[Pål Jensen]

Much cheaper to reapair lenses than bying UV filters. I don't know how many lenses I've owned since 1976 but I haven't owned one UV filter or smashed one single front element, but if I had a repair would have been cheaper than 15 UV filters...

[nater]

I always use hoods (indoors and out, night and day), always use filters (protect/UV/IR/polarizer), and have never broken a filter or lens.

I have a friend who's a professional astronomer and chatted with him about lenses and coatings. He said that at observatories they re-finish the mirror lenses in their scopes every year because of loss of reflectivity over time (this is why you'll find camera people claiming that a mirror lens "isn't really an f/8, but more like an f/11", because they are comparing a typically old mirror lens that hasn't been re-finished with other, often newer lenses). Also, he said that the coatings on the transmissive elements also degrade over time, just not as quickly, and are susceptible to wear from contact.

[Digitalis]

I always use hoods (indoors and out, night and day), always use filters (protect/UV/IR/polarizer), and have never broken a filter or lens.

I have a friend who's a professional astronomer and chatted with him about lenses and coatings. He said that at observatories they re-finish the mirror lenses in their scopes every year because of loss of reflectivity over time ...

Rubbish! unless the mirror is poorly made the mirror itself will not lose it's reflectance - it's all the dust and crud that builds up on the mirror itself that causes this. Dust gets everywhere. Though it never ceases to amaze me how upset some photographers get when they see dust inside their new shiny lens - a piece of dust would have the be the size of a match head to cause any visible degradation in most photographic lenses* - though a piece of dust of that size on a telescope mirror is a much bigger issue.

he said that the coatings on the transmissive elements also degrade over time, just not as quickly, and are susceptible to wear from contact.

again, I would have to have personal experience or read at least two scientific studies on photographic optics confirming this before I would put faith in such a statement. I have lenses that were coated in Magnesium fluoride that despite the fragile nature of MgF2, these lenses have remained in superb condition for more than 50 years.

* Though wide angle lenses because of their great DOF dust can become especially problematic especially when strong light sources are involved.

[Ben_Edict]

Rubbish! unless the mirror is poorly made the mirror itself will not lose it's reflectance - it's all the dust and crud that builds up on the mirror itself that causes this. Dust gets everywhere. Though it never ceases to amaze me how upset some photographers get when they see dust inside their new shiny lens - a piece of dust would have the be the size of a match head to cause any visible degradation in most photographic lenses* - though a piece of dust of that size on a telescope mirror is a much bigger issue.

again, I would have to have personal experience or read at least two scientific studies on photographic optics confirming this before I would put faith in such a statement. I have lenses that were coated in Magnesium fluoride that despite the fragile nature of MgF2, these lenses have remained in superb condition for more than 50 years.

* Though wide angle lenses because of their great DOF dust can become especially problematic especially when strong light sources are involved.

All the larger astronomical facilities have inhouse coating chambers to renew their mirror coating frequently. Today this is not always because of the loss of visual reflectivity, but probably more often to suppress the IR emission from the scope, because the coatings loose this suppression capacity over time. Do a web search and you'll know.

Today coating is often done inhouse, because the large glass mirrors could be scratched or receive rubmarks, when transported from the mirror maker to the observatory. These voyages often involve thousands of kilometres and it easier to coat the mirror at the place - and then there is recoating:

"A recoating schedule of every two years has been discussed, with a time limitation of 2 days from mirror and cell removal to reinstallation in the telescope. " (http://www.google.de/url?sa=t&rct=j&q=frequency%20of%20mirror%20coating%20es...VaTZRg&cad=rja)

It is fine that you always insist on getting scientific proof - but you could be more polite ("rubbish" in this case applies to your post, not the one you quoted), doing this. Please search for the second evidence yourself.

By the way, talking about photographic lens coating as you do and then transfer that to the reflection coating of a mirror is complete nonsense - these are very different things. Mirrors are coated with a deposited metal film. Some have a protective overcoating, but not what you find in your old Takumars.

Ben

[jeff knight]

I've got to say I'm confused why there are discussions of hoods when talking about filters. I use hoods on all my lenses all the time except one (FA 100mm, which has a front element so recessed it doesn't need one). I like contrast in my photos, so why would I welcome light into my lens that won't directly hit my sensor? That light can't help, period. In fact, I would pay money for adapters to make all my full-frame hoods into APS-C hoods. I wish all those hoods were deeper. When I buy an old lens that doesn't come with a hood, I buy a hood for it. When I buy an old lens with an insufficient sliding built-in hood, I buy an appropriate hood for it.

I was in the museum at the International Center for Photography in NYC last week (good Weegee exhibit there), and they had a poster up in their cafeteria advertising their student classes, with a picture of some students in action taking photos. None of them had hoods on their lenses. That certainly didn't sell me on their classes! Hood use should be in photo 101, and doesn't belong in a discussion about filters of any type.

I totally agree! Hoods are much more crucial than UV filters. UV filters are completely obsolete unless they are the finest multi-coated versions. I cringe every time I see some fool put a cheap piece of glass in front of an expensive lens. The only time I use a protective filter is at the beach when the sand is blowing, or when the spray of a waterfall forces me to wipe the lens repeatedly. Use a metal hood if you can, and just watch what you are doing.