When giving eye exam or measuring for glasses what does the minus or plus mean and when is the term used?

When giving eye exam when is the term minus or plus used or negative or positive.

An eyeglass prescription is written in a standardized format so it can be understood globally. The right eye, is generally referred to as "OD" or "R", while the left eye is generally referred to as "OS" or "L". The right eye is almost always on top in a written prescription with the left directly below. Ignoring for sample sake, the right or left eye, let's look at a example below:

-2.00 -1.00 x 90. The first number (-2.00) tells us the spherical refractive diopter (a unit of measurement) needed to correct (farsightedness or nearsightedness). In this example, a minus sign in front of the number indicates a correction for nearsightedness. A plus sign would indicate a correction for farsightedness. This is generally true when you are talking about the first set of numbers.

The plus and minus signs on the second number, generally indicates what professional examined your eyes. An optometrist usually refracts in what's referred to as "Minus Cylinder, while an ophthalmologists refracts in "Plus Cylinder". For example, an optometrists script would be -2.00 -1.00 x 90, while the same prescription written by an ophthalmologists would be; -2.00 +1.00 x 180. Please note that the second number has a plus sign, and the last number (180, the Axis) has been transposed 90 degrees.

The second number (-1.00) is for astigmatism. If there is no astigmatism correction needed then you would not see the third (180) number. Sometimes you might see the following; SPH written for a cylinder correction instead of a number and nothing written for the third number. SPH stands for "Sphere" which indicates that there is no astigmatism correction needed.

The final number (180, the Axis line) is the direction of the astigmatism. Astigmatism can be measured in any direction around the clock. We use the numbers from 001 to 180 to indicate the orientation of the correction needed.

Depending on your need, there may be additional numbers in a eyeglasses prescription as well. If your prescription has a set of numbers, or a single number with a symbol such as a triangle, or the letters " BI, BO, BU, or BD that would indicate a prism correction. BI = Base In, BO = Base Out, BU = Base Up, and BD = Base Down. It is not uncommon to have different base directions for either eye.

Also, you will see "ADD" numbers for those requiring bifocals or reading glasses. The ADD number is exactly what it indicates...; an ADD, or an additional script to an otherwise already existing prescription. For example, your prescription is -2.00 for the first number. (In this example there is no astigmatism). For the "ADD " number you have a +3.00.This would indicate that by 'Adding" the +3.00 to the -2.00, your reading prescription would be +1.00 (adding a greater positive number to a lesser negative number results in a positive answer).

In optics the words "plus" and "minus" are used to teel us the prescription numbers. Plus numbers give you magnification and munus give you minification. A plus lens is thinck in the middls and thin on the outsides and corrects for hypermetropia (Farsightedness) and Presbyopia (Reading Glassses) Minus lenses are thin in the middle and thicker on the outsides, these correct for mostly nearsightedness. The words postitive and negative usually refer to the curvature of a frame, and how it is curved sitting on the face or shapes of something, not to numbers. So we use the words in a different way than what is used by laymen. It is s funky world, but we all have to see and someone has to know this stuff!
Certified optician
- is for correction of being farsighted and + is for correction of being nearsighted
If you take a piece of window glass and light rays pass through it, they pass without deviating.

If light rays pass through a magnifying glass, which is thick in the center and thin at the edges, convex, the center light ray will pass through undeviated. But as you get further from the center, the light rays bend. They bend towards a point a certain distance away. If the light rays are bent a LOT, then the focal point or focal distance will be very close to the lens. Weaker lenses don't bend the light much, so the focal distance is further away.

The power of the lens will equal 1/d where d is in meters.
If that lens focuses light one meter away, that's a +1.00 lens.
half meter away +2.00
third of a meter +3.00
quarter of a meter +4.00
tenth of a meter +10.00

Lenses that are thick on the outside and thin in the middle bend light too. They bend the light out. The light rays that are bent out would 'appear' that they came from a point on the other side of the lens. If it appears that the light rays, if you traced the line backwards, would cross that center, undeviated light ray at a certain distance.

So the power of that lens would be -1.00 if the light rays appeared to have come from a meter away. -2.00 from a half meter away. -3.00 from a third of a meter.

People that are nearsighted have stronger than normal eyes. They are already focused near the eye or nearer than optical infinity (about 20 feet).

So if one were a myope, nearsighted, shortsighted...and they were focused a one meter in front of them, their eyes would be +1.00 too strong. They'd wear a -1.00 lens to move that focal distance out. In wearing that minus lens, the light passing through is deviated out as IF it came from a meter away, which is where they are focused already. So everything comes into focus. The eye things the object of regard is a meter away.

Same for half meter, -2.00
third of a meter -3.00 etc.

When they talk of astigmatism, it just means that at the axis, the power is added to the base number so it's more powerful (+ cylinder), or less powerful (-cylinder) than the first number.

So if the first number is a -3.00 and the next number is a -2.00 at axis such and such, the real power at the axis line is only -1.00. One adds the second number to get the overall power at the axis line.

People that are farsighted have weaker eyes. They don't bend the light enough to focus it on the retina. The image is focused behind the retina. So the "+" power will move the image up towards the retina. If they eye is a +3.00 hyperopic eye, it needs to bend light that much to get to the back of the eye. (third of a meter).

that's basically it...