Mixed astigmatism conversion formula

Everyone has a certain focus when using their eyes to see things in the outside world, and these focuses can make things in the outside world clearer. However, if a person feels blurry and cannot determine the correct focus when looking at some objects, he should consider whether he has astigmatism. Because astigmatism is also a type of myopia, but astigmatism is more complicated to calculate than general myopia. Next, I will explain to you how to convert mixed astigmatism.

Mixed astigmatism

My opinion on the principle of mixed astigmatism lens

(High astigmatism + anisometropia + mixed astigmatism) Optometry and glasses fitting [difficult problems] and treatment methods

Mixed astigmatism: Mixed astigmatism only needs to meet two basic requirements: one is that the spherical and cylindrical light types are different, and the other is that the absolute value of the spherical power is smaller than the absolute value of the cylindrical power.

Therefore, mixed astigmatism may appear as "-/+" light type or "+/-" light type in the eye prescription. Each company will manage the light type of mixed astigmatism as the same type during inventory management. Our company will warehouse all light types of mixed astigmatism as "+/-" light type.

Mixed astigmatism is a combination of simple myopic astigmatism and simple hyperopic astigmatism, referring to two meridians at right angles to each other, one for hyperopia and the other for myopia. When parallel light enters the eye, the foci of the two meridians are respectively in front of and behind the retina.

There is a formula for conversion: add the balls together, change the sign in the middle, if the axis position is 90, subtract 90, if it is less than 90, add 90.

For example: -1.00+2.50/45 is converted to +1.50-2.50/135

-1.25+1.00/50 is converted to -0.25-1.00/140.

First of all, we need to consider whether optical identity transformation is needed. Transformation is required when the absolute value of the spherical lens power is ≥ the absolute value of the cylindrical lens power or the mixed astigmatism shown on the computer display is actually compound astigmatism. The purpose of the identity transformation is to facilitate lens manufacturing, simplify prescription forms and expose the true type of astigmatism. The identity transformation method is: the vertex power of the new spherical lens is the algebraic sum of the vertex powers of the original spherical lens and the cylindrical lens; the vertex power of the new cylindrical lens is the opposite of the vertex power of the original cylindrical lens; the new axis position: if the original axis position is less than or equal to 900, add 900; if it is greater than 900, subtract 900. For example: -2.00DS+2.00DC×70 (seemingly mixed astigmatism) → -2.00DC×160 (actually simple myopic astigmatism); +3.00DS-3.00DC×170 → +3.00DC×80 (actually simple hyperopic astigmatism), +5.00DS-3.00DC×40 → +2.00DS+3.00DC×130 (actually compound hyperopic astigmatism); -5.00DS+2.00DC×70 → -3.00DS-2.00DC×160 (actually compound myopic astigmatism). If it can be completely corrected, then do it. In general, in order to reduce adverse reactions, the principle of minimum circle of confusion can still be applied to reduce half of the astigmatism added to the spherical power. In the above example, after the transformation, +3.00DC×80 is converted into +0.50DS+2.00DC×80; -2.00DC×160 is converted into -0.50DS-1.00DC×160; +2.00DS+3.00DC×130 is converted into +2.75DS+1.50DC×130; -3.00DS-2.00DC×160 is converted into -3.25DS-1.50DC×160.

Generally high astigmatism

In order to reduce adverse reactions, the principle of minimum circle of confusion is often used to appropriately subtract part of the astigmatism for those who are wearing glasses for the first time, and half of it is added to the spherical lens. The astigmatism degree subtracted from both eyes should be the same, so that the difference in astigmatism between the two eyes should be controlled within 1.5D. Start wearing the lenses with 1/2 or 2/3 of the astigmatism, then gradually increase the astigmatism several times until it approaches or reaches the original refractive power and you get satisfactory corrected vision. Example 1, original refractive power -4.00DC×40→-1.00DS-2.00DC×40→-0.50DS-3.00DC×40→final glasses -4.00DC×40; Example 2, -3.00DS-5.00DC×170→-4.00DS-3.00DC×170→-3.50DS-4.00DC×170→final glasses -3.25DS-4.50DC×170. For those who have worn astigmatism glasses in the past, if additional astigmatism is needed, it will naturally be easier to achieve the correction goal.

Oblique astigmatism

The literature (Izumi Yukio. Japan. General refractive examination. Published by the China Vocational Association Light Industry Branch Optical Industry Training Center. Advanced professional textbook. Page 161.) describes the method of eliminating the adverse reactions of wearing glasses under the premise of correcting better vision, which is summarized as follows:

1. For mild astigmatism (such as -0.75DC), use the principle of minimum circle of confusion to incorporate half of the total astigmatism into the spherical lens.

2. The axis remains unchanged and the astigmatism is reduced, but it is less effective for those with large astigmatism.

3. Maintain the degree of astigmatism, change the axis of astigmatism, and move the axis closer to the horizontal or vertical direction; if the previous method is ineffective, it is necessary to reduce the degree of astigmatism and change the axis of astigmatism. Please refer to the attached table to change the axis.