Glass quality and f stop question.

"David J Taylor" wrote in message
om...

"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

Post some data that proves you would need a different exposure time for 2 otherwise identical lenses.


John

"jaf" wrote:
"David J Taylor" wrote in message
. com...

"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

Post some data that proves you would need a different
exposure time for 2 otherwise identical lenses.

Why didn't you bother to read the URL he cited above?
It provides the "data" you request.

Or stop and think about it... for example a pair of
500mm f/8 lenses, have an equal sized aperture.
However, if one of them is a mirror lens and the other
is not, the mirror lens will almost certainly have at
most only half the light transmission efficiency that
the other does. In effect it will f/9 or worse. That
mirror which blocks off a portion the front element is
where the loss occurs. Same aperture and focal length
though, so it has the same f/stop, but with less light
passing through.

Or another very simple experiment will also demonstrate
the fact. Take a single lens (virtually any lens that
you can mount a filter on), and compare it to
itself... with a neutral density filter added. In
either instance the fstop you select will be exactly the
same with or without the filter because the filter does
not change the aperture or the focal length. But it
will change the amount of light going through the lens.

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

"jaf" wrote in message
...

"David J Taylor"
wrote in
message om...

"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

Post some data that proves you would need a different exposure time for
2 otherwise identical lenses.


John

John,

Please look at the reference I gave, and other references to T-stops.
That T-stops were required at all proves the need. With today's optics I
don't think you would be talking about large changes - probably less than
1/3 f/stop - except perhaps in the case of the mirror lens.

This would only apply if you were not metering through the lens.

Cheers,
David

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

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.

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 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'.

"Floyd L. Davidson" wrote in message ...
"jaf" wrote:
"David J Taylor" wrote in message
.com...

"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

Post some data that proves you would need a different
exposure time for 2 otherwise identical lenses.

Why didn't you bother to read the URL he cited above?
It provides the "data" you request.

Or stop and think about it... for example a pair of
500mm f/8 lenses, have an equal sized aperture.
However, if one of them is a mirror lens and the other
is not, the mirror lens will almost certainly have at
most only half the light transmission efficiency that
the other does. In effect it will f/9 or worse. That
mirror which blocks off a portion the front element is
where the loss occurs.


The "mirror which blocks off a portion the front element " is in a blind spot.



Same aperture and focal length
though, so it has the same f/stop, but with less light
passing through.

Or another very simple experiment will also demonstrate
the fact. Take a single lens (virtually any lens that
you can mount a filter on), and compare it to
itself... with a neutral density filter added. In
either instance the fstop you select will be exactly the
same with or without the filter because the filter does
not change the aperture or the focal length. But it
will change the amount of light going through the lens.

Ya! It blocks some of the light REQUIRING a change in the exposure time!

If you measure the light falling on an object with a meter, and you know what your doing, there is ONE aperture setting and ONE
shutter speed that can be used to get a properly exposed picture.
You will get the same image regardless of the "lens glass" used. The coloration may be different, but the exposure is the same.

John

"David J Taylor" wrote in message
m...

"jaf" wrote in message ...

"David J Taylor" wrote in message
om...

"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

Post some data that proves you would need a different exposure time for 2 otherwise identical lenses.


John

John,

Please look at the reference I gave, and other references to T-stops. That T-stops were required at all proves the need. With
today's optics I don't think you would be talking about large changes - probably less than 1/3 f/stop - except perhaps in the case
of the mirror lens.

This would only apply if you were not metering through the lens.

Cheers,
David

I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Relative to what?

Would you use a different EXPOSURE if you are looking a the T stop scale than if you are looking at the f stop scale on the lens?
T stops are just a different scale of measuring light.
EV steps/stops are another.


John

"jaf" wrote:
"Floyd L. Davidson" wrote in message ...
"jaf" wrote:
"David J Taylor" wrote in message
a.com...

"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

Post some data that proves you would need a different
exposure time for 2 otherwise identical lenses.

Why didn't you bother to read the URL he cited above?
It provides the "data" you request.

Or stop and think about it... for example a pair of
500mm f/8 lenses, have an equal sized aperture.
However, if one of them is a mirror lens and the other
is not, the mirror lens will almost certainly have at
most only half the light transmission efficiency that
the other does. In effect it will f/9 or worse. That
mirror which blocks off a portion the front element is
where the loss occurs.

The "mirror which blocks off a portion the front element " is in a blind spot.

No, the mirro *creates* a blind spot, and restricts the amount
of light that can pass to less than it would be if the mirror
were not there.

Same aperture and focal length
though, so it has the same f/stop, but with less light
passing through.

Or another very simple experiment will also demonstrate
the fact. Take a single lens (virtually any lens that
you can mount a filter on), and compare it to
itself... with a neutral density filter added. In
either instance the fstop you select will be exactly the
same with or without the filter because the filter does
not change the aperture or the focal length. But it
will change the amount of light going through the lens.

Ya! It blocks some of the light REQUIRING a change in the exposure time!

Exactly. Even though the lens is "identical" (one has
an extra element, but otherwise the construction is
exactly the same and both have exactly the same focal
length and exactly the same aperture) one requires a
different exposure time if the f/stop is set the same.

If you measure the light falling on an object with a
meter, and you know what your doing, there is ONE
aperture setting and ONE shutter speed that can be used
to get a properly exposed picture.

In the above examples that is simply not true.

The mirror lense will require more exposure time, and
the lense that has a neutral density element added will
also require more exposure time.

You will get the same image regardless of the "lens
glass" used. The coloration may be different, but the
exposure is the same.

You'll need to rethink that, as it is clearly not the
case. Re-examine the above examples.

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

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

Absolute to what?

Relative to what?

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

--
Any man who is a triangle, has thee right, when in Cartesian Space,
to have angles, which when summed, come to know more, nor no less,
than nine score degrees, should he so wish. [TEMPLE OV THEE LEMUR]

"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?

Relative to what?

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.
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.

John

"Floyd L. Davidson" wrote in message ...
"jaf" wrote:
"Floyd L. Davidson" wrote in message ...
"jaf" wrote:
"David J Taylor" wrote in message
ia.com...

"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

Post some data that proves you would need a different
exposure time for 2 otherwise identical lenses.

Why didn't you bother to read the URL he cited above?
It provides the "data" you request.

Or stop and think about it... for example a pair of
500mm f/8 lenses, have an equal sized aperture.
However, if one of them is a mirror lens and the other
is not, the mirror lens will almost certainly have at
most only half the light transmission efficiency that
the other does. In effect it will f/9 or worse. That
mirror which blocks off a portion the front element is
where the loss occurs.

The "mirror which blocks off a portion the front element " is in a blind spot.

No, the mirro *creates* a blind spot, and restricts the amount
of light that can pass to less than it would be if the mirror
were not there.

Same aperture and focal length
though, so it has the same f/stop, but with less light
passing through.

Or another very simple experiment will also demonstrate
the fact. Take a single lens (virtually any lens that
you can mount a filter on), and compare it to
itself... with a neutral density filter added. In
either instance the fstop you select will be exactly the
same with or without the filter because the filter does
not change the aperture or the focal length. But it
will change the amount of light going through the lens.

Ya! It blocks some of the light REQUIRING a change in the exposure time!

Exactly. Even though the lens is "identical" (one has
an extra element, but otherwise the construction is
exactly the same and both have exactly the same focal
length and exactly the same aperture) one requires a
different exposure time if the f/stop is set the same.

If you measure the light falling on an object with a
meter, and you know what your doing, there is ONE
aperture setting and ONE shutter speed that can be used
to get a properly exposed picture.

In the above examples that is simply not true.

The mirror lense will require more exposure time, and
the lense that has a neutral density element added will
also require more exposure time.

You will get the same image regardless of the "lens
glass" used. The coloration may be different, but the
exposure is the same.

You'll need to rethink that, as it is clearly not the
case. Re-examine the above examples.


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

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

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?

No photoshopping allowed.

Post back with the results.


John