The difference in enlarging lenses

In article t, "Nicholas
O. Lindan" wrote:

1) Attracts dust: helps keep the rest of your darkroom dust free

Nope. A large, charged ABS plastic sink attracts the dust. Just wipe it
clean once a day. It's like magic.

2) Creates Newton's rings: Adds color and pattern to your pictures

Never had Newton's rings with the Leitz Focomat IIa enlarger.

3) Decreases contrast: Keeps those pesky highlights under control

Nice try, but not true.

4) Alters light path: Helps achieve that sought-after 'soft focus' look

Nope!

5) Shatters when dropped: Maintains full employment in the glass industry

Now you are frightening me. No more coffee before printing!

jjs wrote:

In article ,
Bob Salomon wrote:



[... ]





In a glass negative carrier as the lenses are designed to be used with?



Bob, are you saying that the glass carrier is part of the optical formula?
Does a glass carrier do more than simply keep the negative flat? Tell me
it does so that I have some rationalization for the dust I put up with.


John,

Search this ng's archives for articles by Michael Gudzinowicz about
enlarging lenses. He gives the optical formulas for depth of field at
the negative stage. Doing the computations for your situation will most
likely convince you that a glass carrier and a "perfectly" aligned
enlarger are necessary for the best results your system can deliver.

Bob

HypoBob wrote:

jjs wrote:

In article ,
Bob Salomon wrote:



[... ]





In a glass negative carrier as the lenses are designed to be used with?



Bob, are you saying that the glass carrier is part of the optical formula?
Does a glass carrier do more than simply keep the negative flat? Tell me
it does so that I have some rationalization for the dust I put up with.


John,

Search this ng's archives for articles by Michael Gudzinowicz about
enlarging lenses. He gives the optical formulas for depth of field at
the negative stage. Doing the computations for your situation will most
likely convince you that a glass carrier and a "perfectly" aligned
enlarger are necessary for the best results your system can deliver.

Bob
Glass between the negative and the lens affects the focus slightly,
depending on the thickness of the glass. It does that by refraction, of
course, which means that the effect varies with thickness of the glass
as well as color of the light. Some lenses are designed to be used with
a glass carrier of certain thickness. Whether any of us mortals could
tell the difference is another matter. In any case, you can see how
glass might affect chromatic aberration and curvature of the field for
better or worse, depending on how the lens is designed.

In article ,
Patrick Gainer wrote:

HypoBob wrote:

jjs wrote:

In article ,
Bob Salomon wrote:



[... ]





In a glass negative carrier as the lenses are designed to be used with?



Bob, are you saying that the glass carrier is part of the optical formula?
Does a glass carrier do more than simply keep the negative flat? Tell me
it does so that I have some rationalization for the dust I put up with.


John,

Search this ng's archives for articles by Michael Gudzinowicz about
enlarging lenses. He gives the optical formulas for depth of field at
the negative stage. Doing the computations for your situation will most
likely convince you that a glass carrier and a "perfectly" aligned
enlarger are necessary for the best results your system can deliver.

Bob
Glass between the negative and the lens affects the focus slightly,
depending on the thickness of the glass. It does that by refraction, of
course, which means that the effect varies with thickness of the glass
as well as color of the light. Some lenses are designed to be used with
a glass carrier of certain thickness. Whether any of us mortals could
tell the difference is another matter. In any case, you can see how
glass might affect chromatic aberration and curvature of the field for
better or worse, depending on how the lens is designed.

Any shift in focus would be compensated for when the enlarger is
focused. It is a non-issue.

--
To reply no_ HPMarketing Corp.

HypoBob wrote:

Glass between the negative and the lens affects the focus slightly,
depending on the thickness of the glass. It does that by refraction, of
course, which means that the effect varies with thickness of the glass
as well as color of the light. [...]

That reads like impressionistic keyboard engineering. I've never seen it
IRL.

"jjs" wrote in message
...
In article

m,
Bob Salomon wrote:

[... ]

In a glass negative carrier as the lenses are designed
to be used with?

Bob, are you saying that the glass carrier is part of the
optical formula?
Does a glass carrier do more than simply keep the negative
flat? Tell me
it does so that I have some rationalization for the dust I
put up with.

The glass is not part of the formula and has insignificant
effect on the light in the optical path. What Bob is getting
at is that high quality enlarging lenses are designed to
image a flat object onto a flat surface. If the negative
bows, as it will in a glassless holder, the corner
performance of the lens is compromised. Despite the extra
bother of the glass sandwich holder it does increase
sharpness, particularly at the corners, and eliminates
motion of the negative during exposure. This last can be the
explanation for the occasional complaint of getting not
quite sharp prints from sharp negatives even though the
lenses are of excellent quality.
The choice is whether these advantages are worth the
considerable extra effort of keeping four glass surfaces
absolutely clean and free of blemishes. For some printing,
particularly where a great amount of magnification is
involved, I think the glass carriers are a must.
There are lenses designed to work from curved surfaces.
Some slide projector lenses, for instance, the Kodak Ektanar
series, are so designed, intended to work with cardboard
mounted slides. The problem is that many times these slides
are much closer to being flat than was assumed in the lens
design so a flat field lens will give better pictures.
Slides are another case where glass, in this case glass
sandwich slide mounts, has advantages in consistent
sharpness despite extra effort being needed to keep
everything clean.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Patrick Gainer" wrote in message
...


HypoBob wrote:

jjs wrote:

In article

m,
Bob Salomon wrote:



[... ]





In a glass negative carrier as the lenses are designed
to be used with?



Bob, are you saying that the glass carrier is part of
the optical formula?
Does a glass carrier do more than simply keep the
negative flat? Tell me
it does so that I have some rationalization for the
dust I put up with.


John,

Search this ng's archives for articles by Michael
Gudzinowicz about
enlarging lenses. He gives the optical formulas for
depth of field at
the negative stage. Doing the computations for your
situation will most
likely convince you that a glass carrier and a
"perfectly" aligned
enlarger are necessary for the best results your system
can deliver.

Bob
Glass between the negative and the lens affects the focus
slightly,
depending on the thickness of the glass. It does that by
refraction, of
course, which means that the effect varies with thickness
of the glass
as well as color of the light. Some lenses are designed to
be used with
a glass carrier of certain thickness. Whether any of us
mortals could
tell the difference is another matter. In any case, you
can see how
glass might affect chromatic aberration and curvature of
the field for
better or worse, depending on how the lens is designed.

Because the light at the negative is essentially
collimated the glass has little if any effect on the image
forming path other than increasing it by approximately 1/3rd
the thickness of the glass. If the glass is not homogeneous
it can creat some problems because any differences in index
will show up as shadows. This is really of academic more
than practical interest because most glass now is pretty
uniform.
I think the main problem with glass type holders is dust.
That is controlable but takes some effort.
I've crossed swords with Bob Salomon here more than once
but must agree with him on this one.


--
---
Richard Knoppow
Los Angeles, CA, USA

"jjs" wrote in message
...
HypoBob wrote:

Glass between the negative and the lens affects the
focus slightly,
depending on the thickness of the glass. It does that by
refraction, of
course, which means that the effect varies with
thickness of the glass
as well as color of the light. [...]

That reads like impressionistic keyboard engineering. I've
never seen it
IRL.


A plane parallel glass sheet will have an effect on image
forming light passing through it. The amount of effect
depends on the "vergence" of the light and the properties of
the glass or other material. If the light is collimated,
that is, if the light waves are parallel, as they are when
coming from a great distance, there is no effect. If the
light is convergent or divergent the sheet will introduce
spherical aberration and chromatic aberration. The former is
because light will be bent depending on its original angle
of incidence. The second is really due to the same thing
except that the index of refraction of glass is not constant
with wavelength. Glass bends, or deviates to use a more
correct term, light more for blue than for red. The amount
of this difference is called dispersion. In lenses this is
compensated by using combinations of positive and negative
lenses of different kinds of glass.
The above is the reason that filters can have a degrading
effect on a lens. When a filter is used between a lens and a
distant object the effect is insignificant. When used where
the light is angled (vergent), as on the back of a lens in
normal photography, or on either side for macro/micro
photography, the effect can be significant. That's why thin
gelatin filters are used where the best quality imaging is
done. Gelatin has a low index of refraction, so its effect
is small, plus gelatin filters are very thin compared to
glass.
In an enlarger, where the glass is close to the negative
or transparcency, it has little effect on the light or on
the quality of the reproduction. If it were next to the
lens, it would have such an effect because the relative
angles of the light passing through it would be much
greater.


--
---
Richard Knoppow
Los Angeles, CA, USA

jjs,

Please be more careful with your snipping. You have made it look like I
made the statement below, which I did not.

Thanks,
The real HypoBob

----------------------------------
jjs wrote:

HypoBob wrote:



Glass between the negative and the lens affects the focus slightly,
depending on the thickness of the glass. It does that by refraction, of
course, which means that the effect varies with thickness of the glass
as well as color of the light. [...]



That reads like impressionistic keyboard engineering. I've never seen it
IRL.

Bob Salomon wrote:

In article ,
Patrick Gainer wrote:

HypoBob wrote:

jjs wrote:

In article ,
Bob Salomon wrote:



[... ]





In a glass negative carrier as the lenses are designed to be used with?



Bob, are you saying that the glass carrier is part of the optical formula?
Does a glass carrier do more than simply keep the negative flat? Tell me
it does so that I have some rationalization for the dust I put up with.


John,

Search this ng's archives for articles by Michael Gudzinowicz about
enlarging lenses. He gives the optical formulas for depth of field at
the negative stage. Doing the computations for your situation will most
likely convince you that a glass carrier and a "perfectly" aligned
enlarger are necessary for the best results your system can deliver.

Bob
Glass between the negative and the lens affects the focus slightly,
depending on the thickness of the glass. It does that by refraction, of
course, which means that the effect varies with thickness of the glass
as well as color of the light. Some lenses are designed to be used with
a glass carrier of certain thickness. Whether any of us mortals could
tell the difference is another matter. In any case, you can see how
glass might affect chromatic aberration and curvature of the field for
better or worse, depending on how the lens is designed.

Any shift in focus would be compensated for when the enlarger is
focused. It is a non-issue.

--
To reply no_ HPMarketing Corp.
Not true. The focus can only be compensated for one wavelength. The
prism in binoculars affects the chromatic aberration of the system and
is accounted for in the design.