How to reduce field width in an extra-wide (1 Viewer)

O

OPTIC_NUT

Well-known member

• Jan 16, 2014

• #1

I'd like to reduce the field width of an extra or ultrawide set of
binoculars. If the eyepiece were a Kellner I'd know what to do
at the field lens, (the image plane seems to be at the field lens)
but extra-wides don't have Kellners. Not sure
where a clean cut of the image can be made for other types.

I have seen a Plossl with a field lens outside the achromatic pair.
The smutz on the 'field lens' didn't seem as sharp as in a Kellner
before the cleaning.

 

Last edited: Jan 16, 2014

H

henry link

Well-known member

• Jan 17, 2014

• #2

You need to find the field stop. In every one of these old binoculars it will be in front of eyepiece lenses. Sometimes it's attached to the eyepiece (where it may be very close to the field lens), sometimes it's in the binocular body between the eyepiece and the prisms.

 

O

OPTIC_NUT

Well-known member

• Jan 17, 2014

• #3

Thanks very much, Henry.
It sounds like I probably don't have to get into the core of the ocular to place things.
That is a huge relief. I'll wait for a decent but not super ex-wide to come along
that needs an internal visit.

--"The Nut"

 

james holdsworth

Consulting Biologist

• Jan 18, 2014

• #4

Generally speaking, what is the field stop made of?

 

O

OPTIC_NUT

Well-known member

• Jan 18, 2014

• #5

james holdsworth said:

Generally speaking, what is the field stop made of?

Click to expand...

Before the light gets to the field lens, typically, it's an obstruction
like an irs or shoulder that restrict the field of view. It is the narrowest
restriction before the field lens.

There are places where a restriction reduces light but not so much the
field, and places where the field can be "cut" and have a real effect
on the image. It has to be at or near an image plane to cleanly cut off
a piece (ring) of image.


My problem was I couldn't dig up where to place them in an Erfle or Plossl.
The answer means, I think
....there will likely be a marker there in the structure because that's the
place for important stopping.

 

Last edited: Jan 18, 2014

H

henry link

Well-known member

• Jan 18, 2014

• #6

james holdsworth said:

Generally speaking, what is the field stop made of?

Click to expand...

Metal or sometimes plastic in cheap binoculars.

 

O

OPTIC_NUT

Well-known member

• Jan 18, 2014

• #7

Oops, I answered too wide.

I have see a field stop that was machined into the body of the ocular as well...

 

Simon S

Well-known member

• Jan 18, 2014

• #8

Fit a washer to the field lens will act as a field stop i'm pretty sure.
Or fit some zoom binocular oculars ;-)

 

O

OPTIC_NUT

Well-known member

• Jan 18, 2014

• #9

Zoom oculars...gack! ...heh
Depressing looking through those...there is this little
disc that gets bigger and a whole lot of blackness instead of field.
Nothing that brutal..

I would start with Erfles or Plossls. 11 or 13 degree.
Erfles I have seen are from the book.
The Plossls I have seen are regular Plossl plus a pseudo field lens up front.

Some astronomical types talk about using "Super Plossls"
for big depth of field in terrestrial observation. It seems what's already in
binocs might already do that, because it has to have wide phase-correction
for extra wide field use anyhow. Just trying to take the deep oculars
and push for more field depth.

There is a risk that 10degree extra-wides are optimized and I can't really
taking advantage of a field reduction just by cutting the width. I'm just
going through the mental exercise right now. I don't have a mini-lathe
and chance washers wouldn't cut it for ocular dimensions. For now,
just pondering. I don't work with guys who have the nice tracing software anymore.

 

Last edited: Jan 18, 2014

R

ronh

Well-known member

• Jan 19, 2014

• #10

I cut circles in thin but substantial plastic sheet with a drafting tool called a divider. You need the kind that adjusts with a screw, which will hold its adjustment through some stressful use. I sharpen the points very sharp with a sharpening stone, and with the sheet of plastic resting on a wooden backing like a kitchen cutting board, I poke a hole with one end of the divider where I want the center of the circle to be, and go round and round, applying force with the "cutting" end of the divider. It doesn't take long to make a neat round hole.

A good choice of material for this project would be a plastic binder from an office supply company.

Round things are the blood of the amateur optics buff!

Ron

 

O

OPTIC_NUT

Well-known member

• Jan 19, 2014

• #11

Aha...the sort of thing I'm looking for...variability.
Chewy HDPE binder plastic is good for experiements, too.
The disc/donut can be pushed in on the end of a dowel.

Scale is a remaining question: what diameters (outer) could this do accurately?