Mineral monocular add-on lens for near vision.

How to proceed

You can:

Buy a trial case with 3 monoculars that fit the optician's standard trial frame
Rent a trial case, for a one-off sale.

Our add-on lens holder, used with its trial box, lets the optician find the wearer's optimal magnification.

It is universal: all you need is one telescope with a trial ring.

No more need for excessive stock.
Placed on the trial frame, the patient's prescription can be added to assess the benefit with correction.
The trial is done in real-life conditions.

With a reading chart or a newspaper.
On a computer screen.
At intermediate distance, a television screen.

To find the focal power of the add-on lens, divide 100 by the eye-to-object distance (in centimetres).
Choose a large "panto" style frame.

The optical centre must be the near-vision centre.
The distance from the optical centre to the frame must be at least 19mm.
Mark the optical centre (near vision) and check it.

For a monocular fitting, order:

The chosen 1.8x, 2.1x or 2.5x telescope (Code BV20.Gxx ?).
The add-on lens determined with its trial box. Code (BV20.Axx ?)

Option: pre-calibrated ophthalmic lens to be edged to 21mm.

The plain mounting ring (Code BV20.B24) or prescription mounting ring (Code BV20.B20), depending on the option above

A mounting disc to be edged. (Code BV20.D30)

A large case. (Code BV20.E01)

For a binocular fitting, order:

2 chosen 1.8x, 2.1x or 2.5x telescopes (Code BV20.Gxx ?).
2 add-on lenses determined with the trial box. Code (BV20.Axx ?)

Option: 2 pre-calibrated ophthalmic lenses to be edged to 21mm.

2 plain mounting rings (Code BV20.B24) or prescription mounting rings (Code BV20.B20), depending on the option above

2 mounting discs to be edged. (Code BV20.Dxx ?).

A large case. (Code BV20.E01)

A mounting kit. (Code BV20.D34)

Or simply mark the near-vision optical centre on the spectacles and LESA FRANCE takes care of everything..............Code BV20.MONT

Simple, isn't it?........

Conversion table for removable add-on lenses
			Telescope magnification
			1.8 x	2.1 x	2.5 x
0.5 D	2 metres	Watching television	2.00	2.40	2.80
1 D	1 metre	Watching television	2.25	2.60	3.10
1.5 D	66 centimetres	Looking at a screen	2.50	2.90	3.40
2 D	50 centimetres	Looking at a screen	2.70	3.10	3.75
3 D	33 centimetres	Looking at a screen, reading	3.10	3.60	4.30
4 D	25 centimetres	Looking at a screen, reading	3.60	4.20	5.00
5 D	25 centimetres	Looking at a screen, reading	4.00	4.70	5.60
6 D	17 centimetres	Looking at a screen, reading	4.50	5.20	6.20
8 D	12 centimetres	Looking at a screen, reading	5.40	6.30	7.50
10 D	10 centimetres	Looking at a screen, reading	6.30	7.30	8.70
12 D	8 centimetres	Looking at a screen, reading	7.20	8.40	10.00
14 D	7 centimetres	Looking at a screen, reading	8.10	9.40	11.20
16 D	6 centimetres	Looking at a screen, reading	9.00	10.50	12.50

The Galilean telescope.

Telescopic systems are multi-lens systems that allow both distance and near vision.

How does it work?

Its basic principle is that of binoculars.

Binoculars are a Keplerian system.

It consists of a convex objective and an eyepiece that is concave..
When the 2 focal points coincide, the system is said to be afocal.
The magnification of such a system is the ratio of the objective focal length to the eyepiece focal length.
For example, with an 80mm objective and a 40mm focal-length eyepiece, the magnification is 80/40=2.
The total length of the system is then 120mm, and the image is also inverted.

Rather difficult to fit on spectacles.

The Galilean telescope is the most convincing alternative.

It consists of a convex objective and a concave eyepiece.
When the 2 focal points coincide, the system is said to be afocal.
The 2 focal points coincide; since the eyepiece is negative, both lenses lie in front of the focal point.
The total length of the system is then 40mm, and the image is upright, so no erecting prisms are needed.

The drawback is that the eyepiece focal length is very long for a negative lens, which limits magnification to 3x at most to keep a good image.
That said, the system is compact and light, so it can be fitted on a spectacle frame.

Near vision.

Adding a lens in front of a telescope makes the focal length of the system that of the add-on lens.
The magnification is the telescope power multiplied by that of the lens.
For binocular vision, angled mounts are used.
The limit for a binocular fitting is 5D.
To correct ametropia, a prescription lens is placed in front of the eyepiece.
We can carry out the complete fitting.

In summary.

The system is light and can be fitted on a frame.
A Galilean telescope without a lens gives vision at infinity when ametropia is corrected
Near vision at the focal length of the add-on lens.
Without corrected ametropia the image is still good; only the focal distance varies slightly.
Strong astigmatism must be corrected.
To correct ametropia, a prescription lens is placed in front of the eyepiece.
By design, our telescopes have a pinhole effect.

The big advantage.

Lets you work hands-free.
Lets you watch television with additions below 1.
Lets you read an entire screen just by moving your head.

Reference	BV20.A005
Diopters	0.5 D
Working distance	2000 mm
Net weight	6 g
Material	Mineral glass

Eklas & Oklas Frame Case

Additional Bonnette Galilee

Power(D)

Simple, isn't it?........

The Galilean telescope.

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Characteristics:

Ophthalmic lenses

LESA France