Oblique Crossed Cylinders

Contact lens power
 0°
Overrefraction
Suggested trial lens

Purpose:
This tool uses your evaluation of a soft toric contact lens and suggests a new trial lens.

Instructions:

  1. Enter the power of your contact lens in the first box.
  2. Enter the rotation of the contact lens as you see it on the eye. You can do this with the buttons below the image or by clicking the contact lens itself.
  3. Enter your overrefraction in the second box.
  4. If desired, enter the brand of trial lens you're working with.

Results:
The powers of the suggested lens will appear in the last box.

Interpretation:
It is assumed that your new trial lens will be of the same brand as the original, and therefore will rotate exactly the same (therefore it's critical that you make an accurate assessment of lens rotation).

Explanation:
This calculator first adds the amount of rotation to the axis of the contact lens. This is the only way to know how the power meridians are going to line up with the meridians of the over refraction. Once we know how these meridians relate to each other we can then apply traditional oblique crossed cylinder equations. After this calculation, we know what power and what meridians make the patient reach their best corrected vision. However, since the trial lens should rotate just like the original, the calculator then subtracts the angle of lens rotation. The final result is a recommendation for a lens that is expected to rotate like the original lens.

The calculator also takes into account the brand of trial lens. Please note that lens brand does not affect the calculation of the ideal next lens. The calculation solely uses the lens power, lens rotation, and overrefraction to make its suggestion - no considerations are made for lens brand in the calculation. However, once the ideal lens power is established the calculator looks up the chosen lens brand and finds the powers that are closest to the ideal lens. For example, if the recommended axis is 87 deg and the lens axes are available in 10 deg steps it will chose the 90 deg option. Similarly, if the recommended cylinder is -2.75 and the max cyl for the lens is -2.25 it'll recommend this power. In this case it will also adjust the sphere power accordingly to maintain the spherical equivalent.