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From: Leon on 4 Sep 2009 22:26

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 !

From: Me on 4 Sep 2009 22:37
Leon(a)nospam.com 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).
From: Randy T. on 4 Sep 2009 22:40
On Fri, 04 Sep 2009 22:26:29 -0400, Leon(a)nospam.com 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.

From: Floyd L. Davidson on 4 Sep 2009 22:57
Leon(a)nospam.com 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) floyd(a)apaflo.com
From: J. Clarke on 4 Sep 2009 22:52
Leon(a)nospam.com 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.


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