Is this possible?

I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

You mean like a bellows?

"Ockham's Razor" wrote in message
...
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

On Mar 18, 10:10 am, Ockham's Razor wrote:
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?


Spherical aberration at less than infinity focus would mean you'd have
to stop the lens
down substantially, or it simply would not be correctable. However,
if you have a extension tube
set, it can be used to obtain close focus without losing the relative
aperture. In order to really work well,
you'd need an an extender with a floating optics group inside to
correct the aberrations that occur from using the lens at less than
infinity = expensive and you'd need a solid mechanism to allow for
expansion and contraction of the extender. With plastic lenses
costing upwards of $1000 today, I would not like to see the kind of
price such a device would be. I once saw a device designed to correct
the chromatic aberration in older superlong lenses. It cost about
$800 and needed dead-on precise alignment.

In article ,
"Ed Ruf (REPLY to E-MAIL IN SIG!)" wrote:

On Sun, 18 Mar 2007 08:10:10 -0700, in rec.photo.digital Ockham's Razor
wrote:

I have been considering a Nikon extender.

First, what do you explicitly mean by an "extender"? Do you mean a lens
element added at the objective portion of the lens or in between the lens
and the camera?

Yes, thats what I mean. A teleconvertor.

Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

Then you might consider the laws of physical optics may present some
limitations even at the level of a thought experiment.

That is what I know exactly nothing about.

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

In article ,
"Rudy Benner" wrote:

You mean like a bellows?

I did not use correct terminology. I was thinking of something like a
tele-convertor between the lens and the camera.

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

"Ockham's Razor" wrote in message
...
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

--
"When fascism comes to America, it will be wrapped in the flag and
carrying a cross."
Sinclair Lewis

Not possible on the negative side as the image circle would not let you get
a full frame from the lens.
John

Ockham's Razor wrote:
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

No, you are not nuts and such things are done, although not
both magnification and focal reducer. The teleconverter
between lens and camera is a negative lens. A focal reducer
is a positive lens. Probably the original telextender was
called (and still is) the Barlow lens used on telescopes.
By adjusting the distance inside the lens focus and the distance
to the camera or eyepiece, you can change the magnification.
Originally the Barlow was a single negative lens, then the
Goodwin Barlow, an achromatic negative lens was introduced
(probably around the 1950s). I have use a multi-element
"Barlow" using an old Minolta 2x TC as a variable Barlow for
several decades on my telescope and have taken a fair number
of pictures with it (none are currently on my web site).
Focal reducers are also used on telescopes.

The challenge for use on any lens is the need for variable
distance inside focus and variable distance to the camera
sensor plane. For a fixed focal length (Barlow is negative focal
length), that range is simply too much for most lens designs
so it is not practical. The other reason is that for faster
optics (faster than about f/8), the aberrations are not well
corrected, especially over a large area like a 35 mm film plane.
Thus fixed magnification TC provide the best results.
You generally don't see focal reducers for lenses because of the shift
in focal plane closer to the lens. Generally, reducing focal length
also has other side effects, like the lens is not able to field the full
field of view that a shorter focal length would normally give.

Roger

"Ockham's Razor" wrote in message
...
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

In a word, yes. Not nuts exactly, but perhaps ignorant of the optics
involved. First, you need to consider that converters are optically less
than perfect. You not only end up magnifying existing optical aberrations in
the prime lens, but the teleconverter introduces aberrations of its own. The
best teleconverters are quite pricey, and the very best ones are
dedicated--designed for specific lenses. It would certainly be possible to
design a "zoom" converter, but very difficult to design one to perform well
optically, especially considering that it would be used with a variety of
lenses.

Second, it is infinitely more difficult to construct a "minus" converter
than a "plus" converter. The only one I am aware of was designed by Nikon to
allow their wide lenses to be mounted on professional videocameras without
the effective 4x magnification that is a result of the smaller image sensor
of the video camera. This converter is the size of a 200mm lens, and
consists of very complex optics that focuses the image of the prime lens on
the front lens of the converter, and then "ports" the image (like a
collimator) down the tube, reducing it in size until it is projected on the
image sensor at 1/4 its original size. The problem here for your idea is
that the image circle is also reduced to 1/4 of its original size. There is
no way to change the image circle of the original lens, so your "minus"
converter would suffer from serious vignetting.

And since the aperture is a function of both the focal length and the
diameter of the lens, there is no way to vary the focal length with a
converter without changing the effective aperture. The only possibility
would be a mechanical coupling that would vary the diameter of the iris as a
function of the zoom ratio of the converter, but that would have serious
limits at both the wide and small end.

That's just for starters...

Toby

On Mar 19, 6:06 am, "Roger N. Clark (change username to rnclark)"
The challenge for use on any lens is the need for variable
distance inside focus and variable distance to the camera
sensor plane. For a fixed focal length (Barlow is negative focal
length), that range is simply too much for most lens designs
so it is not practical. The other reason is that for faster
optics (faster than about f/8), the aberrations are not well
corrected, especially over a large area like a 35 mm film plane.

Roger

Yeah..... but if Stanley Kubrick was still around, I reckon he could
talk Zeiss or Angenieux into trying to build one... (O;
http://www.visual-memory.co.uk/sk/ac/len/page1.htm

I'd like to have a little play with that 'Cine-Pro T9 24-480mm zoom'...

Roger N. Clark (change username to rnclark) wrote:
Ockham's Razor wrote:
I have been considering a Nikon extender. Got to thinking, would it be
possible to build something like a continuously variable "extender" that
would go from say minus 3x to plus 3x. And, include a mechanism that
would not change the aperature of the attached lens.

I have no idea what would be involved and it might be monstrously large
and heavy.

But, such a thing on a 70-200 2.8 would be an almost universal lens.

Am I nuts?

No, you are not nuts and such things are done, although not
both magnification and focal reducer. The teleconverter
between lens and camera is a negative lens. A focal reducer
is a positive lens. Probably the original telextender was
called (and still is) the Barlow lens used on telescopes.
By adjusting the distance inside the lens focus and the distance
to the camera or eyepiece, you can change the magnification.
Originally the Barlow was a single negative lens, then the
Goodwin Barlow, an achromatic negative lens was introduced
(probably around the 1950s). I have use a multi-element
"Barlow" using an old Minolta 2x TC as a variable Barlow for
several decades on my telescope and have taken a fair number
of pictures with it (none are currently on my web site).
Focal reducers are also used on telescopes.

The challenge for use on any lens is the need for variable
distance inside focus and variable distance to the camera
sensor plane. For a fixed focal length (Barlow is negative focal
length), that range is simply too much for most lens designs
so it is not practical. The other reason is that for faster
optics (faster than about f/8), the aberrations are not well
corrected, especially over a large area like a 35 mm film plane.
Thus fixed magnification TC provide the best results.
You generally don't see focal reducers for lenses because of the shift
in focal plane closer to the lens. Generally, reducing focal length
also has other side effects, like the lens is not able to field the full
field of view that a shorter focal length would normally give.

Roger

Thanks for all that.

One question. If I understand correctly, focal reducer also increase
the speed of the lens/telescope, is that right? So if I can get
resolution of about 1arc sec with my rubinar 1000mm f10 lens, while in
large observatories they use telescopes with up to 80x that aperture
(8m) and practicaly achievable resolution for earth based telescope is
at best (if not impossible) 0.1arc sec, do they increase light
gathering using focal reducers or what do they do?

IIRC most of big telescopes are not diffraction limited or at least
are not used at diffraction limit. So the large aperture is used for
light gathering mostly, not magnification. Did I miss something?