Tessar Coating Question

Q.G. de Bakker wrote:
Sarah Brown wrote:

I've got some lovely results, with good contrast with an uncoated Tessar on
Provia 100F. It's only got 6 air/glass interfaces after all.

Plus one glass-glass interface.
Provided the bits of glass do not have the same index of refraction (which
they do not, else only one piece of glass could have been used), that
glass-glass 'interface' will bounce light off it as well as glass-air
'interface'.

The reflection at the glass-glass interface is much weaker than
at the air-glass interface. This is obvious if you look at any
cemented doublet lens, like the rear element of a Tessar, coated
or uncoated; the reflection off the cemented surface is so weak
that it is hard to find. There are several reasons for this.

The difference between glasses' index of refraction is less
than between air and glass index. Typical crown and flint glasses
have indexes in the 1.5-1.7 range while air is 1.0.
And the optical cement is formulated to have an index similar to
that of typical glasses (n=1.5-1.6 or so).

Also, lens elements use different types of glass not only for their
different indexes, but for their different dispersions (variation
of index with wavelength). This is how lenses are corrected
for both aberrations and color.

As a corollary, it is not necessary to put an anti-reflection
coating on lens surfaces that will be cemented together.

I wouldn't spend a lot of money coating an uncoated Tessar,
even if it had magical properties. First I am cheap. Second,
it is only going to perhaps mildly increase contrast, since
Tessars have only 6 air-glass surfaces. Investing
in a good lens shade is more practical. Unless you shoot
heavily backlit scenes a lot, in which case perhaps a different
lens is called for.

If you do disassemble and decement a lens for coating, be
sure you have a plan for getting all the elements back
together with the proper spacing _and_ centering.

Q.G. de Bakker wrote:

Sarah Brown wrote:

I've got some lovely results, with good contrast with an uncoated Tessar
on
Provia 100F. It's only got 6 air/glass interfaces after all.

Plus one glass-glass interface.
Provided the bits of glass do not have the same index of refraction (which
they do not, else only one piece of glass could have been used), that
glass-glass 'interface' will bounce light off it as well as glass-air
'interface'.

Actually that is two glass-balsam interfaces. But anyone who has seen a
Tessar knows that they don't really matter. Since the coefficients of
refraction are so close to each other, reflections are insignificant.

-- Lassi

wrote:

... The coating operation I contacted told me that in order to get a
visible result, I would need to coat 80% of the surfaces. In other words
every element of the Tessar - including the expensive separation and
re-gluing of the rear doublet. Does anyone agree with me that one or two
elements would yield a visible result or are they right in suggesting that
it's all or nothing.

Each uncoated glass-air surface sends about 4% of the light in wrong
direction. With six of those, over 20% gets scattered around. If you coat
four of the surfaces, the proportion drops to 8%.

-- Lassi

Matt Clara wrote:

"Stacey" wrote in message
...

wrote:
I recently got a quote to coat a Tessar - $700 if I'm figuring
right.


Talk to the guys at www.arax.com. Lower labor rate, MUCH cheaper at
multicoating old optics and the people who have had them do this sort of
work have been happy.


http://araxfoto.com


Whoops, thanks.. :-)
--

Stacey

wrote in message
...
I recently got a quote to coat a Tessar - $700 if I'm figuring right. My
original idea was to coat the easily removable front and center elements
only. I would think the center element would be the most important
element
to coat, assuming I'm using a proper lens hood and a coated UV filter up
front. The coating operation I contacted told me that in order to get a
visible result, I would need to coat 80% of the surfaces. In other words
every element of the Tessar - including the expensive separation and
re-gluing of the rear doublet. Does anyone agree with me that one or two
elements would yield a visible result or are they right in suggesting that
it's all or nothing.
Thanks



Let's see, if I had $700 to spend on a lens, would I get an old one coated,
or just buy a new one...?

--
Regards,
Matt Clara
www.mattclara.com

"Matt Clara" wrote in message
m...
wrote in message
...
I recently got a quote to coat a Tessar - $700 if I'm figuring right. My
original idea was to coat the easily removable front and center elements
only. I would think the center element would be the most important
element
to coat, assuming I'm using a proper lens hood and a coated UV filter up
front. The coating operation I contacted told me that in order to get a
visible result, I would need to coat 80% of the surfaces. In other
words
every element of the Tessar - including the expensive separation and
re-gluing of the rear doublet. Does anyone agree with me that one or two
elements would yield a visible result or are they right in suggesting
that
it's all or nothing.
Thanks



Let's see, if I had $700 to spend on a lens, would I get an old one
coated,
or just buy a new one...?

I don't understand the recycling of the supposed outrage over this price.
Again $700 is peanuts compared to the dollars being thrown at digital gear
today. Plus, who, including myself has said that this is a reasonable
price? Don't you think if I had thought it reasonable I would have just
sent the Tessars for coating rather than asking if all of the elements need
coating? And as for bokeh, I've seen a wide variation in effects across
different four-element "Tessar" lenses: from the bizzare swirling pattern
provided by a Wollensak Raptar to the gentle blurring of the Taylor-Taylor
Hobson Micronar - so I don't believe there is a stereotypical Tessar
"bokeh".

"Lassi Hippeläinen" wrote in message
...
wrote:

... The coating operation I contacted told me that in order to get a
visible result, I would need to coat 80% of the surfaces. In other
words
every element of the Tessar - including the expensive separation and
re-gluing of the rear doublet. Does anyone agree with me that one or two
elements would yield a visible result or are they right in suggesting
that
it's all or nothing.

Each uncoated glass-air surface sends about 4% of the light in wrong
direction. With six of those, over 20% gets scattered around. If you coat
four of the surfaces, the proportion drops to 8%.

Thank you for the informative response. This is pretty much what I was
thinking. Also considering that the rear two elements are cemented, I would
think there would be less reflections going on there than with the front two
elements. I'm really scared to have the rear two elements re-cemented also.
It seems to me that coating is fairly straightforward, but re-cementing
could prove to be a problem.
Thanks again

Etaoin Shurdlu wrote:
wrote in message
ups.com...

Etaoin Shurdlu wrote:
wrote in message
...
[...] And I don't for one believe Tessars are "turds".

Terssars are strange - kinda-sharp at the edges, sharp in the center, and
the rest is soft.

"turds", in other words.

What are ya, Dr. Seuss?

I just feel that a person should know what the lens does and use the virtues
it has. Some folks do very well with uncoated lenses; they know where the
light is thrown, how it diminishes contrast, how the sharpness works, and
how to exploit all those characteristics.

There are modern, coated Tessar-design lenses to choose from.

And they will have better optical properties than the old Tessars,
quite apart from the coating. I would suggest that an old Tessar given
a modern coating will fall far short of a newer, coated Tessar. And
these will fall far short of other lens designs, such as the Sonnar,
when it comes to bokeh - Waterhouse (round hole) apertures or not.

There has been some confusion on this newsgroup about the Tessar
design. One of the well-respected posters here seemed to think they
were all the same. They are not. So long as you have a diverging
air-spaced doublet at the front and a strongly converging cemented
doublet at the rear then that counts as a Tessar. There are an infinite
variety of designs possible within this constraint when you think of
glass type, thickness, spacing and surface curvature. The later Tessars
were optical improvements over the early designs due to these factors.