# Optical Forums > Ophthalmic Optics >  Circle of Least Confusion

## Snitgirl

Hello Optiboarders,

I am needing some information on "circle of least confusion"

Does anyone out there have a definition/explaination?

I am studying at the moment and I don't have any information on it.

Thanks, I appreciate your time!

Michelle :Nerd:

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## Darryl Meister

A lens with cylinder power produces an _astigmatic_ focus. This astigmatic focus contains a vertical focal line, corresponding to the focus of the horizontal principal meridian, and a horizontal focal line, corresponding to the focus of the vertical principal meridian. The region between these two lines is known as the _conoid of Sturm_ or _Sturm's interval_. At the dioptric mid-point between these two focal lines, the astigmatic focus forms a circular patch known as the _circle of least confusion_.



The location of the circle of least confusion is equal to the spherical equivalent of the prescription:

*Spherical Equivalent = Sphere + Cylinder / 2*

For example, given a prescription of +2.00 DS -1.00 DC 180, the spherical equivalent (and the location of the circle of least confusion) is given by +2.00 + (-1.00) / 2 = +1.50 D, or 66.7 cm behind the lens.

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## chip anderson

Darrel's explaination is textbook and excellent but if you still don't quite grasp this. Take a high plus (+10.00 or more with some cylinder) and go outside and focus the Sun on the ground. You will find that held at one interval you will see a thin line focused, at another you will see another thin line 90 degrees opposed to the first. Now if the lens is held at a distance halfway between these two focii, you will see a circle.
Now invision what would happen to an astigmatic patient if a spherical lens were made to focus halfway between his meridianal powers.

Chip

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## Darryl Meister

> Take a high plus (+10.00 or more with some cylinder) and go outside and focus the Sun on the ground.


Just try not to fry any ants... ;)

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## Dave Nelson

It is also helpful to understand that, the greater the interval, the more "confused" the circle of least confusion will be. By way of example, rx -2.00 -100x180. Adding half of the cyl to the sphere will place the circle of least confusion on the retina, thus the spherical equivilent= -2.50 with the expected va to be about 20/20-2: usually quite acceptable. If we use -2.00 -300x180 the spherical equivilent is -350, the circle of least confusion is on the retina, but the visal acuity would be very poor, maybe something like 20/40 or so. Hope this helps.

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## FVCCHRIS

Wow Darryl, I want to get on the list of people who want your brain if you happen to pass before your time! Chris

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## Darryl Meister

> I want to get on the list of people who want your brain if you happen to pass before your time!


It would make a really ugly paper weight. ;)

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## HarryChiling

> Wow Darryl, I want to get on the list of people who want your brain if you happen to pass before your time! Chris


Get in line buddy :D

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## OPTIDONN

I get his pants!:D

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## Snitgirl

what about just loaning your cerebrum, can I borrow it till the end of the month? I'll pay top $$$$ dollar!  lol;)

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## Fezz

> It would make a really ugly paper weight. ;)



And a messy one at that!! :drop: 


Fezz
 :cheers:

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## Ory

> It would make a really ugly paper weight. ;)


But just think of all the business cards you could wedge between the hemispheres!

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## Darryl Meister

There will be no "wedging" of anything between my "hemispheres," thank you. ;)

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