Refraction at:  
0 °  
90 °  
45 °  
135 °  

  Have you been in front of a case with such an irregular cornea/astigmatism that you do not get reliable readings from automated refractors and you do not really know where to start the refraction?
  This is frequently happening after keratoplasties, keratoconus, complicated corneal surgeries and any other situation producing severe irregular astigmatism.
  To do a meridional refraction use the stenopeic slit. Place it in the 0°-180° meridian and get the best vision using spheres.
  Do the same by placing the stenopeic slit at the meridians 90°-270°, 45°-225° and 135°-315°.
  Fill the results in the calculator boxes and you will get a refraction to start with.
Calculator communicated by Dr César Albarrán from Valencia University (Spain)