Glass quality and f stop question.

I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !

wrote:
I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !

Perhaps you mean these types of glass:
http://en.wikipedia.org/wiki/SLD_glass
called different things by different makers, "ED" glass by Nikon etc.
It's about refractive index and properties not necessarily related to
"quantity" of light transmission, but reducing aberrations.
Lenses should give more or less the same exposure at the same f-stop
setting, but subject to some variability, misalignment of stop-down
mechanisms or aperture blades, uneven light across the frame
(vignetting), and rounding-off of maximum aperture (a nominal f2.8 100mm
lens may not be exactly f2.8 at widest setting, nor exactly 100mm).

On Fri, 04 Sep 2009 22:26:29 -0400, wrote:


I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !

Every lens is subject to air/glass light losses. Each air to glass surface
can rob an imaging light-path by as much as 3% to 5% of the available
light. Much depends on the multi-coating, cemented elements (no light and
contrast-robbing air-space between glass elements), and overall
construction of each lens design. There are no cut and dried rules between
any two multi-element lens designs.

wrote:
I often see lenses that have "special glass" elements, and some that
don't. They claim better transparency.

My question is, if you set up a certain shot with certain parameters,
and then switch to a better lens, would you have to change the f stop
or speed or something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would
that be wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example,
would give different amounts of light. Is this considered when the
manufacturer makes the lens? Do they compensate somehow?

Unless you're talking about IR or UV, the type of glass used in the lens has
little effect on light transmission. The special glass types used have
properties that help control chromatic aberration, they don't provide
greater transparancy.

wrote:
I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

The "special glass" is not for transparency, but to help
control various focusing distortions for different
wavelength of light.

But yes different kinds of glass have different light
transmission characteristics. Likewise the number of
elements (chiefly the number of air to glass
transitions) affects light transmission, and so do the
coatings of some elements. The overall effect of one or
two elements made from "special glass" might be small;
but the overall differences from all effects might well
be significant!

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

For still photography that is rarely significant, if for
no other reason than the common use of through the lens
metering systems. But for cinematography it can be very
important. Because of that is not uncommon for their
lenses to be calibrated in "T-Stops". F-Stops are the
physical parameter, and T-Stops are the actual light
transmission characteristics.

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !

I'm sure the designers give it "consideration"... but
not much! If there is, say, 1/2 an f-stop difference in
actual light transmission, what difference would that
actually make to you personally?

--
Floyd L. Davidson http://www.apaflo.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)

wrote:
I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

Well, not so much "better transparency" as rather a different refractive
index which allows, when used in the right combination with ordinary
glass, to significantly reduce chromatic abberation.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

In theory maybe yes, but in real life no. The differences are just too
small to make any impact. If you reduce the light loss from let's say 5%
per element to 3% per element (which is a lot), then with e.g. 10
elements (which is a lot) the total light passing is about 60% versus
74%. That's less than 1/4 f-stop difference.

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

The difference is just not significant enough to make a big deal out of
it.

jue

wrote:
I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !


Since the meter reads the light coming through the lens, variations in
light transmission are automatically taken care of by the meter. It is
only of consequence if you use an external meter and set shutter and
aperture manually.

Colin D.

wrote in message ...

I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

Curious !


Two 50mm lenses set at f8 are two 50mm lenses set at f8.
Same focal length.
Same aperture.
Same amount of light.


John

wrote:
I often see lenses that have "special glass" elements, and some that don't. They
claim better transparency.

My question is, if you set up a certain shot with certain parameters, and then
switch to a better lens, would you have to change the f stop or speed or
something to match the better glass?

I'm just wondering about things like "rule of sun on grass" - would that be
wrong with a better lens?

It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light. Is this considered when the manufacturer makes the
lens? Do they compensate somehow?

The variance is less than 1/3 of a stop. With good photometry you would
measure a difference. The variance in in-camera meters is of the same
magnitude (between models or manufacturers) so you're well in the
tolerance basket.

More importantly though, is that unless you're shooting some specific
target in controlled lighting, and instead using your camera for actual
photography, then the variance on exposure is even greater due to your
metering mode, the camera implementation of that mode and technique.

There are a few idiots around who believe "high speed glass" refers to
the glass transparency rather than the whole lens system, usually of
course wide and constant (with zoom) aperture. eg: a 70-200 f/2.8 is
"high speed glass" but not because of the composition of the lens elements.

"jaf" wrote in message
...
[]
Two 50mm lenses set at f8 are two 50mm lenses set at f8.
Same focal length.
Same aperture.
Same amount of light.


John

Not so. f/8 is a mechanical measurement relating to aperture and focal
length. It actually says nothing about the actual amount of light, simply
the /maximum/ light possible. The less than 100% transmission of the
glass will reduce the actual amount of light (albeit, perhaps only by a
small amount).

Two lenses with the same T-stop would pass the same amount of light. See:
http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop

Also: consider a standard and a mirror lens both at f/8. The mirror lens
typically has a central obstruction which reduces the light....

Cheers,
David