Ordinary lens glass allows UV down to about 300 nm, whilst most
common UV filters used in photography cut off at about 400 nm. They are
these 100 nm which normally make the difference.
UV with a wavelength above some 280 nm has virtually no effect on
fungi or micro-organisms. Only UV-A can cross normal optical glass. Hence
UV should be totally useless to get rid of fungi on glass if it has to
cross the glass before reaching the target.
And, of course, attenuation or transmission does not depend on the
number of (optical) elements, but on the total thickness thereof.
Bob Meyer <[EMAIL PROTECTED]>
Sent by: [EMAIL PROTECTED]
13-05-2005 14:54
Please respond to
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Subject
Re: EOS My 80-200 f2.8 got fungus
--- Bob <[EMAIL PROTECTED]> wrote:
> How much UV does the lens glass absorb with no UV filter?
>
Well, there's no simple answer because different types of glass absorb
differently. And plastic aspherics are different still (likely absorbing
even more than glass). But, if I remember correctly my long ago optics
unit in physics, typical optical glass probably absorbs about 50% of UV IN
EACH ELEMENT. So a 4 element lens would pass only about 6% of total UV.
You can imaging a modern zoom lens with 12 to 15 elements passes
essentially none. This begs the qestion: why buy a UV filter, then? Turns
out the absorption is related to the frequency of UV light.
Here's an interesting web page:
http://www.bobatkins.com/photography/technical/filters.html
It says, in part:
"So if you buy a UV filter, you'd expect it to block UV right? Well,
sometimes you'd be wrong as the results of this test show. I've looked at
the range between 350nm and 400nm for UV blocking
*** since the glass used in almost all lenses will itself block any light
with a wavelength shorter than 350nm,*** so you don't need help from a
filter there."
The UV range begins at about 400nm, so there's a tiny range of UV
radiation (350-400nm) that is passed by normal lenses. It's this range
that UV filters are supposed to reduce. But as the web site referenced
above shows, not all do.
Hope this is useful,
Bob Meyer
I don't suffer from insanity... I enjoy every minute of it.
...
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