Full-frame sensors can't do wide angle - NOT! - Page 2

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"Dave Martindale" <davem@cs.ubc.ca> wrote:
> "David J. Littleboy" <davidjl@gol.com> writes:
>
>>Speaking of reverse telephotos, I looked up "telecentric" as best I could,
>>and got something about lenses for industrial inspection/image processing
>>whose magnification was constant with subject distance (obviously not the
>>sort of lens I'm used to).
>
> Here's my understanding of it:

Thanks!

<SNIP>
> Yes, it's going to "waste" a lot of light, since the light cone
> converging on any given image point is going to have a small f/number
> (e.g. f/8 or f/16) despite the large size of the lens elements.
>
> The unique thing about this lens is that as you adjust focus, the image
> remains the same size - because all of the little cones of light are
> parallel to the optical axis. So image size is determined solely by
> subject-lens distance, and you can adjust for best focus without
> messing up the framing.

This makes it pretty clear what the answer is, but you haven't _explicitly_
answered the critical question here. Which is: "Are any of the Oly 4/3
lenses actually telecentric?"

David J. Littleboy
Tokyo, Japan

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[A complimentary Cc of this posting was sent to
Dave Martindale
<davem@cs.ubc.ca>], who wrote in article <dhaqhv$qbu$1@mughi.cs.ubc.ca>:

Minor nitpicking:

> With a non-telecentric lens, all points in the image plane *except* the
> one in the very centre are illuminated by a ray bundle that arrives at
> the surface on a slant. Roughly speaking, this bundle appears to leave
> the lens via the exit pupil and converges on the destination point in
> the image surface.

By definition of exit pupil, this holds for any lens.

> Now, imagine a lens whose rear element is larger than the image area,
> and whose optics are arranged so there is always *some* ray through the
> lens that arrives exactly perpendicular to the image surface. Further,
> imagine the designer has cleverly placed a stop inside the lens so that
> the *only* rays that reach the image plane are this perpendicular ray,
> plus a small and symmetric cone centred on that perpendicular ray.
> This is a lens that is telecentric in image space.

What you appear to imply is that the exit pupil is at infinity. Is
this interpretation right?

> In this environment, it's obviously very useful to be able to align
> images precisely, and then adjust final focus without messing up the
> alignment.

Looks like one of those pre-digitally-controlled-era compromises; like
having equatorial mount for telescopes... "It is easier to design a
very complicated lens, than design a proper coupling between focus
ring and magnification ring." ;-)

Thanks for very interesting explanation,
Ilya

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Ilya Zakharevich <nospam-abuse@ilyaz.org> writes:
>Minor nitpicking:
>
>> With a non-telecentric lens, all points in the image plane *except* the
>> one in the very centre are illuminated by a ray bundle that arrives at
>> the surface on a slant. Roughly speaking, this bundle appears to leave
>> the lens via the exit pupil and converges on the destination point in
>> the image surface.
>
>By definition of exit pupil, this holds for any lens.

The last sentence is true for any lens. But the first sentence is *not*
true of a telecentric lens, since all the bundles are perpendicular to
the surface. That's the difference.

>What you appear to imply is that the exit pupil is at infinity. Is
>this interpretation right?

I believe so. One way to make a telecentric lens is to put a small
stop at the focus of the lens groups that are ahead of (or behind) the
stop. If that stop forms the pupil of the lens as a whole, it would be
at infinity when viewed from the subject (respectively image) side of
the lens.

>> In this environment, it's obviously very useful to be able to align
>> images precisely, and then adjust final focus without messing up the
>> alignment.

>Looks like one of those pre-digitally-controlled-era compromises; like
>having equatorial mount for telescopes... "It is easier to design a
>very complicated lens, than design a proper coupling between focus
>ring and magnification ring." ;-)

In fact, that's an interesting example. Until very recently, it *was*
easier to build a precision equatorial mount that worked properly than
an alt-azimuth mount for a large telescope. The AA mount is lighter
and cheaper, but requires accurately coordinating the movement of 3 axes
to take time exposure images of the stars. The mount has to move in
altitude and azimuth continuously, plus the scope tube (or at least the
camera) must rotate around its own axis, to keep the image stationary.
Doing that with a fraction of an arc second precision is *hard*, as the
early AA telescope builders found out.

Anyway, I don't think you know what an optical printer looks like.
There is no "magnification ring" because the lens is not a zoom.
Magnification is controlled by adjusting the spacing between projector
head, lenses, aerial image head, and camera. All are mounted to a heavy
cast metal or granite bed. Focus is adjusted in exactly the same way -
by moving one of the optical elements on the bed. There is no focus
ring either. The thing is basically a large optical bench with many
adjustments, except that several of the stations on the bench have full
film transports.

Here are some photos. Unfortunately, they don't show the optics very
clearly:
http://www.scifi2k.com/misc_html/a [...] inter.html
http://en.wikipedia.org/wiki/Optical_printer

Dave

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[A complimentary Cc of this posting was sent to
Dave Martindale
<davem@cs.ubc.ca>], who wrote in article <dhcn33$a93$1@mughi.cs.ubc.ca>:
> >Looks like one of those pre-digitally-controlled-era compromises; like
> >having equatorial mount for telescopes... "It is easier to design a
> >very complicated lens, than design a proper coupling between focus
> >ring and magnification ring." ;-)
>
> In fact, that's an interesting example. Until very recently, it *was*
> easier to build a precision equatorial mount that worked properly than
> an alt-azimuth mount for a large telescope.

Depends on what you call "recent". I think the Burakan telescope was
the first big one with azimuth mount. What it was, design of '60s?
(I remember its introduction being postponed again and again; then it
fizzled when it finally was operational).

> Anyway, I don't think you know what an optical printer looks like.
> There is no "magnification ring" because the lens is not a zoom.

This is why I put things into quotes. Thanks, but I have seen many
optical benches in my life. The moment you can have
computer-controlled movement of component, and reliable feedback about
their position, you have digital equivalents of "rings". ;-)

Yours,
Ilya