I'm looking to find out if lens MgF2 anti-reflective coating can be damaged if the lens is submerged frequently in the sea water (salt water).

Just a clarification. This relates ONLY to lens itself. No electronics, no seals or plastic of any kind just bare glass lens with MgF2 coating.


1 Answer 1


Short version : stop the madness. Keep your lens out of salt water. Period.

Salt water is corrosive and fairly close to the worst thing you can get on a lens.

Submerging any lens is an appallingly bad idea, as even sealed lenses are not designed for that. They're designed for, at worst, splashes and typically for light to moderate rain. You'll note that except for specialty products (specifically designed for underwater) manufacturers don't generally specify in any detail how reliable seals are on lenses or bodies.

If you need to submerge a lens and/or camera into salt water "frequently" then I'd strongly suggest investing in a proper underwater case.

So if you submerge a lens in salt water the very least of your worries should be the effect on the coatings.

And the same goes for cameras, BTW. In fact it's sometimes worse as salt water and electronics (which are also in most lenses) do not mix well.

For what it's worth I don't think the coatings are soluble, but, again, salt water is corrosive and, when it dries there will be salt on the lens, waiting to be rubbed by someone and this will erode the coatings. I'd recommend using a lens pen to clean it.

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    \$\begingroup\$ I think most engineers would regard salt water as corrosive. Try googling those terms and see what ya get. But perhaps not the major issue in relation to a camera. \$\endgroup\$ Jul 25, 2017 at 17:35
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    \$\begingroup\$ Salt water accelerates rate of corrosion (most notably, in metals), it is not "corrosive" per se. However, I speak of "corrosion" with regard to the lens...not of the metal bits that constitute a camera (I'm assuming it's sealed pretty well, so salt water doesn't seep into the device). If you want to talk about "corrosion" and its usage more, drop by at the Chemistry.SE ;) I'm not into photography; just thought I'd point out that the "corrosion" bit isn't correct (from a chemist's perspective)... I can't vouch for the rest of the answer, but it seems reasonable to me O:) \$\endgroup\$ Jul 25, 2017 at 17:45
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    \$\begingroup\$ Well seawater is a lot more than just salt and water... lots of other things floating around in there. Isn't it feasible that there may be some trace stuff that will happily react with the coating (especially in the presence of said salt water)? \$\endgroup\$
    – Doktor J
    Jul 25, 2017 at 17:57
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    \$\begingroup\$ {Continuation...} It's been a while since I've studied anti-reflective coatings (ARCs) and how they're employed in optical instruments... but I'm fairly confident that the polymeric binder used here is effectively "shielded" from the external medium by the MgF2 layer over it. So yes, sea-water contains all sorts of ions...but if they can't get past the MgF2 (they can't, because the MgF2 isn't going to come off easily for the reasons I've mentioned), they won't be able to get to the binder, and the coating stays intact. \$\endgroup\$ Jul 25, 2017 at 18:44
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    \$\begingroup\$ @paracetamol That info is very directly on the point, so maybe you could convert that to an answer, as comments can be deleted by the system's automatic housecleaning functions. \$\endgroup\$ Jul 25, 2017 at 20:07

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