Bob Meyer wrote:
--- 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.
Thanks. I appreciate the info.
Bob
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