# Thread: Circle of Least Confusion

1. ## Circle of Least Confusion

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.

Michelle

2. 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.

3. ## Mo's simpler.

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

4. 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... ;)

5. 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.

6. ## Organ donation

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

7. 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. ;)

8. Originally Posted by 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
Get in line buddy :D

9. I get his pants!:D

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

11. Originally Posted by Darryl Meister
It would make a really ugly paper weight. ;)

And a messy one at that!! :drop:

Fezz
:cheers:

12. Originally Posted by Darryl Meister
It would make a really ugly paper weight. ;)
But just think of all the business cards you could wedge between the hemispheres!

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

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