The importance of a uv filter?

On Thu, 12 Aug 2004 14:26:16 +0200, "Bart van der Wolf"
wrote:

"Hans-Georg Michna" wrote in
message ...

On Wed, 11 Aug 2004 09:25:39 -0600, "RonFrank"
wrote:

I have UV filter on all my lenses, and always have.

let's ask this question: How far does the extra UV filter reduce
the contrast?

At least 3% for two glass/air surfaces, which is a lot if you add 3%
of a bright area to a dark area!
Theoretically it could add some RGB 8,8,8 to the linear gamma shadows,
on its own, before the lens groups come into play, and before gamma
adjustment boosts it further.

Bart,

thanks for the quantification. I suspected something like this.

In other words, you don't want any superfluous filters, at least
when you take high-contrast photos. Night shots are just an
extreme example and make the extra reflections very visible.

Hans-Georg

--
No mail, please.

"Bart van der Wolf" writes:


At least 3% for two glass/air surfaces, which is a lot if you add 3%
of a bright area to a dark area!

It depends a lot on the filter coatings. If I remember correctly,
uncoated glass or plastic with a refractive index around 1.5 reflects
about 4% per surface. But a single-layer coating brings that down
dramatically, and good multi-layer coatings are in the 0.5-1% range per
surface.

Dave

"Bart van der Wolf" writes:


At least 3% for two glass/air surfaces, which is a lot if you add 3%
of a bright area to a dark area!

It depends a lot on the filter coatings. If I remember correctly,
uncoated glass or plastic with a refractive index around 1.5 reflects
about 4% per surface. But a single-layer coating brings that down
dramatically, and good multi-layer coatings are in the 0.5-1% range per
surface.

Dave

"Dave Martindale" wrote in message
...
"Bart van der Wolf" writes:


At least 3% for two glass/air surfaces, which is a lot if you add
3%
of a bright area to a dark area!

It depends a lot on the filter coatings. If I remember correctly,
uncoated glass or plastic with a refractive index around 1.5
reflects about 4% per surface.

Per surface, yes that's what I recall as well.

But a single-layer coating brings that down dramatically, and
good multi-layer coatings are in the 0.5-1% range per surface.

Correct, a *good* multi(!)-layer coated UV filter will reduce the loss
of contrast, assuming a good lens hood is used to keep false light
away from the protruding filter :-(
The multi-coating is also not extremely resistant to 'environmental'
influences, although they have improved over the years. Some are hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Bart

"Dave Martindale" wrote in message
...
"Bart van der Wolf" writes:


At least 3% for two glass/air surfaces, which is a lot if you add
3%
of a bright area to a dark area!

It depends a lot on the filter coatings. If I remember correctly,
uncoated glass or plastic with a refractive index around 1.5
reflects about 4% per surface.

Per surface, yes that's what I recall as well.

But a single-layer coating brings that down dramatically, and
good multi-layer coatings are in the 0.5-1% range per surface.

Correct, a *good* multi(!)-layer coated UV filter will reduce the loss
of contrast, assuming a good lens hood is used to keep false light
away from the protruding filter :-(
The multi-coating is also not extremely resistant to 'environmental'
influences, although they have improved over the years. Some are hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Bart

"Dave Martindale" wrote in message
...
SNIP
If I remember correctly, uncoated glass or plastic with a
refractive index around 1.5 reflects about 4% per surface.

In fact the formula for normal (!) incidence is (with n as refractive
index):
R=((n-1)/(n+1))^2, per air to glass surface, so it is exactly 4%.

At larger angles of incidence the reflection increases up to total
reflection, and the efficiency of AR coatings decreases with deviation
of the lightpath from the quarter wavelength thickness.

Bart

"Bart van der Wolf" writes:

The multi-coating is also not extremely resistant to 'environmental'
influences, although they have improved over the years. Some are hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Have you ever scratched a lens surface doing this? I'm always leery of
the possibility of a tiny bit of grit in the cloth causing noticeable
damage if I rub the lens using any significant pressure.

I've always gotten good results using liquid cleaning, which takes very
little pressure. I normally start with a detergent-based lens cleaner
like Kodak lens cleaner, applying it with a new cotton Q-tip. Then wipe
off (using little pressure) with some sort of absorbent cloth. If the
cleaning solution evaporates and dries before you get it soaked up by
the cloth, there will be some detergent residue left on the lens, but it
comes off nicely in a second pass using distilled water.

Occasionally, I'll see gunk on a lens that isn't water-soluble, and the
Kodak cleaner won't touch it. Then I experiment with pure isopropyl
alcohol and/or petroleum naphtha based lighter fluid. Both of these
evaporate with no residue from a clean surface, and they're not too
agressive so they're unlikely to damage plastic or paint if contact is
limited to a few seconds.

Dave

"Bart van der Wolf" writes:

The multi-coating is also not extremely resistant to 'environmental'
influences, although they have improved over the years. Some are hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Have you ever scratched a lens surface doing this? I'm always leery of
the possibility of a tiny bit of grit in the cloth causing noticeable
damage if I rub the lens using any significant pressure.

I've always gotten good results using liquid cleaning, which takes very
little pressure. I normally start with a detergent-based lens cleaner
like Kodak lens cleaner, applying it with a new cotton Q-tip. Then wipe
off (using little pressure) with some sort of absorbent cloth. If the
cleaning solution evaporates and dries before you get it soaked up by
the cloth, there will be some detergent residue left on the lens, but it
comes off nicely in a second pass using distilled water.

Occasionally, I'll see gunk on a lens that isn't water-soluble, and the
Kodak cleaner won't touch it. Then I experiment with pure isopropyl
alcohol and/or petroleum naphtha based lighter fluid. Both of these
evaporate with no residue from a clean surface, and they're not too
agressive so they're unlikely to damage plastic or paint if contact is
limited to a few seconds.

Dave

"Dave Martindale" wrote in message
...
"Bart van der Wolf" writes:

The multi-coating is also not extremely resistant to
'environmental'
influences, although they have improved over the years. Some are
hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Have you ever scratched a lens surface doing this?

Not yet.

I'm always leery of the possibility of a tiny bit of grit in
the cloth causing noticeable damage if I rub the lens using
any significant pressure.

Yes, grit and grease require a different approach. My common
procedure, which is rarely needed, is to use a (blower) brush to
remove loose surface grit/lint, vigorously shake the *clean*
microfiber cloth (optical grade) to unfold, breath on the surface of
the lens/filter, and wipe gently in a motion that rotates the cloth
from the surface as I move forward. Usually the (blower) brush is
sufficient, unless an accidental fingerprint finds it way to the
surface. In extreme cases one could consider lens cleaner on the
cloth.

Bart

"Dave Martindale" wrote in message
...
"Bart van der Wolf" writes:

The multi-coating is also not extremely resistant to
'environmental'
influences, although they have improved over the years. Some are
hard
to clean without leaving smudges, but clean micro-fiber cloth will
achieve it.

Have you ever scratched a lens surface doing this?

Not yet.

I'm always leery of the possibility of a tiny bit of grit in
the cloth causing noticeable damage if I rub the lens using
any significant pressure.

Yes, grit and grease require a different approach. My common
procedure, which is rarely needed, is to use a (blower) brush to
remove loose surface grit/lint, vigorously shake the *clean*
microfiber cloth (optical grade) to unfold, breath on the surface of
the lens/filter, and wipe gently in a motion that rotates the cloth
from the surface as I move forward. Usually the (blower) brush is
sufficient, unless an accidental fingerprint finds it way to the
surface. In extreme cases one could consider lens cleaner on the
cloth.

Bart