Helping finding a converging lens with focal length of several feet

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Date: 26 Apr 2007 08:31:04
From: Dave
Subject: Helping finding a converging lens with focal length of several feet

I am building a large camera obscura.

Can anyone recommend where or how to obtain a converging lens with a
focal length of several feet (for less than $200)?

I am open to unique suggestion,s even if they result in some
distortion of aberration.

Thanks
Dave

Date: 30 Apr 2007 02:12:59
From: Chris.B
Subject: Re: Helping finding a converging lens with focal length of several feet

On Apr 27, 9:26 am, Ben <bet71...@netzero.com > wrote:
> On Apr 26, 8:37 pm, William Hamblen <wrhamb...@comcast.net> wrote:
>
>
>
> > On 26 Apr 2007 08:31:04 -0700, Dave <davidberge...@gmail.com> wrote:
>
> > >I am building a large camera obscura.
>
> > >Can anyone recommend where or how to obtain a converging lens with a
> > >focal length of several feet (for less than $200)?
>
> > >I am open to unique suggestion,s even if they result in some
> > >distortion of aberration.
>
> > Can you get a spectacle lens from a local eyeglasses shop? A +1
> > diopter lens would have a focal length of 1 meter (about 40 inches).
>
> > Bud
> > --
> > The night is just the shadow of the Earth.
>
> They could also start with a plano (0.0 diopters) and put
> it on the finer/polisher and give a more tractable focal
> length. They would certainly want to use a CR-39
> (optical plastic) blank rather than the more fragile
> crown or flint glass.
>
> See if an optician will refer you to the optical laboratory
> he uses.
>
> You will also have more of a choice of the diameter
> of the blank, from ca. 35mm to 80mm.
>
> Ben

Be prepared to stop the lens down from full aperture for a shaper and
less colourful view. My first "real" astro telescope was a 1/2
diopter spectacle lens of abut 60mm OD. I built the whole thing myself
to Huygen's aerial design and hung it over a clothes post in the
garden.I still have the lens somewhere but wouldn't even consider
using it for a camera obscura. It "obscures" far too much detail! ;-)

A long focus achromat would be infinitely preferable because you need
such a huge field compared with any telescope. Field curvature is also
a problem if you want to do this well. Amateur Telescope Making (1,2
or 3?) had a short article about Dall's camera obscura. A mirror to
bring the view down to a rise and fall round tabletop is nice because
the inverted image thrown onto a wall or card is a pain in the neck.
(literally) With a table you can orientate yourself so the image is
seen correctly and enjoy the view. A rotating tabletop supported on a
stout screwed rod (studding) is all you really need to focus the table
sharply. The table top can be curved into a gentle bowl to match your
lens' field curvature using plaster of paris or perhaps a preformed
bowl shape. (Old satellite dish?) The image in a decent camera
obscura has a magical quality unmatched by modern (electronic)
displays or by actually looking through a telescope. It's a shame you
don't see the old seaside and viewpoint camera obscuras open for
business these days. Too costly to hire the staff for the tiny income
generated I suppose. Security would be a problem if an obscura was
left unattended.

Date: 27 Apr 2007 00:26:19
From: Ben
Subject: Re: Helping finding a converging lens with focal length of several feet

On Apr 26, 8:37 pm, William Hamblen <wrhamb...@comcast.net > wrote:
> On 26 Apr 2007 08:31:04 -0700, Dave <davidberge...@gmail.com> wrote:
>
> >I am building a large camera obscura.
>
> >Can anyone recommend where or how to obtain a converging lens with a
> >focal length of several feet (for less than $200)?
>
> >I am open to unique suggestion,s even if they result in some
> >distortion of aberration.
>
> Can you get a spectacle lens from a local eyeglasses shop? A +1
> diopter lens would have a focal length of 1 meter (about 40 inches).
>
> Bud
> --
> The night is just the shadow of the Earth.

They could also start with a plano (0.0 diopters) and put
it on the finer/polisher and give a more tractable focal
length. They would certainly want to use a CR-39
(optical plastic) blank rather than the more fragile
crown or flint glass.

See if an optician will refer you to the optical laboratory
he uses.

You will also have more of a choice of the diameter
of the blank, from ca. 35mm to 80mm.

Ben

Date: 26 Apr 2007 20:37:50
From: William Hamblen
Subject: Re: Helping finding a converging lens with focal length of several feet

On 26 Apr 2007 08:31:04 -0700, Dave <davidberger79@gmail.com > wrote:

>I am building a large camera obscura.
>
>Can anyone recommend where or how to obtain a converging lens with a
>focal length of several feet (for less than $200)?
>
>I am open to unique suggestion,s even if they result in some
>distortion of aberration.

Can you get a spectacle lens from a local eyeglasses shop? A +1
diopter lens would have a focal length of 1 meter (about 40 inches).

Bud
--
The night is just the shadow of the Earth.

Date: 26 Apr 2007 15:58:35
From: Rich
Subject: Re: Helping finding a converging lens with focal length of several feet

Date: 26 Apr 2007 15:54:32
From:
Subject: Re: Helping finding a converging lens with focal length of several feet

Date: 26 Apr 2007 15:27:08
From: Helpful person
Subject: Re: Helping finding a converging lens with focal length of several feet

On Apr 26, 11:31 am, Dave <davidberge...@gmail.com > wrote:
> I am building a large camera obscura.
>
> Can anyone recommend where or how to obtain a converging lens with a
> focal length of several feet (for less than $200)?
>
> I am open to unique suggestion,s even if they result in some
> distortion of aberration.
>
> Thanks
> Dave

You may want to experiment with a round sheet of acrylic. Warp it
with a tightened ring around the edge (a hose clamp should work). It
will distort into a spherical shape and end up with a small amount of
optical power. This may work. (You should try doing this after it
has been heated to a few hundred degrees.)

Even if this doesn't work it should be fun. If it does work you'll
have an interesting lens for your camera.

Anyone else got a wackier idea?

Please visit my web site at www.richardfisher.com

Date: 26 Apr 2007 16:38:35
From: Chris L Peterson
Subject: Re: Helping finding a converging lens with focal length of several feet

On 26 Apr 2007 08:31:04 -0700, Dave <davidberger79@gmail.com > wrote:

>I am building a large camera obscura.
>
>Can anyone recommend where or how to obtain a converging lens with a
>focal length of several feet (for less than $200)?
>
>I am open to unique suggestion,s even if they result in some
>distortion of aberration.

How about combining a positive and negative lens that differ in focal
length only slightly?

Anchor Optics sells stock lenses with focal lengths up to a couple of
feet.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com