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Apertu geometric vs. real



 
 
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  #1  
Old January 2nd 11, 12:39 AM posted to rec.photo.darkroom
Anton Shepelev[_3_]
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Posts: 4
Default Apertu geometric vs. real

Hello all,

As I know, and I hope am correct about it, a lens's
aperture is the ratio of the front pupils' diameter
to the focus length. And it is this number that is
indicated as the maximum aperture and is also marked
on the aperture ring, while the actual illuminance
produced by a lens on its focal plane depends on
many factors like (multi)coating, the number of
optical elements e.t.c.

Taking this into accout, the reading of an ideal
external light-meter may differ from that of an
ideal internal (say TTL) one. My question is whether
this correction coefficient (or difference, if
expressed in logarithmic terms) has a name for it in
the photographic slang.

Thanks in advance,
Anton

P.S.: Asking this because I recently got a tele-
scopic lens with two mirrors in it, and the
declared apperture of f/8 seems to be about
one step too light...

  #2  
Old January 2nd 11, 07:13 AM posted to rec.photo.darkroom
Richard Knoppow
external usenet poster
 
Posts: 751
Default Apertu geometric vs. real


"Anton Shepelev" wrote in
message ...
Hello all,

As I know, and I hope am correct about it, a lens's
aperture is the ratio of the front pupils' diameter
to the focus length. And it is this number that is
indicated as the maximum aperture and is also marked
on the aperture ring, while the actual illuminance
produced by a lens on its focal plane depends on
many factors like (multi)coating, the number of
optical elements e.t.c.

Taking this into accout, the reading of an ideal
external light-meter may differ from that of an
ideal internal (say TTL) one. My question is whether
this correction coefficient (or difference, if
expressed in logarithmic terms) has a name for it in
the photographic slang.

Thanks in advance,
Anton

P.S.: Asking this because I recently got a tele-
scopic lens with two mirrors in it, and the
declared apperture of f/8 seems to be about
one step too light...


You are correct, the definition of f/stop is the ratio
of the diameter of the entrance pupil to the focal length.
The entrance pupil is the _image_ of the stop as seen from
the front of the lens. The actual transmission through the
lens depends on various factors that can attenuate the
light, you've named them pretty well. In the case of a
reflecting objective there will also be some loss at the
mirror or mirrors although that is generally low. It is
unusual for the transmission to be full stop less than the
f/stop but possible.
Its easy to measure the diameter of the entrance pupil
by autocollimating the lens to find the exact infinity focus
and placing a point source light there, then place a
translucent screen over the lens. The diameter of the circle
of light projected on it is the diameter of the entrance
pupil. It can be smaller or larger than the physical
aperture depending on whether the lenses in front of it are
magnifying or diminishing. I've posted instructions for
autocollimating to this list several times in the past, its
not difficult especially on a view camera which can be used
as an elementary optical bench. If you can't find them write
and I will repeat it.


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



  #3  
Old January 2nd 11, 04:55 PM posted to rec.photo.darkroom
No spam please
external usenet poster
 
Posts: 67
Default Apertu geometric vs. real

"Anton Shepelev" wrote in message
...
Hello all,

As I know, and I hope am correct about it, a lens's
aperture is the ratio of the front pupils' diameter
to the focus length. And it is this number that is
indicated as the maximum aperture and is also marked
on the aperture ring, while the actual illuminance
produced by a lens on its focal plane depends on
many factors like (multi)coating, the number of
optical elements e.t.c.

Taking this into accout, the reading of an ideal
external light-meter may differ from that of an
ideal internal (say TTL) one. My question is whether
this correction coefficient (or difference, if
expressed in logarithmic terms) has a name for it in
the photographic slang.

Thanks in advance,
Anton

P.S.: Asking this because I recently got a tele-
scopic lens with two mirrors in it, and the
declared apperture of f/8 seems to be about
one step too light...


Hello Anton.
I've used several makes of mirror lens. I agree that their "f8" seems
optimistic. I reckon, judging from the shutter speeds I use, that the lenses
are no better than f11.

Best wishes,
Rog.



--- news://freenews.netfront.net/ - complaints: ---
  #4  
Old January 3rd 11, 05:36 PM posted to rec.photo.darkroom
Nicholas O. Lindan
external usenet poster
 
Posts: 1,227
Default Apertu geometric vs. real

"Anton Shepelev" wrote

I recently got a tele-
scopic lens with two mirrors in it, and the
declared apperture of f/8 seems to be about
one step too light...


To be expected.

There is nothing subtle about this - you don't need
to worry about the calibration of TTL Vs. handheld
meters or the effects of flare.


A mirror lens has part of the front entrance pupil
blocked by the mirror that is glued to the back of
the front element. Then part of the rear mirror is
missing because of the hole cut in the center for the
exit pupil. To arrive at the largest f number, the
manufacture simply divides the diameter of the front
lens by the focal length and doesn't correct for either
of these light stops.

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/da-main.htm
n o lindan at ix dot netcom dot com


  #5  
Old January 3rd 11, 10:04 PM posted to rec.photo.darkroom
Howard Lester[_3_]
external usenet poster
 
Posts: 24
Default Apertu geometric vs. real

"Nicholas O. Lindan" wrote

A mirror lens has part of the front entrance pupil
blocked by the mirror that is glued to the back of
the front element. Then part of the rear mirror is
missing because of the hole cut in the center for the
exit pupil. To arrive at the largest f number, the
manufacture simply divides the diameter of the front
lens by the focal length and doesn't correct for either
of these light stops.


The hole in the rear (i.e., primary) mirror is irrelevant because it's
already blocked by the secondary mirror, which is rather large. (If the hole
through the primary is larger than the secondary, then the manufacturer's
designer's got his/her head up the you know what.)

  #6  
Old January 3rd 11, 11:24 PM posted to rec.photo.darkroom
Howard Lester[_3_]
external usenet poster
 
Posts: 24
Default Apertu geometric vs. real (correction)

The secondary *could* be smaller than the primary hole (plus the primary's
surrounding baffle!) that is sized according to the required diameter of the
camera's lens mount....

The hole in the rear (i.e., primary) mirror is irrelevant because it's
already blocked by the secondary mirror, which is rather large. (If the
hole through the primary is larger than the secondary, then the
manufacturer's designer's got his/her head up the you know what.)


  #7  
Old January 4th 11, 07:03 AM posted to rec.photo.darkroom
Nicholas O. Lindan
external usenet poster
 
Posts: 1,227
Default Apertu geometric vs. real

"Howard Lester" wrote
"Nicholas O. Lindan" wrote
loss of light due to hole in primary mirror

The hole in the rear (i.e., primary) mirror is irrelevant because it's
already blocked by the secondary mirror


Hmmm...yes, I should know that. If the hole in the primary was visible from
the front of the lens it would be a bit of a bummer with 'non image forming
light' getting to the film.

--
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/da-main.htm
n o lindan at ix dot netcom dot com


  #8  
Old January 5th 11, 12:44 AM posted to rec.photo.darkroom
Anton Shepelev[_3_]
external usenet poster
 
Posts: 4
Default Apertu geometric vs. real

Thanks to everybody for their replies.

Richard Knoppow:

I've posted instructions for autocollimating to
this list several times in the past, its not dif-
ficult especially on a view camera which can be
used as an elementary optical bench.


Do I understand correctly that you refer to a device
that produces a parallel beam of light? If so, this
is really very interesting for camera/lens measur-
ing/adjusting purposes. I'll try find your post(s)
in the archieve and will get back to you if I fail.

I currently don't do any optical measurements except
for calibraing viewfinders in SLR cameras to get
correct focusing. For that I use a darkened glass in
the film plane, with some scratches in it. A bright
light is placed behind the glass and the lens' focus
is adjusted to get the sharpest possible image of
the scrathes on the opposite wall. This is the
"true" focus.

Anton

  #9  
Old January 6th 11, 09:22 AM posted to rec.photo.darkroom
Richard Knoppow
external usenet poster
 
Posts: 751
Default Apertu geometric vs. real


"Anton Shepelev" wrote in
message ...
Thanks to everybody for their replies.

Richard Knoppow:

I've posted instructions for autocollimating to
this list several times in the past, its not dif-
ficult especially on a view camera which can be
used as an elementary optical bench.


Do I understand correctly that you refer to a device
that produces a parallel beam of light? If so, this
is really very interesting for camera/lens measur-
ing/adjusting purposes. I'll try find your post(s)
in the archieve and will get back to you if I fail.

I currently don't do any optical measurements except
for calibraing viewfinders in SLR cameras to get
correct focusing. For that I use a darkened glass in
the film plane, with some scratches in it. A bright
light is placed behind the glass and the lens' focus
is adjusted to get the sharpest possible image of
the scrathes on the opposite wall. This is the
"true" focus.

Anton

Its pretty easy to set a lens for its exact infinity
focus by autocollimating. This is done with the aid of a
flat mirror that fits over the front of the lens. A
first-surface mirror is ideal but a flat shaving mirror will
work OK. The idea is that the light passes through the lens
twice so that it comes to focus at the source. Since the
path from source to image is folded by the mirror the lens
is at its infinity focus and the light emerging from the
front is collimated. The method is simple especially if done
using a view camera as an optical bench. The source should
be a small lamp. I penlight flashlight will do. For rough
measurements such as finding the diameter of the entrance
pupil of a view camera lens, its enough to put the end of
the light against the ground glass, near, but exactly at,
the center. The mirror is placed over the lens. The distance
is not important but it is important that the mirror not be
tilted. If its held against the rim of the lens cell it will
be quite perpendicular to the optical axis. The image of the
source is brought to focus, the lens is now exactly at
infinity focus and several measurements can be made on it.
If something approaching a point source is now placed
at the center of the axis at the focal plane the light will
be projected from the front of the lens as a parallel beam,
that is, a collimated beam. This will have constant diameter
regardless of distance (not counting scattering by the air
for you purists). A translucent screen placed over the lens
will have the image of the entrance pupil focused on it. The
diameter of this circle of light is the diameter of the
entrance pupil, which determines the f/stop of the lens.
This is a simple method of calibrating irises.
Since you have the infinity focus point you can also
determine the focal length of the lens. This is done by
adjusting the lens to produce an exact equal size image of
some object. At exatly 1:1 the focal length can be
calculated from two things: One is the distance the lens
moves from its infinity focus. That is by definition one
focal length. It can also be determined by measuring the
distance from the image to the object, that will be exactly
four times the focal length. These two can be used to check
each other.
Now, knowing the exact focal length and the diameter of
the entrance pupil makes is possible to determine the
geometric f/stop.
Also, when the lens is set to exactly infinity focus
the rear principle point will be exactly one focal length
from the image measured back toward the lens. The front
principle point is found by turning the lens around,
refocusing, and again measuring. You now know the exact
focal length, the true f/stop, the location of both
principle points.
Now, its easy to find the location of the stops or
pupils. The pupils are the images of the stop as seen from
the outside of the lens. The front pupil is called the
entrance pupil and teh rear is called the exit pupil. The
pupils are the _images_ of the stop as seen from the outside
of the lens. They can be displaced in space from the
physical stop depending on the power of the lenses beween
the stop and the outside in just the same way as the size of
the pupils is chaned by the lens power.
To find the location of the pupils one needs a camera
capable of being focused at a close distance. The procedure
is simple: First set the camera up close to the lens and
facing it. This is done so that a small change in distance
can be accurately determined. Focus the camera on a
convenient reference point, for instance on the rim of the
lens mount. Record the location of the camera. Then _move
the entire camera_ until until the iris is in focus. The
camera may move toward the lens or away from it. Now, record
the location of the camera and measure the distance and
direction its has moved. Now lay this distance against the
lens from the reference point you used for the first
measurement. That is the location of the pupil. Pupils can
lie inside or outside the lens as can principle points. The
entrance pupil is important both because it determines the
light gathering power of the lens and because it is the
correct point of rotation for panoramic pictures to prevent
relative movement of forground and background parts of the
image. It is often thought that the front principle point is
the proper pivot point but it is not, rather the front
entrance pupil is the correct point. The entrance pupil is
also the correct point to use when calculating the
magnification of the lens.
These simple measurements allow you to amost completely
characterize a lens. While precise measurements are
difficult without a proper optical bench very close
measurements can be made.
Its interesting to experiment with the location of the
principle points of various lenses becuse they may not be
where you would think. For instance the single cells of
meniscus convertible lenses like the Dagor or Protar have
the principle points displaced so that the lens is somewhat
retrofocus when in the usual position with the concave side
facing the object and somewhat telephoto when placed with
the convex side facing the object. While the best correction
is gotten when the concave side facing out with the stop in
front the shortening of the required bellows draw will often
allow a long single objective to work on a camera with
limited bellows capacity when reversed from ideal practice.
Whew, that's a lot of typing...


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



  #10  
Old January 6th 11, 09:29 AM posted to rec.photo.darkroom
Richard Knoppow
external usenet poster
 
Posts: 751
Default Apertu geometric vs. real


"Bob AZ" wrote in message
...
On Jan 1, 4:39 pm, Anton Shepelev
wrote:
Hello all,

Anton et al

FWIW

From the book "The Focal Encyclopedia of Photography"

"the focal point of the lens is the point at which the lens
can be
revolved such that the focus plane remains stationery. The
focus
point is not always within the body of the glass/lens. Also
the
revolvement to try this is limited by the field of view of
the lens.

Bob AZ

Optical benches are equipped with what is called a focal
slide. This allows small fore and aft movement of the lens
while wiggling it. The image of a very distant object, such
as the target in a collimator, is viewed and the slide
adjusted until there is no motion of the image as the lens
is wiggled. The axis or rotation is now exactly at the
principle point. I just posted another way of finding the
principle points but this is the academically correct one. A
view camera is actually a simple optical bench but without
the calibrations for all movements and without a microscope
(or telescope depending the distance of the virtual image)
to examine the aerial image. A view camera can give you very
useful data on longer lenses. For short focal length lenses
one really needs the precision of a true optical bench.


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



 




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