1. ## Eyepiece Focal Length?

Can someone draw me a word-picture or direct me to a resource showing
just how the focal length of an ep is measured? That is, if one could
cut an ep in half lengthwise showing everything intact, from which
part to which part is the f/l measured?

David Neal Minnick
Lake Elsinore, CA

"The meek shall inherit the gulag."

2. ## Eyepiece Focal Length?

"David Neal Minnick" <dmfopticsdavid@verizon.net> wrote in message

This website shows calculations for 2-3 element eyepieces:
http://www.astronomyboy.com/eyepieces/ep_calc.html

Regards,
Ed T.

3. ## Eyepiece Focal Length?

The FL of an eyepiece is not measured from anyplace in particular, unless it is
a single element EP. The EFL is based on the magnification with a particular
objective, not a measurment from somewhere in it. There are formulas for the
EFL of an eyepiece, depending on the design and glass used. The formula for a
2-lens EP is pretty straightforward (using the FL of each single lens and the
distance between them), but adding more elements complicates this. Generally,
the FL printed on an EP is an approximation.
(And considerate neighbors!!!)

4. ## Eyepiece Focal Length?

On 27 Dec 2003 19:03:29 -0800, dmfopticsdavid@verizon.net (David Neal
Minnick) wrote:

If you know the curvatures, thickness and type of glass for each
element in the eyepiece you can calculate the focal length of the
eyepiece. That's what lens designers do. If you had an optical bench
you could measure it directly. There are natural variations in
manufacturing tolerances so the focal length engraved on the eyepiece
may not be exact. Or the design can change, but the markings stay the
same.

Lenses have imaginary planes called the principal planes where the
rays of light that enter and exit seem to bend when you trace the
rays. The locations for any given eyepiece depends on the design.
The focal length is measured from those planes.

5. ## Eyepiece Focal Length?

"William Hamblen" <william.hamblen@earthlink.net> wrote in message
news:0a1uuvos29dvhcgg630t977pg49lfjnc6a@4ax.com...

I believe the OP question was _not_ answered. The way I understood it, he
was asking where is the _reference point_ from where the focal length is
considered for a multi-element eyepiece . For example let's say we have a 4
element ep , which are spread (due to their thickness and their spacing)
over a 1" length .Let's say the eyepiece is a 7mm FL. Where is the resultant
focal plane exactly ? Depending where is this "reference point" situated
lengthwise , one could have the focal place very close or very far from the
eyepiece end , or last/first element . This question is important if one
wants to know how to connect various accessories, tube length, focuser
length , etc . I'd like to see other answers than "just measure it" or "it's
hard to explain" , "do a google search" etc .

Thank you,
Matt

6. ## Eyepiece Focal Length?

On Sun, 28 Dec 2003 11:53:43 -0500, "matt"
<mariusrf@removethisbellsouth.net> wrote:

In eyepieces with an external field stop, the first principal plane is
one focal length away from the field stop. You could locate the
second principal plane by catching the image of the moon on a piece of
card stock and measuring one focal length in from that point.

To measure any better would take a well equipped optical shop. The

7. ## Eyepiece Focal Length?

Note: If you take your eyepieces apart to measure the distances between the
elements in order to use this calculator, the chance that you will ever get
them back together again correctly is rather small.

8. ## Eyepiece Focal Length?

David,
Here is a way to find the focal length of your eyepieces by direct
experiment that I posted a while back. It worked very well for me.

"If you want to try measuring the focal lengths of your eyepieces here is

a simple way that seems to give me good accuracy of about +-0.1 mm in
the focal length even for the short focal length eyepieces. It also
works if the eyepiece is used with a Barlow and gives the effective
focal length of the eyepiece+Barlow combination so you can thus get the
amplification factor.

Its based on measuring the magnification seen when you use the eyepiece
to project a Ronchi grating on to a distant screen (im my case a white
wall). To use it you put a Ronchi grating behind the eye end of the
eyepiece, with a flashlight behind the Ronchi to illuminate it. You then
carefully adjust the separation of the eyepiece from the Ronchi so that
a sharp (greatly enlarged) image of the grating is shown on the wall.
You the measure the separation of the projected lines on the wall (I
measure the span of about 5 cycles and then divide by 5). Knowing the #
of lines/inch on the grating you divide the separation on the grating
itself into the measured separation as projected to get the
magnification. The only thing left is to measure the separation of the
eyepiece from the wall. To a first approximation the eyepiece focal
length is that distance divided by the magnification. If the distance to
the wall is much larger than the focal length the error will be small.
To get a better approximation you have to allow for the separation of
the eyepiece principle planes. You can correct for that by changing the
distance to the wall and repeating the measurement. The math for that is
not hard but perhaps a bit long for a post here. By the way I posted on
this in sci.astro.amateur back on about Feb 10,2000.

ruler with a mm scale can also be used but the projected mm separation
is large for short focal length eyepieces and distortion may then throw
off the result.

Have fun if you try it,
Don Taylor"

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9. ## Eyepiece Focal Length?

Don <europa@bendcable.com> wrote in message news:<3FEF7F2F.8F0562FD@bendcable.com>...

Thanks, Don.

I was more interested however, in knowing just how the designers come
up with the measurement in a diagrammatical, drafting table
visualization of it.

DM

10. ## Eyepiece Focal Length?

"David Neal Minnick" <dmfopticsdavid@verizon.net> wrote in message
news:<3FEF7F2F.8F0562FD@bendcable.com>...
Your best bet, if you want to try 'playing' with some optics to see what
happens, is to download OsloLT, which is a complete optical design program,
with the 'LT' version being limited on the number of elements that can be
used (but being free). Programs like this do all the 'hard work'. Simple
physics gives the way that a lens element behaves, but when the different
elements are based on different glasses, the surfaces on each side of the
element may differ in curvature, and may use unusual curves, calculating the
effects for different light frequencies and paths, involves a _lot_ of
arithmetic. Design programs that do the calculations for you, have become
the 'norm' because of this.
Also try looking at:
http://www.atmsite.org/
This is the 'amateur telescope makers' homepage, and there are some articles
on there, illustrating some eyepiece designs, and the optical formulae.

Best Wishes

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