Apertures and focal length

Maybe this is a silly question, but I'll ask anyways.

Suppose I have a shutter from a 210mm Trashagon with accurate
aperture markings. If I instead screw in the elements of a 210mm
Expensivegon will the same markings apply? In other words, is
the physical aperture opening the same for all lenses of the same
focal length? I _think_ it should be...

TIA!

Steve

Suppose I have a shutter from a 210mm Trashagon with accurate
aperture markings. If I instead screw in the elements of a 210mm
Expensivegon will the same markings apply? In other words, is
the physical aperture opening the same for all lenses of the same
focal length? I _think_ it should be...

--------------------------------------------------------------------------
------------------------------------------------
Not so. If you had two one-element lenses made out of the same glass and of the
same focal length, it would be true.

But modern camera lenses are composed of many different elements of different
glass types, each with its own light-transmission peculiarities.

The physical size of the iris at any particular f-stop value would likely be
different from one lens type to another, even if their focal lengths were
identical.

I'm sure there are many other considerations apart from glass type that enter
into the equation. I'm no optical expert but I know from experience that the
apertures will vary.




Bob G

The physical size of the iris at any particular f-stop value would likely be
different from one lens type to another, even if their focal lengths were
identical.

The above statement is NOT true of f/stops.
The old T/stop was an effort to factor in the light transmission of a given
lens design, but the f/stop is defined as the simple ratio of the 'effective
aperture' size:focal length. The 'effective aperture' measures the diameter of
the beam of light entering the aperture and must be measured. But the
effective aperture has nothing to do with the light transmission of the lens.
The variability of light transmission in different lenses was minimized via
coatings, but critical exposure differences are not eliminated entirely with
the f/stop system.


--Wilt

"Stephan Goldstein" wrote in message
...
Maybe this is a silly question, but I'll ask anyways.

Suppose I have a shutter from a 210mm Trashagon with
accurate
aperture markings. If I instead screw in the elements of
a 210mm
Expensivegon will the same markings apply? In other
words, is
the physical aperture opening the same for all lenses of
the same
focal length? I _think_ it should be...

TIA!

Steve

The _effective_ size of the stop is affected by the
magnification of the lenses in front of it. The calibrations
are for this effective size, not the physical size of the
hole. The image of the physical stop as seen from the front
of the lens is called the Entrance Pupil. In general, its
size and position will be different from the physical size
and position of the stop. An example is the speed of a
single element of a convertible lens like a Dagor. When the
cell is located at the rear of the stop (its optimum
position) the speed is about f/13 and the entrance pupil is
at the stop. The _effective_ size of the stop is its
physical size and position its physical position because
there is no glass in front of it to either magnify or reduce
it. When the half-Dagor is placed in front of the stop the
speed increases to about f/12, not a big difference, but
different. The reason is that the stop is now magnified by
the lens. The location of the stop will also change. Since
the half lens is positive in power it will make the stop
seem closer to the front than the physical position.
The effect on the stop will be opposite where the front
cell is negative in power, as it is in a Tessar type lens.
There, the entrance pupil will be smaller than the stop and
seem to be behind it.
The size of the entrance pupil can be measured by placing
a small light source exactly at the focal plane of the lens.
The focal plane is where it focuses light from an infinit
source. The exact infinity focus position can be found by
autocollimating. To do this you need a card with a small
hole in it and a small light, like a penlight. You also need
a flat mirror to place over the lens. You can do this in a
view camera. First, place the mirror over the lens, a
shaving mirror will do but make sure the flat side is
against the lens. Place the card behind the lens and focus
the reflected image of the lighted hole on the card. You
will have to adjust it so that the image does not fall
exactly on top of the light of course. The lens is now
focused exactly at infinty. Now, place a translucent screen
over the lens, thin paper will do. The circle of light
projected onto the paper is the outline of the entrance
pupil. If you measure its diameter you will have the
diameter of the _effective_ stop. If the front cell doesn't
have a lot of power this diameter will not be much different
than the physical size of the stop, but, for some lenses it
can be quite different.
The above method works well when a new stop scale must be
made for a lens. Its not even necessary to remove the cells
for this.
If you want to measure the location of the entrance pupil
use a camera that can focus close, the closer the better.
First, focus the camera on a reference surface, the rim of
the cell is a good place. Now, move the whole camera until
the stop, as seen through the lens, is in focus. The
distance the camera moved is the distance from the rim of
the cell (or whatever reference surface you used) to the
entrance pupil. The location of the entrance pupil is
sometimes useful, for instance, it is the correct point to
rotate a camera to take panoramic pictures.
Do you still think the question is silly? :-)

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

Not so. If you had two one-element lenses made out of the same glass and
of the
same focal length, it would be true.

But modern camera lenses are composed of many different elements of
different
glass types, each with its own light-transmission peculiarities.

The physical size of the iris at any particular f-stop value would likely
be
different from one lens type to another, even if their focal lengths were
identical.

I don't think this is correct. Lens apertures represent the diameter of the
lens opening expressed as a fraction of the focal length. So I would think
that at any given f stop the physical aperture size in two lenses of the
same focal length should be the same regardless of the number of elements or
the type of glass used. I think it's simply a mathematical calculation made
by measuring the diameter of the aperture and focal length of the lens and
then expressing the result as a fraction. But I could be wrong, I'm not an
optical expert and some of the people who participate here are so they can
correct me if necessary.

In any event, lenses marked as a given focal length often, maybe always,
aren't acutally that exact focal length. One lens sold as a 150mm lens might
actually by 148mm and another might actually be 153mm, and in that case the
physical size of the apertures at the same f stop presumably would be
slightly different.

"Bob G" wrote in message
...
Suppose I have a shutter from a 210mm Trashagon with accurate
aperture markings. If I instead screw in the elements of a 210mm
Expensivegon will the same markings apply? In other words, is
the physical aperture opening the same for all lenses of the same
focal length? I _think_ it should be...

--------------------------------------------------------------------------
------------------------------------------------
Not so. If you had two one-element lenses made out of the same glass and
of the
same focal length, it would be true.

But modern camera lenses are composed of many different elements of
different
glass types, each with its own light-transmission peculiarities.

The physical size of the iris at any particular f-stop value would likely
be
different from one lens type to another, even if their focal lengths were
identical.

I'm sure there are many other considerations apart from glass type that
enter
into the equation. I'm no optical expert but I know from experience that
the
apertures will vary.




Bob G

Perhaps I misunderstood the question but I thought he was simply asking
whether at F2.8, for example, the physical size of the aperture in lenses of
the same focal length will be the same. I thought yes, you seem to be saying
no but I'm not sure. In other words, have I been corrected? : - )

"Richard Knoppow" wrote in message
link.net...

"Stephan Goldstein" wrote in message
...
Maybe this is a silly question, but I'll ask anyways.

Suppose I have a shutter from a 210mm Trashagon with
accurate
aperture markings. If I instead screw in the elements of
a 210mm
Expensivegon will the same markings apply? In other
words, is
the physical aperture opening the same for all lenses of
the same
focal length? I _think_ it should be...

TIA!

Steve

The _effective_ size of the stop is affected by the
magnification of the lenses in front of it. The calibrations
are for this effective size, not the physical size of the
hole. The image of the physical stop as seen from the front
of the lens is called the Entrance Pupil. In general, its
size and position will be different from the physical size
and position of the stop. An example is the speed of a
single element of a convertible lens like a Dagor. When the
cell is located at the rear of the stop (its optimum
position) the speed is about f/13 and the entrance pupil is
at the stop. The _effective_ size of the stop is its
physical size and position its physical position because
there is no glass in front of it to either magnify or reduce
it. When the half-Dagor is placed in front of the stop the
speed increases to about f/12, not a big difference, but
different. The reason is that the stop is now magnified by
the lens. The location of the stop will also change. Since
the half lens is positive in power it will make the stop
seem closer to the front than the physical position.
The effect on the stop will be opposite where the front
cell is negative in power, as it is in a Tessar type lens.
There, the entrance pupil will be smaller than the stop and
seem to be behind it.
The size of the entrance pupil can be measured by placing
a small light source exactly at the focal plane of the lens.
The focal plane is where it focuses light from an infinit
source. The exact infinity focus position can be found by
autocollimating. To do this you need a card with a small
hole in it and a small light, like a penlight. You also need
a flat mirror to place over the lens. You can do this in a
view camera. First, place the mirror over the lens, a
shaving mirror will do but make sure the flat side is
against the lens. Place the card behind the lens and focus
the reflected image of the lighted hole on the card. You
will have to adjust it so that the image does not fall
exactly on top of the light of course. The lens is now
focused exactly at infinty. Now, place a translucent screen
over the lens, thin paper will do. The circle of light
projected onto the paper is the outline of the entrance
pupil. If you measure its diameter you will have the
diameter of the _effective_ stop. If the front cell doesn't
have a lot of power this diameter will not be much different
than the physical size of the stop, but, for some lenses it
can be quite different.
The above method works well when a new stop scale must be
made for a lens. Its not even necessary to remove the cells
for this.
If you want to measure the location of the entrance pupil
use a camera that can focus close, the closer the better.
First, focus the camera on a reference surface, the rim of
the cell is a good place. Now, move the whole camera until
the stop, as seen through the lens, is in focus. The
distance the camera moved is the distance from the rim of
the cell (or whatever reference surface you used) to the
entrance pupil. The location of the entrance pupil is
sometimes useful, for instance, it is the correct point to
rotate a camera to take panoramic pictures.
Do you still think the question is silly? :-)

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

Reciprocity Failure wrote:
Perhaps I misunderstood the question but I thought he was simply asking
whether at F2.8, for example, the physical size of the aperture in lenses of
the same focal length will be the same. I thought yes, you seem to be saying
no but I'm not sure. In other words, have I been corrected? : - )

Read Richard Knoppow's explanation again. As he remarks, it is the
effective aperture which is used in calculating the f-stop, not the
physical size of the opening in the diaphragm.


"Richard Knoppow" wrote in message
link.net...

"Stephan Goldstein" wrote in message
...

Maybe this is a silly question, but I'll ask anyways.

Suppose I have a shutter from a 210mm Trashagon with

accurate

aperture markings. If I instead screw in the elements of

a 210mm

Expensivegon will the same markings apply? In other

words, is

the physical aperture opening the same for all lenses of

the same

focal length? I _think_ it should be...

TIA!

Steve

The _effective_ size of the stop is affected by the
magnification of the lenses in front of it. The calibrations
are for this effective size, not the physical size of the
hole. The image of the physical stop as seen from the front
of the lens is called the Entrance Pupil. In general, its
size and position will be different from the physical size
and position of the stop. An example is the speed of a
single element of a convertible lens like a Dagor. When the
cell is located at the rear of the stop (its optimum
position) the speed is about f/13 and the entrance pupil is
at the stop. The _effective_ size of the stop is its
physical size and position its physical position because
there is no glass in front of it to either magnify or reduce
it. When the half-Dagor is placed in front of the stop the
speed increases to about f/12, not a big difference, but
different. The reason is that the stop is now magnified by
the lens. The location of the stop will also change. Since
the half lens is positive in power it will make the stop
seem closer to the front than the physical position.
The effect on the stop will be opposite where the front
cell is negative in power, as it is in a Tessar type lens.
There, the entrance pupil will be smaller than the stop and
seem to be behind it.
The size of the entrance pupil can be measured by placing
a small light source exactly at the focal plane of the lens.
The focal plane is where it focuses light from an infinit
source. The exact infinity focus position can be found by
autocollimating. To do this you need a card with a small
hole in it and a small light, like a penlight. You also need
a flat mirror to place over the lens. You can do this in a
view camera. First, place the mirror over the lens, a
shaving mirror will do but make sure the flat side is
against the lens. Place the card behind the lens and focus
the reflected image of the lighted hole on the card. You
will have to adjust it so that the image does not fall
exactly on top of the light of course. The lens is now
focused exactly at infinty. Now, place a translucent screen
over the lens, thin paper will do. The circle of light
projected onto the paper is the outline of the entrance
pupil. If you measure its diameter you will have the
diameter of the _effective_ stop. If the front cell doesn't
have a lot of power this diameter will not be much different
than the physical size of the stop, but, for some lenses it
can be quite different.
The above method works well when a new stop scale must be
made for a lens. Its not even necessary to remove the cells
for this.
If you want to measure the location of the entrance pupil
use a camera that can focus close, the closer the better.
First, focus the camera on a reference surface, the rim of
the cell is a good place. Now, move the whole camera until
the stop, as seen through the lens, is in focus. The
distance the camera moved is the distance from the rim of
the cell (or whatever reference surface you used) to the
entrance pupil. The location of the entrance pupil is
sometimes useful, for instance, it is the correct point to
rotate a camera to take panoramic pictures.
Do you still think the question is silly? :-)

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

Thanks, it's nice to learn something new. I always thought it was the
physical aperture, perhaps because of Ansel Adams' saying in "The Camera"
that "the lens aperture is simply the diameter of the lens opening expresed
as a fraction of its focal length." I guess he was wrong or I misunderstood
what he was saying or he was oversimplifying or something else. But reading
Richard's explanation again led me to Stroebel's explanation of the same
thing and between the two I think I get it.

"Leonard Evens" wrote in message
...
Reciprocity Failure wrote:
Perhaps I misunderstood the question but I thought he was simply asking
whether at F2.8, for example, the physical size of the aperture in
lenses of
the same focal length will be the same. I thought yes, you seem to be
saying
no but I'm not sure. In other words, have I been corrected? : - )

Read Richard Knoppow's explanation again. As he remarks, it is the
effective aperture which is used in calculating the f-stop, not the
physical size of the opening in the diaphragm.


"Richard Knoppow" wrote in message
link.net...

"Stephan Goldstein" wrote in message
...

Maybe this is a silly question, but I'll ask anyways.

Suppose I have a shutter from a 210mm Trashagon with

accurate

aperture markings. If I instead screw in the elements of

a 210mm

Expensivegon will the same markings apply? In other

words, is

the physical aperture opening the same for all lenses of

the same

focal length? I _think_ it should be...

TIA!

Steve

The _effective_ size of the stop is affected by the
magnification of the lenses in front of it. The calibrations
are for this effective size, not the physical size of the
hole. The image of the physical stop as seen from the front
of the lens is called the Entrance Pupil. In general, its
size and position will be different from the physical size
and position of the stop. An example is the speed of a
single element of a convertible lens like a Dagor. When the
cell is located at the rear of the stop (its optimum
position) the speed is about f/13 and the entrance pupil is
at the stop. The _effective_ size of the stop is its
physical size and position its physical position because
there is no glass in front of it to either magnify or reduce
it. When the half-Dagor is placed in front of the stop the
speed increases to about f/12, not a big difference, but
different. The reason is that the stop is now magnified by
the lens. The location of the stop will also change. Since
the half lens is positive in power it will make the stop
seem closer to the front than the physical position.
The effect on the stop will be opposite where the front
cell is negative in power, as it is in a Tessar type lens.
There, the entrance pupil will be smaller than the stop and
seem to be behind it.
The size of the entrance pupil can be measured by placing
a small light source exactly at the focal plane of the lens.
The focal plane is where it focuses light from an infinit
source. The exact infinity focus position can be found by
autocollimating. To do this you need a card with a small
hole in it and a small light, like a penlight. You also need
a flat mirror to place over the lens. You can do this in a
view camera. First, place the mirror over the lens, a
shaving mirror will do but make sure the flat side is
against the lens. Place the card behind the lens and focus
the reflected image of the lighted hole on the card. You
will have to adjust it so that the image does not fall
exactly on top of the light of course. The lens is now
focused exactly at infinty. Now, place a translucent screen
over the lens, thin paper will do. The circle of light
projected onto the paper is the outline of the entrance
pupil. If you measure its diameter you will have the
diameter of the _effective_ stop. If the front cell doesn't
have a lot of power this diameter will not be much different
than the physical size of the stop, but, for some lenses it
can be quite different.
The above method works well when a new stop scale must be
made for a lens. Its not even necessary to remove the cells
for this.
If you want to measure the location of the entrance pupil
use a camera that can focus close, the closer the better.
First, focus the camera on a reference surface, the rim of
the cell is a good place. Now, move the whole camera until
the stop, as seen through the lens, is in focus. The
distance the camera moved is the distance from the rim of
the cell (or whatever reference surface you used) to the
entrance pupil. The location of the entrance pupil is
sometimes useful, for instance, it is the correct point to
rotate a camera to take panoramic pictures.
Do you still think the question is silly? :-)

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

In article .net, "Richard Knoppow" wrote:
A substantial tome in answer to what I thought was a
simple question (snipped for brevity).

Do you still think the question is silly? :-)


Indeed not! Thank you, Richard, for your clear and careful explanation.
I understand now - the physical aperture corresponding to a given
f-stop for a given lens is a function of the actual entrance pupil as
magnified or reduced by the power of the front elements. So I can't
just willy-nilly swap element sets between shutters and expect the
aperture markings to be correct.

A pity, but with your instructions I can make my own scales.
They won't be as nice as those from the factory or from SK Grimes,
but they'll be less expensive.

Thanks again,

Steve

"Stephan Goldstein" wrote

I understand now - the physical aperture corresponding to a given
f-stop for a given lens is a function of the actual entrance pupil as
magnified or reduced by the power of the front elements.

Yup, though now it can be told: for 'government work' the old
quick and dirty method works reasonably well:

Hold the lens at arm's length and measure the entrance pupil with
a ruler held against the front rim of the lens.

Experimental results:

Lens Measured Calculated Marked
150mm 'Gauss' 26mm f5.8 f5.6
16mm fisheye 5.5mm f2.9 f2.8
200mm tele 45mm f4.4 f4.0

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.