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My own experiences with Sunscreen...
I once avoided using sunscreen when shooting on a beach for what I thought would be three hours (that became four) one summer in 47.7C (114F) heat during summer ... because I knew how destructive the compounds in sunscreen would be to the camera I was using and the L-lenses I was carrying.... I foolishly assumed that a baseball cap with T-shirt and jeans would be enough. I had sunglasses on. But this exposure resulted in 2nd degree burns to my face and arms. The doctors warned me that if the burns progressed or were any worse, they'd have had to consider skin grafts to my face.
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From my experience as a prospector in the Australian bush, sunscreen has properties that can strip the reflective coatings from sunglasses and cause damage to the soft rubber eye-piece of cameras. I've also been aware that it can discolor some plastics and cause damage to rubber (eg focus rings and zoom rings). The only solution is to try and source a less "corrosive" (for lack of a better term) sunscreens when shooting outdoors. We need to protect our skin and we need to protect our lenses.
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Some relevant online posts relating to this lens below.
It's surprising how many people have been vocal about this problem online and considering how important this lens is to wedding and commercial photographers it's a concern that these coatings are so fragile. You don't need a lecture from anyone here and your mishap is not your fault.
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