"Hybrid" or polycarbonate lenses and aging

John Bean wrote:
On Mon, 04 Dec 2006 08:49:47 GMT, "David J Taylor"

wrote:

jeremy wrote:
[]
About a month ago I decided to return to those classic 50mm f/1.4
lenses, and I wrapped one in aluminum foil, pointed the rear element
out a window and let it sit. I checked about 10 days ago and found
that the color cast had been significantly lessened. So I put 2
additional 50mm f/1,4 lenses on the windowsill and they, too are
exhibiting a much reduced color cast after a week and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?

David

"David J Taylor"
wrote:

John Bean wrote:
On Mon, 04 Dec 2006 08:49:47 GMT, "David J Taylor"

wrote:

jeremy wrote:
[]
About a month ago I decided to return to those classic 50mm f/1.4
lenses, and I wrapped one in aluminum foil, pointed the rear element
out a window and let it sit. I checked about 10 days ago and found
that the color cast had been significantly lessened. So I put 2
additional 50mm f/1,4 lenses on the windowsill and they, too are
exhibiting a much reduced color cast after a week and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?


Ultra violet light.

In article , Tom Ross
wrote:

You really are an idiot.

That's a pretty accurate description of RichA.

Tony Polson wrote:
"David J Taylor"
wrote:

John Bean wrote:
On Mon, 04 Dec 2006 08:49:47 GMT, "David J Taylor"

wrote:

jeremy wrote:
[]
About a month ago I decided to return to those classic 50mm f/1.4
lenses, and I wrapped one in aluminum foil, pointed the rear
element out a window and let it sit. I checked about 10 days ago
and found that the color cast had been significantly lessened.
So I put 2 additional 50mm f/1,4 lenses on the windowsill and
they, too are exhibiting a much reduced color cast after a week
and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?


Ultra violet light.

But Jeremy said he wrapped the lenses in aluminium foil, which would not
pass UV. Or if the read element is exposed, how does adding more UV to a
lens cure the damage done by UV in the first place?

I know that UV can degrade certain plastics...

David

David J Taylor wrote:
Tony Polson wrote:
"David J Taylor"
wrote:
John Bean wrote:
"David J Taylor"wrote:
jeremy wrote:

About a month ago I decided to return to those classic 50mm f/1.4
lenses, and I wrapped one in aluminum foil, pointed the rear
element out a window and let it sit. I checked about 10 days ago
and found that the color cast had been significantly lessened.
So I put 2 additional 50mm f/1,4 lenses on the windowsill and
they, too are exhibiting a much reduced color cast after a week
and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?

Ultra violet light.

But Jeremy said he wrapped the lenses in aluminium foil, which would not
pass UV. Or if the read element is exposed, how does adding more UV to a
lens cure the damage done by UV in the first place?

I know that UV can degrade certain plastics...

I think these are old glass lenses with radioactive elements and the
foil is probably just on the back side.

On Mon, 04 Dec 2006 10:38:44 +0000, David J Taylor wrote:

John Bean wrote:
On Mon, 04 Dec 2006 08:49:47 GMT, "David J Taylor"

wrote:

jeremy wrote:
[]
About a month ago I decided to return to those classic 50mm f/1.4
lenses, and I wrapped one in aluminum foil, pointed the rear element
out a window and let it sit. I checked about 10 days ago and found
that the color cast had been significantly lessened. So I put 2
additional 50mm f/1,4 lenses on the windowsill and they, too are
exhibiting a much reduced color cast after a week and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?

If you google "Takumar Yellowing" you'll find numerous references to the
phenomenon and to its cure by exposure to UV. The notion that it is
somehow caused by the radiation from thorium in the lens however seems to
be speculation--there's no mention that I could find of the mechanism by
which this yellowing occurs or that by which exposure to UV reverses
it--it could be a purely chemical reaction rather than anything related
to radiation and the thorium might be totally uninvolved.

David

Paul Furman wrote:
David J Taylor wrote:
Tony Polson wrote:
"David J Taylor"
wrote:
John Bean wrote:
"David J Taylor"wrote:
jeremy wrote:

About a month ago I decided to return to those classic 50mm
f/1.4 lenses, and I wrapped one in aluminum foil, pointed the
rear element out a window and let it sit. I checked about 10
days ago and found that the color cast had been significantly
lessened. So I put 2 additional 50mm f/1,4 lenses on the
windowsill and they, too are exhibiting a much reduced color
cast after a week and a half.

Did I miss the smiley?

Why? The method works, I've used it successfully on an old
Pentax S-M-C Takumar 50/1.4 that had yellowed over the
years. It didn't end up perfect but it was much improved.

OK. What's the mechanism?

Ultra violet light.

But Jeremy said he wrapped the lenses in aluminium foil, which would
not pass UV. Or if the read element is exposed, how does adding
more UV to a lens cure the damage done by UV in the first place?

I know that UV can degrade certain plastics...

I think these are old glass lenses with radioactive elements and the
foil is probably just on the back side.

Thanks. Can't really comment. I sold off all my off 35mm stuff a few
years back, but I'd never seen any discolouration of the lenses.

David

Gisle Hannemyr wrote:
[]
I think maing such lenses became illegal in the mid-1960ies, but some
samples are still around and pop up on eBay.

Thanks, Gisle and J.Clarke for the explanations. I was only just starting
out with photography around that time, so perhaps helps explain why I'd
never heard of the problem or the cure.

You really do learn something every day!

Cheers,
David

On Mon, 04 Dec 2006 15:54:16 +0100, Gisle Hannemyr
wrote:

"David J Taylor":
Thanks. Can't really comment. I sold off all my off 35mm stuff a
few years back, but I'd never seen any discolouration of the lenses.

Thorium lenses were rare and expensive. The most common one was a
very fast (for the time) 50mm f/1.4 Super Takumar (but the even more
common 50mm f/1.4 SMC Takumar contained no Thorium element and shows
no discolouration).


Also some of the "Super Multi Coated" (not "SMC") Takumars
like the one I have suffer from yellowing. As you rightly
say the later SMC Takumars didn't have this problem.


--
John Bean

"David J Taylor"
wrote in message news:R8Wch.11513

But Jeremy said he wrapped the lenses in aluminium foil, which would not
pass UV. Or if the read element is exposed, how does adding more UV to a
lens cure the damage done by UV in the first place?



The technique of leaving the rear element facing the sun does work over
time, but the sun's energy is absorbed by the black finish of the lens
barrel, and the lens is at risk of heating up. This might cause the
internal lubricants to thin and to run, which would damage the internal
optical elements.

The aluminum foil is to reflect the light back, rather than to let the lens
heat up.

Another possible benefit of the foil is to cover the FRONT element, and to
reflect the light back toward the rear element, hopefully maximizing the UV
that passes through the rear element. My understanding is that SMC coatings
absorb most UV, so I have my doubts as to whether that is of much benefit.

Other than acting as a reflector of light, the aluminum foil does nothing to
bleach the rear element.

When I first heard of the technique I had my reservations, but it seems to
be working just fine. I'm looking forward to my four 50mm normal lenses
returning to active duty. It is taking a long time, but it took 30 years
for them to yellow. If it takes 60 or even 90 days to bleach, I have no
basis to complain.