Glass quality and f stop question.

David J Taylor wrote:

"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

The difference is very slight amongst lenses, and plays within less than
1/3 of a stop of light. Given metering differences, technique and so
on, it is quite unimportant. (I do wonder if the manufacturers bias the
aperture slightly to account for light transmission - perhaps some do).

The few SLR lenses that have a T-stop these days are a specialty matter
(such as the superb Sonyolta 135 f/2.8 [T4.5]) while in the film making
industry T-stops are not exceptional at all.

"jaf" wrote:

Nikon 500mm
http://www.nikonusa.com/Find-Your-Ni...F4G-ED-VR.html

Nikon 500mm mirror
http://www.kenrockwell.com/nikon/5008.htm

You've got a very good example of exactly what I've been
talking about! Indeed, you've cited a (generally less
than reliable, but in this case acceptable) web page
that tells you exactly what you are then asking me.

Take a camera.
Mount each lens, shoot a picture of a Color Chart http://figitalrevolution.files.wordp...or-checker.jpg

100 ASA film speed. f8 aperture. 500mm focal length.

What shutter speed is required to get a correct exposure for each lens?

The mirror lens will require a shutter speed that is
twice as long.

The reason, as Rockwell explains in the cited URL, is
because the mirror lens only passes half as much light
as the regular 500mm lens when they are both set to f/8.
The mirror lens passes the same light at f/8 as the
other lens does at about f/11.

No photoshopping allowed.

Post back with the results.

Stop and *think* about this. Do more research.

Post back with the results.

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

"jaf" writes:

You missed the point.
The questions were rhetorical.
A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same regardless of which scale you look at.

Obviously, but the scales are different on some lenses -- the
relationship between the scales is only fixed for a given lens, not
generally, whereas the relationship between the t-stop numbers and the
"amount of light" passed through is fixed for all lenses.

Frankly, I'm not sure _what_ you're trying to argue...

The meaning of t-stop and f-stop seems obvious, and the difference is
clearly useful in some applications. Your tone gives the impression
that you somehow disagree, but it's not clear how or why.

-Miles

--
"She looks like the wax version of herself."
[Comment under a Paris Hilton fashion pic]

Miles Bader wrote:
"jaf" writes:

You missed the point.
The questions were rhetorical.
A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same regardless of which scale you look at.

Obviously, but the scales are different on some lenses -- the
relationship between the scales is only fixed for a given lens, not
generally, whereas the relationship between the t-stop numbers and the
"amount of light" passed through is fixed for all lenses.

Frankly, I'm not sure _what_ you're trying to argue...

The meaning of t-stop and f-stop seems obvious, and the difference is
clearly useful in some applications. Your tone gives the impression
that you somehow disagree, but it's not clear how or why.

F/stops are measured with a ruler.
T-stops are measured with a light meter.

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam

Paul Furman writes:
The meaning of t-stop and f-stop seems obvious, and the difference is
clearly useful in some applications. Your tone gives the impression
that you somehow disagree, but it's not clear how or why.

F/stops are measured with a ruler.
T-stops are measured with a light meter.

Right, exactly.

I still am confused about what the [jaf's] argument is here -- there
doesn't seem anything at all controversial or vague about this
subject...

-Miles

--
x
y
Z!

In article , jaf
writes

"Miles Bader" wrote in message
...
"jaf" writes:
I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Absolute to an aperture with 100% transmission. That is what the "T" in
"T-stop" stands for!

Relative to what?
Dimensionally relative to the Focal length. That is what the "f" in
"f-stop" stands for, and is completely independent of actual optical
transmission.

I think the meaning is that t-stops have a fixed relationship to an
external reference -- e.g., using t-stops, you can calculate exposure
using an external meter, and directly use the indicated settings on your
camera. That can't always be done using f-stops, but if you calculate
the correct exposure at a single f-stop, then other f-stops using the
same lens etc will have a fixed relationship to that (so f2 always lets
through twice as much light as f2.8).
-miles


You missed the point.
No, he hit the point precisely. You seem to have some difficulty making
a point.

The questions were rhetorical.
That may be so, but they were questions and could be answered factually
and meaningfully.

Common f-stops on similar lenses produce common depth of field but not
necessarily the same exposure.

Common t-stops on similar lenses produce common exposures but not
necessarily the same depth of field.

A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same regardless of which
scale you look at.

That is true for a SINGLE lens. However it is certainly untrue for
different lenses. Since you seem to have lost the plot, the OP was
asking about how one lens compares to another.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)

wrote:
On Sat, 05 Sep 2009 10:07:53 -0400, Alan Browne

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.

I see now that even though the manufacturers sometimes refer to special glass as
'better transmission of light' they probably mean better quality light rather
than quantity.

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.

I imagine "high speed glass" refers to the fact you can shoot at higher shutter
speeds since the lens is at 2.8 instead of 5.6 at 200mm, like a kit lens.

That's exactly what the term means.

That reminds me of people who thought that the statement; "That DSLR is much
quieter than the other one." was referring to the sound of the mirror and
shutter! When I discuss my camera with 'non hip' people I avoid 'noise' and
refer to 'snow like a poor TV picture'.

Well, ... I suppose. "Video" noise and image noise in a still image
have a really different flavour to my eyes, but if your analogy works
with these folks, then fine...

"Kennedy McEwen" wrote in message ...
In article , jaf
writes

"Miles Bader" wrote in message
...
"jaf" writes:
I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Absolute to an aperture with 100% transmission. That is what the "T" in
"T-stop" stands for!

Relative to what?
Dimensionally relative to the Focal length. That is what the "f" in
"f-stop" stands for, and is completely independent of actual optical
transmission.

I think the meaning is that t-stops have a fixed relationship to an
external reference -- e.g., using t-stops, you can calculate exposure
using an external meter, and directly use the indicated settings on your
camera. That can't always be done using f-stops, but if you calculate
the correct exposure at a single f-stop, then other f-stops using the
same lens etc will have a fixed relationship to that (so f2 always lets
through twice as much light as f2.8).
-miles


You missed the point.
No, he hit the point precisely. You seem to have some difficulty making
a point.

The questions were rhetorical.
That may be so, but they were questions and could be answered factually
and meaningfully.

Common f-stops on similar lenses produce common depth of field but not
necessarily the same exposure.

Common t-stops on similar lenses produce common exposures but not
necessarily the same depth of field.

A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same regardless of which
scale you look at.

That is true for a SINGLE lens. However it is certainly untrue for
different lenses. Since you seem to have lost the plot, the OP was
asking about how one lens compares to another.
--


The OP stated "It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light."

I stated they do not.
My reply was directed at the spec's in question. 50mm @ f8.

Some other pinheads decided to hijack the the thread into a babbling idiot contest about using different scales.
As if different scales make a difference even though the aperture can only be set on one reading.

Now we a blessed by your inability to see the forest because the trees are in the way.
Thanks for stopping by.



John

"jaf" wrote:
"Kennedy McEwen" wrote in message ...
In article
, jaf
writes

"Miles Bader" wrote in message
...
"jaf" writes:
I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Absolute to an aperture with 100% transmission. That
is what the "T" in "T-stop" stands for!

Relative to what?
Dimensionally relative to the Focal length. That is
what the "f" in "f-stop" stands for, and is completely
independent of actual optical transmission.

I think the meaning is that t-stops have a fixed relationship to an
external reference -- e.g., using t-stops, you can calculate exposure
using an external meter, and directly use the indicated settings on your
camera. That can't always be done using f-stops, but if you calculate
the correct exposure at a single f-stop, then other f-stops using the
same lens etc will have a fixed relationship to that (so f2 always lets
through twice as much light as f2.8).
-miles


You missed the point.
No, he hit the point precisely. You seem to have some
difficulty making a point.

The questions were rhetorical.
That may be so, but they were questions and could be
answered factually and meaningfully.
Common f-stops on similar lenses produce common depth
of field but not necessarily the same exposure.
Common t-stops on similar lenses produce common
exposures but not necessarily the same depth of field.

A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same
regardless of which scale you look at.

That is true for a SINGLE lens. However it is
certainly untrue for different lenses. Since you seem
to have lost the plot, the OP was asking about how one
lens compares to another.
--

The OP stated "It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light."

I stated they do not.

And that was of course a mistake, because they *can*
have different light transmission characteristics. One
easy way to visualize that is to use the exact same lens
as your example: set it to f/8, and use it with and
without an ND filter on the front. Same focal length,
aperture, and f/8 either way, but a different "lens
design" in terms of light transmission with and without
the ND filter. As someone else pointed out, the DOF
will be the same in both cases because the f/stop is the
same, but the exposure will be different because the
t/stop is different.

My reply was directed at the spec's in question. 50mm @ f8.

As has been noted, f/8 refers to a physical set of
measurements, not to how much light actually passes
through the lens.

Some other pinheads decided to hijack the the thread
into a babbling idiot contest about using different
scales. As if different scales make a difference even
though the aperture can only be set on one reading.

The *fact* is that T-stops are a measure of light
transmission. Hence when the aperture of one lens is
set to f/8, it might be t/9, while another lens might be
t/11.

If both lenses are set to f/8, the exposure will be
*different*. To get an identical exposure using the two
lenses it is necessary to set both to the same T-stop
rather than the same F-stop.

The point of course is that on any single lens there is
as you say a constant and direct relationship between
f/stops and t/stops, but any two different lens designs
can have *different* relationships. On one the t/stop
might be, across the board, 1/3 of an f/stop lower than
the f/stop scale; but on another lense it may be 2/3 of
an f/stop lower than the f/stop scale. That means
setting the two at the same f/stop will result in 1/3 of
an f/stop *different* exposure!

Or, as one example that I posted earlier... with mirror
lenses it might be significantly greater. 1/3 of an
f/stop isn't much to most people, but an f/8 mirror
lense might be more than 1 whole f/stop lower when light
transmission is measured, and actually could be a t/11
when set to f/8.

Now we a blessed by your inability to see the forest
because the trees are in the way. Thanks for stopping
by.

At some point this all might actually become more clear
to you, but you really are going to have to slow down
and *look* at what people are saying. Several people have
tried different ways of describing it, but you are refusing
to read what they say... and *you* are the only one who
is not agreeing!

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

"Floyd L. Davidson" wrote in message ...
"jaf" wrote:
"Kennedy McEwen" wrote in message ...
In article
, jaf
writes

"Miles Bader" wrote in message
...
"jaf" writes:
I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Absolute to an aperture with 100% transmission. That
is what the "T" in "T-stop" stands for!

Relative to what?
Dimensionally relative to the Focal length. That is
what the "f" in "f-stop" stands for, and is completely
independent of actual optical transmission.

I think the meaning is that t-stops have a fixed relationship to an
external reference -- e.g., using t-stops, you can calculate exposure
using an external meter, and directly use the indicated settings on your
camera. That can't always be done using f-stops, but if you calculate
the correct exposure at a single f-stop, then other f-stops using the
same lens etc will have a fixed relationship to that (so f2 always lets
through twice as much light as f2.8).
-miles


You missed the point.
No, he hit the point precisely. You seem to have some
difficulty making a point.

The questions were rhetorical.
That may be so, but they were questions and could be
answered factually and meaningfully.
Common f-stops on similar lenses produce common depth
of field but not necessarily the same exposure.
Common t-stops on similar lenses produce common
exposures but not necessarily the same depth of field.

A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same
regardless of which scale you look at.

That is true for a SINGLE lens. However it is
certainly untrue for different lenses. Since you seem
to have lost the plot, the OP was asking about how one
lens compares to another.
--

The OP stated "It just seems odd to me that two, 50mm lenses, set at f8 for example, would give
different amounts of light."

I stated they do not.

And that was of course a mistake, because they *can*
have different light transmission characteristics. One
easy way to visualize that is to use the exact same lens
as your example: set it to f/8, and use it with and
without an ND filter on the front.

Hey PINHEAD,
Why don't you try it with the lens cap on.
See if you get the same light measurement.
Filters are irrelevant to the question the OP asked!

Same focal length,
aperture, and f/8 either way, but a different "lens
design" in terms of light transmission with and without
the ND filter.

Hey PINHEAD,
Filters are irrelevant to the question the OP asked!

As someone else pointed out, the DOF
will be the same in both cases because the f/stop is the
same, but the exposure will be different because the
t/stop is different.

My reply was directed at the spec's in question. 50mm @ f8.

As has been noted, f/8 refers to a physical set of
measurements, not to how much light actually passes
through the lens.


Hey PINHEAD,
Not relevant whether it's physical, metaphysical or fixation on your part.
It has no relevance to the question the OP asked!

Some other pinheads decided to hijack the the thread
into a babbling idiot contest about using different
scales. As if different scales make a difference even
though the aperture can only be set on one reading.

The *fact* is that T-stops are a measure of light
transmission.

The fact is, you are a expletive deleted PINHEAD!
T stops are irrelevant to the question the OP asked!
Filters are irrelevant to the question the OP asked!

Hence when the aperture of one lens is
set to f/8, it might be t/9, while another lens might be
t/11.

Hey PINHEAD,
T stops are irrelevant to the question the OP asked!


If both lenses are set to f/8, the exposure will be
*different*.

Hey PINHEAD,
NO THEY WON'T

To get an identical exposure using the two
lenses it is necessary to set both to the same T-stop
rather than the same F-stop.

Hey PINHEAD,
T stops are irrelevant to the question the OP asked!


The point of course is that on any single lens there is
as you say a constant and direct relationship between
f/stops and t/stops, but any two different lens designs
can have *different* relationships. On one the t/stop
might be, across the board, 1/3 of an f/stop lower than
the f/stop scale; but on another lense it may be 2/3 of
an f/stop lower than the f/stop scale. That means
setting the two at the same f/stop will result in 1/3 of
an f/stop *different* exposure!


Hey PINHEAD,
T stops are irrelevant to the question the OP asked!

Or, as one example that I posted earlier... with mirror
lenses it might be significantly greater. 1/3 of an
f/stop isn't much to most people, but an f/8 mirror
lense might be more than 1 whole f/stop lower when light
transmission is measured, and actually could be a t/11
when set to f/8.


Hey PINHEAD,
T stops are irrelevant to the question the OP asked!

Now we a blessed by your inability to see the forest
because the trees are in the way. Thanks for stopping
by.

At some point this all might actually become more clear
to you, but you really are going to have to slow down
and *look* at what people are saying. Several people have
tried different ways of describing it, but you are refusing
to read what they say... and *you* are the only one who
is not agreeing!

Hey PINHEAD,
T stops are irrelevant to the question the OP asked!
Try answering the OP's question without T stops.


John